Note: Descriptions are shown in the official language in which they were submitted.
i96
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to analytical measurement an~
in particular to electrodes for determining specific ion
concentrations in solution. More specifically, this invention
relates to multilayer elements for use in the potentiometric
determination of ion concentrations in aqueous liquids,
particularly body fluids such as blood sera.
Description of Related Art
The related art is replete with a great variety of
electrode types and structures for the measurement of various
ions in solution. Typically, devices for obtaining such
measurements include a reference electrode and a separate ion-
selective electrode. The ion selective electrode incorporates
a reference cell. When simultaneously immersed into the same
body of solution to be analyzed, the reference and ion-selective
electrodes constitute an electrochemical cell, across which
a potential develops. This potential is proportional to the
logarithm of the activity of the ion of choice which is
related to concentration in the solution of the ion of choice
to which the ion-selective electrode is sensitive. The
foregoing relationship between the potential and ionic activity
in solution is described by the well-known Nernst equation.
An electrometric device, usually either a direct reading
circuit or a null-balance potentiometric circuit, is employed
for measuring the potential between the electrodes.
In the past, the ion-sensitive electrode generally
comprised an electrode body (usually some type of glass
container) containing a known reference solution in contact
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with a half-cell of known potential, generally Ag/Agcl/llxM
and an ion-selective glass membrane mounted in an aperture
in the electrode body in such a fashion that, when the
electrode was immersed in the unknown solution, the glass
membrane contacted both the reference solution within the
elec~rode body and the unknown solution. An appropriate
metal probe coated with a layer of an insoluble salt of the
metal immersed in the contained reference solution served as
the contact for the electrode. The selectivity of the
electrode was determined by the composition or components
of the glass membrane. Such electrodes are referred to herein
as "barrel" electrodes. U.S. Patent Nos. 3,598,713 and
3,502,560 provide detailed descriptions of electrodes of
this type.
More recently, the development of synthetic,
polymeric ion-selective membranes as substitutes for the ion-
selective glass membrane has broadened the list of ions which
can be determined potentiometrically using "barrel" electrodes.
Such membranes generally comprise a polymeric binder or support
impregnated with a solution of an ion-selective carrier or
ionophore in a solvent for the ionophore. Membranes of this
type can be custom-designed to transport selectively a
particular ion by careful selection of the ionophore, solvent,
etc. Membranes of this type and "barrel" electrodes containing
such membranes as substitutes for the glass membranes are
described in detail in the following U.S. Patent Nos.:
3,562,129 to Simon issued February 9, 1971,
3,691,01l7 to Ross et al issued September 12, 1972, and
3,753,887 to Kedem et al issued August 21, 1973.
The principle advantage of the "barrel" electrodes,
in addition to their high selectivity, is that if certain
11~6696
rigid conditioning procedures are applied between measurements,
the electrode can be used repeatedly for measuring the
concentration of the same ion in different solutions.
The major shortcomings of some conventional ion-
selective electrodes include:
(1) cost: generally a single electrode will cost
several hundred dollars;
(2) fragility: the body and the membrane of glass
electrodes are fragile; and
(3) reproducibility: even with the most carefully
performed conditioning procedures, after the
first use of the electrode to determine the
anionic activity of unconditioned fluids such as
body fluids, the exact composition of the
electrode membrane (glass or polymeric) is
unknown due to the potential for contamination
by earlier test solutions, and for this reason
the results are often suspect.
In an attempt to solve some of the foregoing problems,
20 Cattrall, R.W., and Freiser, H., Anal. Chem., 43, 1905 (1971),
and James, H., Carmack, G., and Freiser, H., Anal. Chem., 44?
856 (1972), described calcium ion-selective "coated wire"
electrodes comprising a platinum wire coated with a layer of
a polyvinyl chloride solution of, for example, calcium
didodecylphosphate (see also British Patent No. 1,375,785
published November 27, 1974). These authors make no mention
of the use of an internal standard reference electrode or an
internal reference solution and, in fact, specifically exclude
these components. These electrodes are evaluated in
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Stworzewicz, T.~ Cyapkiewicz, J., and Lesko, M., "Selectivity
of Coated Wire and Liquid Ion-Selective Electrodes" at the
Symposium on Ion-Selective Electrodes at Mutrafured, Hungary,
October, 1972 (Proceedings reported in Ion-Selective Electrodes,
edited by Pungor, E., Budapest, 1973, at pp. 259-267). The
elect~odes exhibit significant drift in electrical potential
which requires frequent restandardization and hence makes
their commercial use difficult.
Other known ion-selective electrodes are the
reference and hydrogen ion-selective electrodes described
in U.S. Patent Nos. 3,833,495 issued September 3, 1974 and
3,671,414 issued June 20, 1972, both to W.T. Grubb. These
electrodes use a sllver-silver halide reference electrode
immersed in a thickened reference solution of a suitable
"solvent medium," for example agar, carboxymethyl cellulose,
polyvinyl alcohol, etc., and an ionic salt, e.g., KCl, in a
shrinkable tube structure open at one end to the solution
to be tested. In use, the reference solution contacts the
solution under test directly with no intervening ion-selective
membrane. The reference solution contains substantial
quantities of water as evidenced by the fact that the
recommended procedure for preparing the electrode involves
injecting the electrolyte into the structure using a syringe.
French Patent Publication No. 2,158,905 published
June 15, 1973, describes an ion-selective electrode which
utilizes as the internal reference electrolyte solution a
solution of a suitable salt (e.g., KCl) in a hydrated
methylcellulose gel or, alternatively, a hydrophobic poly-
styrene ion-exchange resin overcoated with an ion-selective
3 membrane comprising, for example, an organo polysiloxane or
1~.66g6
polycarbonate binder having a suitable ion carrier, for
example, valinomycin dissolved or dispersed therein. The
internal reference electrode described in this element
comprlses a metal wire (e.g., Ag) having a controlled coating
of salt (e.g., AgCl) thereon. Whichever of the two alternative
refercnce electrolyte materials is used (i.e., the gel or
the ion-exchange resin)~ it is "hydrated" prior to application
of the overlying ion-selective membrane.
In the case of hydrophobic ion-exchange materials
prepared as described in U.S. Patent No. 3,134,697 to Niedrach
which is cited in French Patent No. 2,158,905 as disclosing
the preparation of such materials, the water content of
these ion exchangers is between 15 and 50 percent. As is
recognized by the skilled artisan, this water of hydration
can be removed from such ion-exchange materials only with
great difficulty.
U.S. Patent No. 3,730,868 to Niedrach issued May 1,
1973, describes a carbon dioxide-sensitive electrode which
uses a silver/silver halide internal reference electrode
and a quinhydrone electrode as a pH sensor to detect changes
in pH induced by C02 which penetrates an overcoated carbon
dioxide-permeable membrane. There is no suggestion in this
patent that useful electrodes can be obtained by overcoating
the redox electrode directly with an ion-selective membrane
to obtain an ion-selective electrode. Rather, an ion-exchange
resin is used as an electrolyte solution to quantify variations
in C02 concentration as permitted by the C02-permeable
membrane. The electrode is therefore similar to those
described in French Patent No. 2,158,905, except that in
one aspect a solid quinhydrone electrode is used as a pH sensor.
.. ,, ~ .. . ~ . . , . .. .. .. _ . .. . . . .
1~166~
~ .S. Patent No. 3,856,649 lssued December 24,
1974, to Genshaw et al and a paper by the same authors
entitled "Miniature Solid State Potassium Electrode for
Serum Analysis", Analytical Chemistry, 45, pp. 1782-~4
(1973), describe a solld state lon-selectlve electrode ~or
potasslum ion detection, which electrode ~omprlses, on a
wlre, an electrically conductive inner element with a salt
disposed on a surface portion thereof havlng as a cation, a
cationic form of the inner element and also having an anion, a
solid hydrophilic layer in intimate contact with the salt and
lncluding a water-soluble salt of the anion and a hydro-
phobic layer in intimate contact with the hydrophilic layer
whereby the hydrophilic layer is shielded from contact with
the ion-containing aqueous solution under test when the
electrode is immersed therein. The patent refers to the
importance of maintaining the electrode in a "hydrated"
state during the course of manufacture and states at column
3, lines 27-29, "This hydrated state is considered important
to the proper functioning of the electrode Or thls inven-
tion-"
Although the Genshaw et al patent makes no specific
and clear reference to it, the publication clearly states, and
applicants have found in their evaluations of such electrodes,
as demonstrated in the examples below, that, if accurate and
reproducible results are to be obtained, electrodes of this
type ~ust be hydrated prior to use if stored dry (l.e.,
under ambient conditions, RH 40-50%) for extended periods
after manufacture. Such hydration requires that the electrode~
be stored in an aqueous solution or preconditioned in an
3 aqueous solution prior to use in an ion-activity-determining
operation. Failure to use such preconditionin~ or storage
, . . . . .. _ ... .. . ~ . .. . ~
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techniques will result in the generation of non-Nernstian
responses which exhibit substantial random drirt as described
hereinafter, at least until such time as the electrode is
hydrated by the sample solution. Furthermore, lf the electrode
is used in a "dry" or unpreconditioned state to quantlfy ions
in a small sample of liquid (on the order of less than
about 100 ~1), the absorption of the substantial amounts of
water which are necessary to bring the wire electrode to
equilibration may result in a substantial change in the actual
ion concentration before a reproducible potentiometric
reading can be obtained.
Thus, although the "solid-state" electrodes described
by Genshaw et al offer substantial advantages of size and
the quantity of sample required for measurement, as compared
with electrodes of the prior art, they retain one very
significant shortcomin~, namely, they must generally be
either stored "wet" or hydrated (i.e., preconditioned) ~or
some period prior to use.
Israeli Patent No. 35,473 publ~shed May 16, 1974
to Dr. Reinhold Cohn and Partners and assigned to Hydronautics -
Israel Limited entitled "Ion-Specific Measuring Electrodes"
describes an ion-selective electrode comprising an ion-
selective membrane in conducting contact with a "conductlve
solid material," namely graphite, (particulate or solid)
which in turn contacts a wire lead for the electrode. No
reference solution or redox couple is described or suggested.
U..~. ~atent Nos. 3,6~19,506 and 3,718,569 issued
October 14, 1969 and February 27, l973 respectively "solid-
state" glass electrodes in which a conductor having a surface
3 layer of an electrochemically active metal is coated with a
first coating of a mixture of a glass and a halide of the
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- active metal and a second outer coating of ion-sensitive
glass. Presumably such electrodes require the same precondi-
tioning techniques as conventional glass electrodes.
U.S. Patent No. 3,900,382 issued August 19, 1975
describes a miniaturized electrochemical electrode which
functions as both an oxygen or carbon dioxide electrode and
an ion-selective electrode. At Column 2, lines 43-53 it is
suggested that the various layers could be applied by dipping
the metal wire core of the electrode in various organic
solutions after which each solution solvent was evaporated.
Quite obviously this description cannot apply to the "elec-
trolyte layer" designated 17 which comprises a solution of
sodium bicarbonate and sodium chloride with a thickening
agent. Such a layer could not be provided from an "organic
solution" and hence the suggestion as to manufacture is
inapplicable. Furthermore, at Column 4, lines 49-58 the
electrolyte is explicitly described as an aqueous solution.
Description of the Drawin~
~igures 1 and 2 are cross-sectional views of
ion-selective electrodes as described herein.
Figure 3 shows typical traces of potential vs. time
obtained using the ion-selective electrodes of the present
invention as described in Example 47.
Summary of the Invention
According to the present invention, there is
provided a dry-operative ion-selective electrode comprising:
(a) a dried internal reference electrode, and
(b) in contact with the reference electrode,
a hydrophobic ion-selective membrane.
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The ion-selective electrodes described herein
require no preconditioning prior to use in an ion-sensing
operation.
The reference electrode may comprise either a
metal metal-salt reference half-cell or a dried single- or
mult~ple-layer redox couple reference electrode which is
similarly wetted upon application of an aqueous sample as
described hereinafter. The term "dried" as used herein is
defined below.
According to a further preferred embodiment,`the
hydrophobic membrane includes an inert ion carrier dissolved
in a suitable carrier solvent dispersed in a hydrophobic
binder. A most preferred embodiment provides for electrodes
of the type described which provide a substantially planar
surface for contacting with a sample for testing and comprise
a hydrophobic membrane of predetermined uniform thick~ess in
regions intended for contact with a sample for analysis.
The electrode optionally includes a support. A novel
technique for making measurements of ion concentration by
reading the electrode prior to substantial hydration thereof
is also described.
The novel ion-selective electrode of the present
invention, which is designed for use in the potentiometric
analysis of liquids, is simple in structure, easily manufactured
at a reasonable cost and therefore disposable, and highly
accurate due to its economically feasible single-use capability
which insures the integrity of the ion-selective membrane for
each new measurement. As will be described in somewhat greater
detail below, the electrode can be prepared in a variety of
formats and geometries.
For a complete understanding of the invention
described herein it is necessary to have an understanding of
the phenomenon of electrode drift. As is well known to those
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skilled in the art, electrode drift is the variation in the
potential sensed by an ion-selective electrode in contact
with an ion-containing solution over a period of time.
Electrode drift is apparently due to a number of
factors such as permeation of the ion-selective membrane by
test solution solvent (generally water) with the passage of
time, variations in ion concentration in the test solution
in the region of the solution proximate the electrode, which
variation is caused by the aforementioned solvent permeation,
etc.
All ion-selective electrodes demonstrate some drift,
however, the phenomenon is minimized in conventional electrodes
by preconditioning the electrode to bring the electrode to
an equilibrium state approximating that expected to be
encountered in a testing situation. In this fashion the user
diminishes the factors which cause drift and consequently
reduces drift in the testing situation. One might expect,
therefore, that the use of a totally "unconditione~" ion-
selective electrode would result in severe drift of potentially
catastrophic proportions which would prohibit such use of
the ion-selective electrode until the equilibrium state
usually achieved by preconditioning had been reached in the
test situation. Quite unexpectedly, it has now been discovered
that ion-selective electrodes can be prepared which can be
used without preconditioning of any sort and that the drift
exhr~ted by these electrodes, although sometimes
substantial, can be calibrated to provide accurate and repro-
ducible determinations of the concentration of specific ions i~
test solutions. The features which impart this unusual
performance capability to the electrodes of the present
invention as well as the techniques for their manufacture and
use will be elaborated hereinafter.
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As used herein, the term "dry-operative" describes
an ion-selective electrode which provides reproducible
potentiometric determination of ionic activity which can be
rela~ed to the ion concentration of aqueous test solutions
with no requirement for "wet" storage (i.e., keeping in an
aqueous solution) or preconditioning (i.e., soaking in a
salt solution) prior to use. Essentially, what this means
is that a "dry-operative" electrode produces accurate,
reproducible and detectable determinations of potential
which can be calibrated and thereby related via ionic activity
to ionic concentration in an aqueous test solution without
having first to be substantially hydrated or brought to the
aforementioned equilibrium state. Many of the electrodes
described herein perform in this manner even when used
immediately after storage at RH 20%. The practical application
of this definition will be made more apparent from the dis-
cussion and examples which follow.
The term "thin" when used in reference to individua]
layers of preferred embodiments of the electrodes of the
present invention describes individual electrode layers
having a maximum thickness of about 50 mils. Preferably,
such "thin" layers are on the order of less than about lO mils
in thickness. Most preferred are layers on the order of less
than about 2 mils.
The term "predetermined uniform" when used herein in
refe~ence to the thickness of the ion-selective membrane of a
"dry-operative" electrode describes a thickness tolerance in
regi-ons of the layer intended for contact with a sample for
analysis. This tolerance is met if the drift exhibited by
the electrode incorporating such a layer can be calibrated,
. i.e.~ can provide reproducible and detectable determinations of
potential related to concentration by calibration within an
error tolerance acceptable for the particular measurement
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without preconditioning or permitting the electrode to reside
in the test solution for a period sufficient to achieve
conditioning by the test solution. Electrodes which do not
possess "predetermined uniform" thicknesses will exhibit a
random drift which cannot be calibrated to provide results
which are directly related to ionic concentration. Uniformity
of thickness as exhibited by dry operativeness will generally
call for a maximum variation in the thickness of the membrane
of at most about 20% in regions thereof intended for contact
with a sample for analysis.
The term "dried" when used in reference to layers
of electrodes described herein refers to a physical state of
such layers brought about by subjecting, in manufacture, the
layer to drying conditions, i.e., conditions of temperature,
reduced vapor pressure or whatever, adequate to accomplish
removal of sufficient solvent or dispersing medium as to
render the layer non-tacky, as this term is commonly inter-
preted in the coating arts, prior to the applieation of any
overlying layer(s). This drying to drive off solvent or
dispersing medium is a major factor imparting the "dry-
operative" capability to the electrodes of the present
invention. Although the mechanism of this phenomenon is not
fully understood, and applicants do not wish to be bound
by any theory of operation for their electrodes, it appears
that the shri~king which the "dried" layer undergoes with loss
of ~quid in drying assures intimate contact between the "drie~"
layer and the conti~uous superposed ion-selective membrane
even under relatively harsh storage conditions of very low
relative humidity, i.e., 20% Rll or less. In this reGard, it
is generally desirable to relate the relative humidity of
drying conditions in manufacture to the expected conditions
of use as this will provide an optimized state of hydration
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for the ion-selective electrode. This relation is, however,
not necessary to obtain useful electrodes. More specific
conditions and requirements for certain dried layers of the
electrodes of this invention will be stated hereinafter.
Typical of the "dried" layers described herein are
those o~tained by forming layers under the following condi-
tions:
(A) A solution comprising from about 5 to about
9% by weight gelatin is coated at a level of
about 64 g/m2 and dried under the following
conditions:
(l) chill set for about six mlrl~;tes at 4C
and a dew point of 50~ R~; and
(2) dry for about four minutes at 21C and
50% RH;
(B) Solutions of from about 5 to about 9% of
polyvinyl alcohol and poly(2-hydroxyethyl
acrylate) are coated at a level of about
64 g/m2 and dried under the following condi-
tions:
(l) heat set for about six minutes at 55C
and 50% RH; and
(2) dried for about four minutes at 35C
and 50% RH.
These drying conditions are not required to obtain layers
of the type described herein~ however~ they are typical of
conditions which may be used to obtain dried layers of the
type described herein using a variety of polymer matrixes
suitable as the binder for the reference electrolyte layer.
Such layers demonstrate the "dryness" required of the reference
electrode layer prior to application of the ion-selective
membrane.
.
14
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The electrodes described herein are generally
capable of producing concentration determinations which
demonstrate a coefficient of variation of less than about
10%: ~lectrodes prepared in accordance with preferred
embodiments hereof demonstrate coefficients of variation of
less-than about 3~ and in certain highly preferred embodiments,
coefficients of variation of below about 2% have been
achieved.
Description of the Preferred Embodiments
As described hereinabove, previous so-called
"solid-state" electrodes required the incorporation as an
internal electrolyte of either an aqueous salt solution,
a hydrated salt, or a layer of salt impregnated glass to
achieve operative measurements of ionic concentration. All
such electrodes require preconditioning prior to use in an
ion-sensing operation. It has now surprisingly been
discovered that electrodes having a "dried" internal
electrolyte and ion-selective membrane of predetermined
uniform thickness can be used to achieve stable levels of
precision and accuracy in potentiometric ionic determinations
similar to those until now achievable with electrodes which
required preconditioning, at ambient conditions without any
substantial preconditioning or wet storage. The ion-
selective electrodes of the present invention present a dry,
soli~-appearance and require only a drop (i.e., below about
50 ~1 and preferably about 10 ~1) of solution to produce an
accurate measurement. They require no preconditioning prior
to use, measurements can generally be made in less than five
minutes and because of their low cost they can be discarded
1~166~6
,.
after a single measurement, thereby avoiding contamination
due to prior use thereby insuring integrity of the ion-
selective membrane for each new measurement. Furthermore,
a novel technique for using the ion-selective electrodes
of this invention permits rapid yet accurate quantitative
ionic determinations.
Although the layers described hereinafter are
generally referred to as being "coated" one over another,
it should be understood that the term "coating" is meant
to include laminating or otherwise depositing the various
strata one over another, as well as actually coating using
conventional coating, dipping or extrusion techniques to
achieve layering of the various strata.
The dry-operative ion-selective electrodes of
the present invention comprise:
(a) a dried, internal reference element,
(b) in contact with the reference electrode,
a hydrophobic ion-selective membrane (of
predetermined uniform thickness in areas
thereof intended for contact with a test
solution), and
(c) an optional support.
Reference Electrode
As with any ion-selective electrode useful in the
determination of ionic activity and consequently ionic concentration
in solution, the electrodes of the present invention have an
internal reference electrode which exhibits a fixed reference
potential a~ainst which the potential occurring at the
interface between the ion-selective electrode and the
3 solution under test is measured.
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According to the present invention, the reference
electrode may be of two distinct types, both of which exhibit
the required fixed potential necessary to achieve useful
resu~s. The useful reference electrodes are:
(1) metal/metal-salt electrodes (see Figure 1),
and
(2) redox couple electrodes (see Figure 2).
Metal/Metal-Salt Electrodes
_ .
A commonly used internal reference electrode
comprises a metal in contact with an insoluble salt of the
; metal which is in turn in contact with an electrolyte, i.e.,
a solution containing the anion of the salt. An example of
a very commonly used such element is represented as Ag/AgCl/"XMCl "
(XMCl indicating a solution of known Cl concentration) and
comprises a silver wire having a coating of silver chloride
applied thereto dipping into an aqueous solution of
known chloride concentration. A calomel electrode,
Hg/Hg2C12/Cl , is another example of this type of electrode.
This type of internal reference electrode is used in most
barrel electrodes and in the known, so-called "solid-state"
~as referred to in Genshaw) electrodes. In known "solid-
state" electrodes, the electrolyte solution comprises a
hydrated gel, hydrated PVA, hydrophobic ion-exchange resin,
etc., as described above. The reference electrodes of the
prese~t invention are dried during manufacture and, unexpectedly3
do not require conditioning prior to use.
According to the present invention, the metal/metal-
salt reference electrode comprises a conductive layer of a
metal in conducting contact with a layer of a salt of the
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metal as used in known electrodes and a dried electrolyte
layer in contact with the metal-salt layer.
The conductive metal layer may comprise any
suitable conductive metal of the well known types which have
been used in such electrodes, and which is compatible with
the structure, particularly the formats described herein.
Yarticularly useful conductive metal layers include suitably
thin layers of silver, nickel, platinum and gold.
The salt layer in contact with the conductive
layer may comprise substantially any insoluble salt of the
metal of the conductive layer which establishes a fixed
interfacial potential with the metal of the conductive
layer. Such layers, which are well known and thoroughly
described in the aforementioned patents and publications,
generally comprise a salt of the metal which is a product
of the oxidation of the metal, as, for example, AgCl, Hg2C12,
etc. A highly preferred embodiment of the present invention
utilizes the aforementioned well known Ag/AgnX (wherein X = S ,
Cl , Br or I and n = 1 or 2) interface to establi$h
the potential of the reference electrode. Electrode
elements of this type can be prepared using a number of
well known techniques which include, by way of example,
dipping a layer of silver as a wire, foil or supported
thin layer into a solution of molten silver halide.
According to a preferred embodiment of the present invention,
the silver-silver halide couple is produced by vacuum
depositing silver onto a suitable support of the type
-18-
described below, preferably an insulating film, and then
chemically converting a surface stratum of the silver layer
to s~ver halide. Generally, techniques for chemically
converting metal to metal halide involve exposure or contact
of the surface of the metal, in this case silver, with a
solution of a salt of the halide to be formed for a period
and at a temperature sufficient to cause the desired
conversion. Typical conditions for this sort of chemical
conversion are well known~ and examples of simple and preferred
techniques are shown in the examples below. Other useful
techniques for preparing such electrodes are described in
U.S. Patent Nos. 3,591,482 to Neff et al issued July 6, 1971,
3,502,560 to Wise issued March 24, 1970, and 3,806,439 to
Light et al issued April 23, 1974. Although the teachings
of all of these references are directed primarily to the
preparation of wire electrodes, the application of ordinary
engineering skill will render their application to the
manufacture of electrodes constructed on thin films of
polymeric support apparent. Alternatively, a discrete layer
20 of silver halide may be coated over the silver layer so long
as appropriate contact between the silver and silver halide
is maintained.
Although it is possible to obtain the metal
metal-salt interface with substantially any ratio
of metal layer to salt layer thickness, in a preferred
embodiment which assures a sufficiently dense layer of metal
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salt it is preferred that the insoluble metal-salt layer
have a thickness equal to at least 10~ of the overall
thickness of the conductive metal layer. According to a
preferred embodiment of the present invention wherein a
surface layer of a vacuum-deposited silver layer is converted
to a suitable salt, from about 10 to about 20~ of the
thickness of the silver layer is converted to silver salt
using chemical conversion techniques.
The second member of the metal/metal-salt reference
electrodes of the present invention comprises the electrolyte
layer. According to a preferred embodiment of the present
invention, the electrolyte layer is a dried hydrophilic
layer.
The dried electrolyte solution of the present
invention comprises a hydrophilic binder having a salt in
solid solution therewith. According to a preferred embodiment,
the anion of the salt is common to the salt of the metal -
salt layer and at least a portion of the cation of said salt
comprises the ion which the electrode is designed to detect.
"Dried" hydrophilic electrolyte solutions as
described herein are specifically distinguished from the
hydrated polyvinyl alcohol layers described in UOS. Patent
No. 3,856,649. The "dried" reference solution of this
invention comprises the dried residue of a solution of a
salt and a suitable hydrophilic polymeric binder in a solvent
for the polymer and the salt. This distinction will be
made more apparent by the discussions of making and using the
electrodes of the instant invention which are presented
hereinafter.
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The binder for the "dried" reference electrolyte
solution may comprise any hydrophilic material suitable for
the formation of continuous, coherent, cohesive layers
compatible with the salt of the electrolyte layer and, if
formed by coating, a solvent system for both the ionic salt
and the polymeric binder. Preferred materials of this type
are hydrophiiic natural and synthetic polymeric film-forming
materials such as polyvinyl alcohol, gelatin, agarose~ deionized
gelatin, polyacrylamide, polyvinyl pyrrolidone, hydroxyethyl
acrylate, hydroxyethyl methacrylate, polyacrylic acid, etc.`
Specifically preferred from among these materials are the
hydrophilic colloids such as gelatin (especially deionized
gelatin), agarose, polyvinyl alcohol and hydroxyethyl acrylate.
Some residual solvent for the ionic salt must
remain in the "dried solution" to permit electrolytic con-
ductivity within the polymeric layer. Thus, the layer must
not be so thoroughly dried as to remove all residual solvent.
As a general rule, when water is the solvent, the residual
water comprises less than about 20% of the total weight of
the "dried solution" and the "dried" electrolyte layer is non-
tacky. Similar residual solvent levels are desirable with
solvents other than water.
The ionic salt which is dissolved in the polymeric
binder solution will be determined by the composition of the
metal~metal-salt portion thereof. For example, in a potassium
selective electrode which uses AgCl as the insoluble metal
salt, potassium chloride is a logical choice although sodium
chloride, etc. may also be used. For sodium ion determinations
in a similar configuration, sodium chloride would be useful,
etc. Thus, the salt will generally be a water-soluble salt
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.... ..
` 1~16696
having a cation selected from ammonium, alkali metals and
alkaline earth metals, mixtures of same or any other suitable
cation to which the electrode responds, and as anion a halogen
or sulfur depending upon the composition of the metal-salt
layer. Conductive metal salts of these anions are commonly
insoluble.
Appropriate solvents for the polymeric binder and
ionic salt will depend largely on the nature of the polymer
and the salt. Generally, polar solvents suitable for
dissolving the salt and the polymer are satisfactory. Thus,
water is a preferred solvent for layers of hydrophilic
materials such as polyvinyl alcohol and gelatin.
Since the thickness of the "dried" electrolyte
layer will to some extent determine the response characteristics
: of the electrode, it is generally desirable to maintain the
"dried" layer rather thin. Layers having dry thicknesses
on the order of from about 0.1 to about 0.5 mil have been
found useful. A preferred thickness is about 0.2 mil. Of
course, where electrode response characteristics are not
critical, the thickness of the layer may vary over a wide
range and only the application of sound engineering skills
; and the use requirements of the finished electrod_ will
determine its limits.
The concentration of ionic salt in the "dried
elec~olyte layer" may also be varied widely~ depending upon
response time desired, etc. and especially the level or amount
of polymer used. In the preferred embodiments described
herein wherein the binder level ranges from about 2.4 g/m2
to about 10 g/m2, the concentration of the salt ranges
30 from about 1.40 to about 2.5 g/m2. Below this level~
-22-
. ~
, . . ... ... . , .. , , . . _ , . _ _._ _.. . , _ _ _. ,.. ... . _, ... ... .. _ ~ _ .. .. . .
6{~6
, . .
electrode drift may be a problem as elaborated below, and
above this level coating of the layer becomes somewhat
difficult. Of course, where drift is not critical, layers
of s~stantially greater thickness are used, or layers are
prepared by some technique other than coating, concentrations
of salt outside these ranges may be similarly useful.
Generally, salt concentrations of from about 30 to about
50 percent by weight total solids in the layer are
preferred.
When the reference electrode is prepared by coating
the various layers one over another, it may be desirable
to include surfactants or coating aids in the coating solution
during manufacture. Such materials should generally be
nonionic and, whatever their composition, they should not
include ions which introduce variants into the fixed potential
differences existing at the various electrode layer interfaces
and are most preferably potentiometrically inert. Of course,
where additives which do introduce variants into the potentials
exhibited at the various interfaces are used, it is possible to
compensate for these using a differential mode of measurement
which compares the readings of two identical ion-selective
electrodes, one of which is contacted with a solution of
known ion concentration and the other of which contacts the
unknown test solution. Among the materials found useful for
this purpose are natural surfactants such as saponin and
synthetic materials such as poly(ethylene glycol) and a
material commercially available from Olin Mathieson Company
under the tradename Surfactant lOG. Other useful materials
of this type include octyl phenoxy polyethoxy ethanols such as
... . .. ,, ., . . _ . .. , .. . _ .. _ . . ... , .. . _ , , . ,. . . ... _ . .. . . . .
~1~66~6
TX-100, TX-405, etc. commercially available from Rohm & Haas
Company.
In an alternative embodiment useful metal/metal salt
(specifically Ag/AgX) reference electrode elements can be
prepared using techniques common to the manufacture of photo-
graphic film.
According to such ~rocedures either or both of the
metal (i.e. silver) and metal salt (i.e. silver halide) are
prepared by coating suitable silver halide photographic
emulsions and processing as required. For example, a useful
silver halide layer can be prepared applying to a vacuum
deposited silver layer by coating a conventional fine grain
silver chloride-gelatin emulsion at coverages of from .054 to
.54 g/m2 of gelatin and 1.16 to 1.83 g/m2 of silver as silver
chloride. In evaluations with standard chloride solutions,
such electrodes demomstrated substantially Nernstian
response (i.e. slopes of about 59 mv/dec).
Useful silver layers which can be overcoated with
silver halide layers as just described have been prepared by
coating a poly(ethylene terephthalate) support with a layer
of fine grain silver chloride, gelatin emulsion at a coverage
of 2.02 g/m2 of silver as silver chloride and 95 mg/m2 of
gelatin using conventional photographic film manufacturing
techniques. The silver chloride layer was then developed
for five minutes in a standard black and white developer
solution known as Kodak Developer D-l9 at room temperature
and under white light conditions. ~fter thorough washing
and drying this layer was overcoated with a silver chloride
emulsion as just described. Samples of this electrode
responded acceptably to standard chloride ion solutions.
696
Useful electrodes have also been obtained by coating
the silver chloride emulsion over evaporated layers of gold,
copper and nickel and using fine grain silver bromide emulsions
to prepar~ the metal salt layer.
Oxidation-Reduct;ion Electrodes
The second type of internal reference electrode
useful in the successful practice of the present invention
is the so-callecl oxidation-reduction electrode (hereinafter
redox electrode). Redox electrodes have been described
and generally in,clude an inert metal wire dipping into a
solution containing two different oxidation states of a
chemical species. An example of such an electrode comprises
a platinum wire dipping into a solution containing ferrous
and ferric ions. Such a cell is abbreviated Pt/Fe++, Fe
The electrode reaction is Fe+++ + e ~____' Fe++. Redox
electrodes can also be made with organic molecules that can
exist in two different oxidation states. The most widely
used of this type is the so-called quinhydrone electrode in
which the redox system is:
HO
+ 2H++ 2e
ll l
o HO
~ and ~he cell is represented as:
+
Redox electrodes of this type can also be prepared in a
I'solid-state'' format to provide the internal reference
eIement of the composite ion-selective electrodes of the present
invention. Alternatively such electrodes may be used as
~ -25-
.
ii96
external reference electrodes in the overall determination
of ion concentrations in solution in place of conventional
external reference electrodes such as the saturated calomel
(i.e., Hg/HgC12) electrode. U.S. Patent No. 3,730,868 also
describes such a redox electrode.
The redox electrode of the present invention
comprises:
~a) a solid, electrically conductive layer in
contact with
(b) a redox couple.
The redox couple may be dissolved or dispersed in the
electrically conductive layer or be provided as a discrete
solid layer comprising the redox couple dissolved or dispersed
in a suitable binder and in conducting contact with the
conductive layer.
The Conductive Layer
The conductive layer of the redox reference electrode
comprises an electrically conductive material or conductor
(as this term is conventionally understood in the art). It
20 will be appreciated that the conductive material should not
interact with the redox composition except in the desired
and controlled electrochemical fashion required for operation
of the electrode, i.e., to establish a stable reference
potential. Useful results have been obtained with such
inert conductors as carbon, platinum, gold and nickel. So
long as the conductor is selected so that no unstable
electrochemical or other undesired interaction with the
redox couple is observed, the choice is not critical. A
particularly useful conductor is carbon (in particular,
-26-
, "
. . , ., _ _ .
1~166~ -
particulate carbon), as will be shown in the examples and
described in greater detail below.
In certain embodiments, as in the case of carbon
where the inert conductor may be in the form of discrete
conductive particles, it may be necessary that such particles
be maintained in electrically conductive contact in a solid
layer by means of some binder or matrix. The binder may
comprise any material which permits intimate particle-to-
particle contact and conductive contact between the conductor
and the redox couple as described hereinafter. Generally,
such binders comprise relatively low concentrations of hydro-
philic polymers such as gelatin, polyvinyl alcohol and polyvinyl
pyrrolidone. It is, however, possible to use hydrophobic
polymers such as silicone rubber for the binder. Whatever the
binder used, the ratio of conductor to binder must be suffi-
ciently high that the resistance of the layer is low enough to
insure adequate electrical conductivity. Such resistances
are obtainable with weight ratios of conductor to binder of
between about 1:1 and about 3:2.
The Redox Couple Composition:
The redox couple composition comprises the soluble
redox couple and whatever other means are required to maintain
the composition in a solid form until such time as the electrode
is wetted and at least some portion of the redox couple is
- dissolved and contacts the conductor. This other means generally
comprises a matrix or binder of one sort or another which con-
tains the redox couple as a solid solution or dispersion.
The redox couples of the present invention, as
alluded to above, comprise pairs of the same chemical species
3 (usually ions) in differing oxidation states.
-27-
.
6~
The formal potential of the reference electrode
of the present invention, i.e., the electrical potential of
the redox couple at equal concentrations of its reduced and
_.
oxidized components at some defined finite value of ionic
strength, is determined by:
(1) the redox couple chosen and
(2) the ratio of activities of oxidized to reduced
components.
According to a preferred embodiment of the present invention,
the ratio of the oxidized to the reduced component (i.e.,
the molar ratio of material in one oxidation state to
material in the other oxidation state) is about unity (1),
since the redox buffer capacity is largest at this ratio.
Of course, depending upon the type of measurement to be made
using the electrodes described herein, this ratio may be
varied quite broadly.
When the electrode is wetted with a sample solution,
the redox couple must be capable of establishing a stable
interface with the conductive layer to establish a stable
and reproducible potential; i.e., the redox couple must be
capable of exchanging electrons with the conductive
layer in a constant fashion when the potentiometric circuit
is completed. It is important that the conductive layer and
the redox couple together poise the potential of the redox
; chemistry in a fast electrochemical exchange reaction between
the redox couple and the conductor. It is this capability
to establish a constant potential which is referred to herein
as the "compatibility" of the redox couple with the conductive
layer. A redox couple which readily establishes such a fixed
3 potential with a given conductor is said to be "compatible"
therewith.
-28-
. ~
1~16169~
According to a preferred embodiment, it is, of
course, desirable that in order for the electrode to possess
an extended shelf-life capability, the oxidized and reduced
forms of the couple should be stable for the desired shelf-
life.
Redox couples which have been found part~cularly
useful in the successful practice of the present invention
include ferric/ferrous ion couples such as Fe(CN)6 3/Fe(CN)6
and cobaltic/cobaltous couples such as Co(terpyridyl)2 3/Co-
(terpyridyl)2 wherein terpyridyl is 2,6-di-2'-pyridylpyridine.
Any redox couple capable of exchanging electrons
with a compatible conductive layer and sufficiently stable
against aerial oxidation as to provide a useful shelf-life
is useful in the successful practice of the invention.
Although some redox couples may be applied as a
solid layer directly to the conductive layer without a matrix
or binder, in view of the high solubility of many of the
useful redox couples and the difficulty with which materials
of this type are applied to the conductive layer in their
solid form (i.e., as crystals, etc.), it is generally desirable
to apply the redox couple as a dispersion or solution in a
suitably porous or water permeable binder or matrix.
The preferred water permeable matrixes for the
redox couple comprise a hydrophilic colloid such as gelatin,
polyvinyl alcohol, polyacrylamide, polyvinyl pyrrolidone,
etc., which colloid is most preferably:
(a) sufficiently hardened or cross-linked to
prevent substantial dissolution thereof by
water which may contact it, and
-29-
~ ~ .
. ~_. .. , .. ., .. ~ . . .. .
lil669~
(b) sufficiently hydrophilic to permit
electrolytic contact with the conductive
metal layer.
As alluded to hereinabove, it is also possible to
use highly porous layers of hydrophobic material wettable by
virtue of their porosity and permitting conducting contact
between particulate members of the redox couple also by
reason of this porosity. Thus, water permeable, highly
porous layers (i.e., comprising over about 60% and preferably
over about 75~0 void volume) of such hydrophobic materials
as cellulose acetate or poly(n-butylmethacrylate-co-2-
acrylamido-2-methylpropane sulfonic acid-co-2-acetoacetoxy-
ethyl methacrylate) can be used as the binder or matrix for
the redox couple.
Although the redox reference electrodes are
generally prepared in a two-layer configuration (i.e., a
solid layer of inert conductor in conducting contact with a
superimposed solid dried redox couple layer), it has also
been found that both the inert conductor and the redox couple
may be incorporated into a single layer to provide a useful
electrode. In this configuration, it is preferred to use
a hydrophilic matrix of the type described above in connection
with the redox couple layer for the combined layer; however,
hydrophobic binders are also useful. Embodiments of single-
layer reference electrodes are described in the Examples below.
The techniques for their preparation and use are identical
to those of the two-layer or double-layer electrodes described
herein.
-30-
., , . .. . , . . _ ... .. .. . . . . . ... ... . . . .. . . . . . . . .
Ion-Selective Membrane
Whichever of the foregoing internal reference
electrodes is used, the ion-selective membrane is laminated,
coated or otherwise applied directly thereover. It is important
to the successful practice of the present invention that the
ion-selective membrane be applied at the time of manufacture
so as to assure intimate and uniform contact with the surface
of the reference electrode contiguous with the ion-selective
membrane at least in those areas intended for contact with
a test solution to obtain a "dry-operative" electrode. Such
intimate uniform contact of the ion-selective membrane with
the dried internal reference electrode at the time of
manufacture produces a reference electrode-ion-selective
membrane interface which will respond almost immediately
upon contact of the ion-selective membrane with a test
solution.
Among the patents and publications which describe
ion-selective membranes of the type useful in the instant
invention, the contents of all of which are incorporated
herein by reference to the extent that they are ~ertinent,
are:
U.S. Patent No. 3,562,129 to Simon issued February 9,
1971;
U.S. Patent No. 3,753,887 to Kedem et al issued August 21,
1973;
IJ.S. Patent No. 3,856,649 to Genshaw et al issued
December 24, 1974;
~ 6 9 ~
British Patent No. 1,375,446 lssued November 27, 1974;
German OLS 2,251,287 issued April 26, 1973;
Morf, W. E., Kohr, G., and Simon, W., "Reduction Or the
Anion Interference in Neutral Carrier Llquld-Membrane Elec-
trodes Responsive to Cations", Analytical Letters, Vol. 7,
No. 1, pp. 9-22 (1974);
Morf, W. E., Ammann, D., Pretsch, E., and Simon, W.,
"Carrler Antibiotics and Model Compounds as Components of
Ion-Sensitlve Electrodes", Pure and Applled Chemlstry, Vol.
36, No. 4, pp. 421-39 (1973);
Ammann, D., Pretsch, E., and Simon, W., "Sodium Ion-
Selective Electrode Based on a Neutral Carrier", Analyti-
cal Letters, Vol. 7, No. 1, pp. 23-32 (1974);
Cattrall, R. W., and Freiser, H., Anal. Chem., 43, 1905
(1971); and
James~ H., Carmack G., and Freiser? H., Anal. Chem.,
44, 856 (1972).
Membranes of this type which are well-known generally
include an inert hydrophobic binder or matrix having dispersed
therein an ion carrier or selector commonly referred to as an
ionophore which imparts selectivity to the membrane dissolved
in a carrier solvent to provide adequate ion mobility in the
membrane. The carrier solvent generally also serves as a
plasticizer for the membrane binder.
Ion-Selective Membrane Binder
Binders for use in the ion-selective membrane of the
instant invention include any of the hydrophob~ natural or
synthetic polymers capable of forming thin films of sufficient
permeability to produce, in combination with the ionophores and
ionophore solvent(s), apparent ionic mobility thereacross.
- -32-
. ,~ .
... .. . . .. ..
1~66~6
Specifically, polyvinyl chloride, vinylidene chloride,
acrylonitrile, polyurethanes (particularly aromatic polyure-
thanes), copolymers of polyvinyl chloride and polyvinylidene~
chloride, polyvinyl butyral, polyvinyl formal, polyvinylacetate,
silicone elastomers, polyvinyl alcohol and copolymers of
polyvinyl alcohol, cellulose esters, polycarbonates,
carboxylated polymers of polyvinyl chloride and mixtures
and copolymers of such materials have been found useful.
Films of such materials which include the ionophores and
carrier solvents may be prepared using conventional film
~0 coating or casting techniques and, as shown in the examples
below, may be formed either by coating and film formation
directly over the internal reference electrode or some
suitable interlayer or by formation separately and lamination
thereto.
Ion Carrier
,
The ion carrier used in the ion-selective membrane
is generally a substance capable of selectively associating
or binding to itself preferentially a desired specific alkali
metal, alkaline earth, ammonium or other ions. The manner
in which the ion becomes associated with the carrier is not
; fully understood but it is generally thought to be a steric
trapping phenomenon complexing by coordination or ion exchange.
Suitable ion carriers are more fully described below.
The selectivity of the electrode for a particular
ion is due to the chemical nature of the ion carrier and,
thus, the use of different chemical components as the
uncharged ion carrier provides different membranes for use
1~16~6
in ion-selective electrodes specific to different ions.
Exemplary of such components are a large number of substances,
some of them known to be antibiotics~ which includes:
(1) valinomycin, a potassium-selective (over
sodium), ion carrier that imparts to a
membrane constructed in accordance with
this invention a potassium ion selectivity
of the order of 10 , and an ammonium ion
selectivity (over sodium) of the order of
lo-2;
(2) cyclic polyethers of various constitution
which make the membrane selective to lithium,
rubidium, potassium, cesium or sodium; and
(3) other substances having ion selectivity
similar to valinomycin such as other
substances of the valinomycin group, tetra-
lactones, macrolide actins (monactin, nonactin,
dinactin, trinactin), the enniatin group
(enniatin A, B), cyclohexadepsipeptides,
gramicidine, nigerici~, dianemycin, nystatin,
monensin, antamanide and alamethicin (cyclic
polypeptides).
There can also be used either a single substance or
mixtures of substances of the formula:
-34-
66q~
R2 R1
\N~
/ ~
H C O
o
H2~C
H2C~
o
\C~
N
R 2 R
where in:
1: Rl -CH3
R2 - ( CH2 ) n-C00-CH2-CH 3
wherein n = 1 ~r 10
I]: Rl -CH3
R2 (CH2)6 CH3
II]: Rl = R2 : -CH2-CH2-CH 3
I~T Rl -CH2-CH2-CH3
2 -CH2-C- (CH3) 3
V Rl = R2
VI ~ 2 CJ I ~
6~6
Other useful ionophores include tetraryl borates (especially
tetraphenyl boron) and quarternary ammonium salts. Compounds
such as trifluoroacetyl-p-alkyl benzenes are described in
U.S. Patent No. 3,723,281 issued March 27, 1973, for HC03 .
Compounds of the following structural formulas are
also useful as ionophores:
~ OR X OR ~ OR
OR OR Cis and Trans
OR ~ OR ~ OR
OR
` wherein:
a) R = CH2CON(CH2CH2cH3)2
b) R = CH2coN-(cH2)ll-co2cH2cH3
CH3
; These materials are described by Amman, D., Bissig5 R.,
Guzzi, M., Pretschj E., Simon W., Borowitz, I.J., Weiss5 L.,
in Helv. Chim. Acta, 58, 1535 (1975).
-36-
. . .
.
;
. :
. . .
1~161~9~
Numerous other useful materials are described in
the foregoing publications and patents, as well as other
literature on this subject.
The concentration of ion carrier in the membrane
will, of course, vary with the particular carrier used, the
ion undergoing analysis, the carrier solvent, etc. It has
generally been found, however, that ~on-carrier concentrations
of below about 0.1 g/m2 of membrane assuming the membrane
thicknesses preferred herein result in marginal and generally
undesirable responses. Ion-carrier concentrations of between
about 0.3 and about o.5 g/m2 are preferred. The ion carrier
can be incorporated at levels much higher than this, however,
because of the cost of many of these materials, use of such
levels is not economically sound.
Ion Carrier Solvent
The carrier solvent provides ion mobility in the
membrane and, although the ion-transfer mechanism within
such membranes is not completely understood, the presence of
a carrier solvent is apparently necessary to obtain good ion
transfer.
The carrier solvent must, of course, be compatible
with the membrane binder and be a solvent for the carrier.
In the structure of the present invention, two other charac-
teristics are most desirable. One is that the carrier solvent
be sufficiently hydrophilic to permit rapid wetting of the
membrane by an aqueous sample applied thereto to permit ionic
mobility across the interface between the sample and the
membrane. Alternatively, the carrier must be rendered hydro-
philic by the action of a suitable noninterfering surfactant
-37-
,,
_ .. _ . _ _ . . . . _ ... _ _ _ . _ , ... . , , . ., . _ , .. .. .
~1166~6
which improves contact between the sample in contact with
the membrane and the carrier.
The other highly desirable characteristic is that
the carrier solvent be sufficiently insoluble in water that
it does not migrate significantly into an aqueous ~ample
contacted with the surface of the membrane as described
hereinafter. Generally, an upper solubility limit in water
would be about 4.0 g/l with a preferred limit lying below
about 1 g/l. Within these limits, substantially any solvent
for the ionophore which is also compatible with the binder
may be used. As mentioned above, it is, of course, preferred
that the solvent also be a plasticizer for the binder. It
is also desirable that the ion carrier solvent be substantially
non-volatile to provide extended shelf-life for the electrode.
Among the useful solvents are phthalates, sebacates, aromatic and
aliphatic ethers and adipates. As shown in Example 8 below~
specific useful carrier solvents include bromophenyl phenyl ether,
3-methoxyphenyl phenyl ether, 4-methoxyphenyl phenyl ether,
dimethylphthalate, dibutylphthalate, dioctylphenylphosphonate,
bis(2-ethylhexyl)phthalate, octyldiphenyl phosphateg tritolyl
phosphate and dibutyl sebacate. Particularly preferred among
this class is bromophenyl phenyl ether for potassium electrodes
using valinomycin as the carrier. A large number of other
useful solvents are specified in the references mentioned
above which describe the preparation of ion-selective membranes
and any of these which permit assembly of electrodes of the
type described herein may be used in the successful practice
of the instant invention.
The concentration of carrier solvent in the membrane
3o ~Jill also vary greatly with the components of a given membrane;
._ ... . . _, ,., . --. _ . .. ... _, . .. . _, . ..
16~g6
t
however, weight ratios of carrier solvent to binder of bet~een
about 1:1 to about 5:2 provide useful membranes. m e thickness
of the membrane will affect electrode response as described in
somewhat more detail below, and it is preferred to maintain
the thickness of this layer below about 5 mils and preferably
about 1 mil. As also described in greater detail below, the
uniformity of thickness of the ion selective membrane plays
an important role in the optimum utilization of electrodes
of the type described herein. Thus, if maximum advantage in
terms of storage capability and brevity of response time
are to be obtained, the ion-selective membrane should be of
relatively uniform thickness as defined above.
SuPport
According to preferred embodiments, the ion-selective
electrodes of the present invention include a support which
may be comprised of any material capable of bearing, either
directly or by virtue of some intervening adhesion-improving
layer, the other necessary portions of the electrode which
are described in detail hereinafter. Thus, the support may
comprise ceramic, wood, glass, metal, paper or cast, extruded
or molded plastic or polymeric materials, etc. The composition
of the support is relatively unimportant, so long as it is
capable of carrying the overlying electrode components and
it is inert; i.e., it does not interfere with the indicating
potentials observed as~ for example, by reacting with one of
the overlying materials in an uncontrolled fashion. In the
case of porous materials such as wood, paper or ceramics, it
may be desirable to seal the pores before applying the over-
lying electrode components. The means of providing such a
~39-
., ~
g6
sealing are well known and no further discussion of the same
is necessary here. Electrically insulating supports are
preferred although, as described hereinafter, metallic
conductive supports which serve multiple purposes are equally
useful and may in fact simplify the structure of the electrode.
According to a highly preferred embodiment of the
present invention, the support comprises a sheet or film of
an insulating polymeric material. A variety of film-forming
polymeric materials are well suited for this purpose, such
as, for example, cellulose acetate, poly(ethylene terephthal-
ate), polycarbonates, polystyrene3 etc. The polymeric
support may be of any suitable thickness typically from
about 2 to about 20 mils. Similarly thin layers or surfaces
of other materials mentioned above could be used. Methods
for the formation of such layers are well known in the art.
In certain cases, a separate and distinct support
need not be provided. Such a case occurs when one or more
layers of the electrode demonstrate sufficient mechanical
strength to support the remaining portions of the electrode.
For example, when a metal-insoluble metal-salt electrode is
used as the internal reference electrode as described below,
the metal layer may be in the form of a self-supporting foil.
The metal foil serves as the support, an integral portion of
the internal reference electrode, as well as a contact for
the electrode.
Preparation of the Electrode
The solid-state electrodes of the prior art are
commonly ~anufactured using a conductive wire as the starting
material and dipping the wire sequentially into generally
-40-
6g~
highly viscous solutions of the components of the indlvidual
finished electrode layers to construct a bulbous multilayer
"solid-state" electrode. See, for example, U.S. Patent No.
3,856,649. Alternatively, as shown in U.S. Patent No.
3,649,506, individual layers of ion-selective glass are applied
over the tip of a conductive wire. In either of these
situations, the resulting ion-selective membrane is of
relatively non-uniform thickness in those areas intended for
contact with an aqueous solution whose ionic activity is to be
10 determined.
There is a suggestion in U.S. Patent No. 3,856,649
(col. 2, lines 1-3) that similar multilayer solid-state
electrodes could be prepared in a sheet or web-form configuration
as on a metallized film of a nonconducting support or a metal
foil; however, there is no demonstration of such an electrode
and certainly no appreciation of the unique and novel storage
and use characteristics of carefully prepared electrodes of
uniform layer structure which are described herein. Wire
electrode configurations are within the scope of the present
invention. However, when such electrodes are prepared, care
must be exercised to reduce discrepancies in layer thickness
(to within thetolerances described herein), etc., which might
provide undesirable results in the novel measuring methods
described hereinafter.
Electrodes of the present invention are prepared
by coating, laminating or otherwise applying the various
individual layers one over another in any conventional fashion.
Thus, a typical manufacturing procedure for a metal-
insoluble metal-salt reference element electrode would involve
-41-
. .
6~6
chemically converting or otherwise applying a layer of an
insoluble metal salt to a layer of a compatible conductive
metal in the form of a coating on a nonconductive substrate
or a metallic foil, overcoating the metal-salt layer with an
electrolyte solution layer, drying the thus applied layer to
remove solvent (see definition of "dried" hereinabove), and
subsequently overcoating with a solution of the components
of the ion-selective membrane and drying to provide a
complete electrode. Alternatively, the layers can be laminated
so long as intimate contact between layers is achieved and
maintained, and uniformity of thickness of the ion-selective
membrane is attained.
The particular drying conditions which must be applied
to the internal reference electrode in the manufacture of any
specific ion-selective electrode will, of course, vary greatly
depending upon the composition of the electrode layers,
- particularly the binder used, the solvent or dispersing medium
used to form the layer and these can be readily determined
by the skilled artisan. Typical such conditions are described
in the examples below for layers of the composition described
therein.
~ oating of the various electrode layers provides a
uniquely simple yet efficient method for preparing electrodes
as described herein. Using well known techniques, the various
layers can be deposited under very carefully controlled
conditions which provide highly accurate layer composition,
degree of dryness and layer thickness, all of which are
extremely important to the successful preparation of electrodes
as described herein. Once prepared by coating, which will
JO usually take place in a planar or substantially planar
-42-
~669Ç`~
configuration, if the electrode has been prepared on pliant
support, it may be configured into almost any useful geometry
by cutting, bending, etc. which will permit contact of the
lon-selective membrane with fi test golution. As described
below, a preferred technique for using the electrode is in a
substantially planar configurstion by the application Or a
drop (less than about 50 ~1) of test solution to the ~on-
selective membrane.
Other additives such as dyes, plasticizers, etc.,
which may be desired for one reason or another may also be
incorporated into the layer, so long as they do not interfere
with the functions of the layer or components of the electrode~
cince the hydrophobic membrane layers dcscribed
below are generally coated directly over the h~drophilic
reference electrode, it is not entirely unexpected that, with
certain embodiments of the electrodes described herein,
adhesion problems between these two layers sometimes occur. In
such instances, it may be useful to incorporate thin adhesion-
improving or subbing layers between the reference electrode nd
the hydrophobic membrane. Care must, of course, be exercised
to insure that such layer(s) do not interfere with the conductive
contact between the membrane and the internal reference elec-
trode and that no materials are introduced which might interfere
with the fixed potential established by the reference.
` It ~s important that the ~lectrolyte layer be dried
prior to application of the overlying io~ selective membrane
-43-
6~6
if the electrode is to be dry operative. If the hydrophobic
ion-selective membrane is applied over the reference electrode
while it i5 still wet or fully hydrated as suggested in the
prior art then upon storage of the electrode at ambient
conditions the water present in the reference electrode will
migrate out of the electrode. Since the electrolyte layer is
hydrophilic, i.e., water swellable, upon evaporation therefrom,
the electrolyte layer apparently contracts while the overcoated
hydrophobic membrane does not undergo any substantial con-
traction. Thus, the possibility exists for the occurrence of
gaps or voids (i.e., reticulation) between the reference
; electrode and the hydrophobic membrane which will at least
partially remove them from electrolytic contact until such
time as the hydrophilic electrolyte solution is
rehydrated and once again swells to a point where contact
between the internal reference electrode and the membrane is
reestablished. This phenomenon may lead to the requirement
in the Genshaw et al patent that the membrane be coated over
the electrolyte layer while the latter was still hydrated
and which manifested itself in the Genshaw et al publication
as a blistering or splitting of the membrane when the hydro-
phobic membrane was applied over a "dry" hydrophilic reference
electrode and subsequently hydrated for use.
Electrodes using redox reference elements are
prepared using techniques similar to those described above
for the metal-insoluble metal-salt reference electrodes. Thus,
the inert conductive layer, which may be a metal wire or
foil or, alternatively, a dispersion of a particulate conductor
such as carbon, is coated with a solution or dispersion of
the redox species-containing layer, this latter layer dried
_41~_
and an ion selective membrane applied thereto as described
above. Alternatively, the inert conductor and the redox
species may both be incorporated into a matrix or binder
composition and a single layer coated to provide the desired
reference element. Of course, individual layers may be
laminated in conducting contact to provide a similarly useful
structure.
~se:
The ion selectivity of membrane electrodes can be
observed by measuring the steady-state difference in electrical
potential between solution 1 and solution 2 (both usually
aqueous) in the cell arrangement schematically represented
by the following:
Reference electrode l/solution l//membrane//
solution 2/reference electrode 2
The calculations required to determine the ionic
activity of solution 2 (generally the solution of unknown
concentration) are derived from the well-known Nernst equation
and are discussed in detail in a paper entitled "Cation
Selectivity of Liquid Membrane, Electrodes Based upon New
Organic Ligands" of Simon and Mor~ reported in the Pungor-
edited reference cited above.
The electrode described herein incorporates within
its structure substantially all of the components needed for
making a potentiometric determination with the exception of
a second reference electrode, the potential-indicating
device and associated wiring so that in use the user merely
needs to provide for contacting the sample with the ion-
selective membrane, preferably by application of a small
_L,5_
~,
1~166~6
quantity of the sample to be analyzed (on the order of c50
~1) thereto and connection Or appropriate lead wlres.
Automated dispensers for applying controlled Amounts Or
sample to the electrode at the appropriate location are
known and any such dispenser, or for that matter careful
manual dispensing, may be used tv contact the sample with
the electrode. Specifically, dispensers of the type dis-
closed in U.S. Patent No. 3,572,400 to Casner et al issued
March 23, 1971, may be adapted for applying small quantities
(i.e., drops) to the surface sf the electrode of the present
invention. Other suitable dispensers are described in U.S.
Patent 4,041,995 of R. Columbus issued August 16, 1977.
Alternatively, when wires, cylinders, rods, etc., i.e.,
structures comprising other than planar surfaces which
can be spotted, are used for the electrode, the electrode
may actually be immersed in or contacted with the surface
of the solution under analysis.
Second reference electrodes such as saturated
calomel electrodes for use in combination with the integral
electrodes Or the present invention are also well known. In
^ addition to such electrodes, reference elements of the type
described herein as the internai references may also be used
as the second or external reference electrode.
Similarly, potentiometers capable of reading the
potentials generated in the ion-selective electrodes of the
present invention are well known and, when properly connected
as described hereinafter, can be used to give a sensory
indication of the potential from which the ionic acti~ity in
the unknown solut~on may be calculated.
-46-
1~66C~
By incorporating computing capability into the
potentiometric device it is, of course, possible to obtain
direct readings of specific ionic concentrations in æolution
as a function of ionic activity.
As referred to numerous times herein, it is in
their use that the electrodes of the present invention
demonstrate their highly unexpected properties. Thus, while
many prior art electrodes require preconditioning, wet
storage or an equilibration period prior to use, the
electrodes of the present invention, apparently because of
their dried internal reference electrodes and the predetermined
uniform thickness of their ion-selective membrane can be
used without any need for conventional preconditioning, wet
storage or equilibration protocols.
It has now been discovered that~ when electrodes of
the type described herein are stored under ambient conditions
of the type normally encountered in a laboratory environment
; (most generally at or below RH about 65%) and subsequently
- spotted or otherwise contacted with samples of an aqueous
ion-containing liquid as described above, under reproducible,
known conditions, reproducible traces of the potential
exhibited by these electrodes will define traces of potential
vs. time as shown in Figure 3. The phenomenon which is
represented by this curve shape is "drift" which is defined
hereinafter.
The shape of the curve produced by any specific
electrode is determined by its composition and configuration.
As described above, it is theorized that drift, particularly
in electrodes as described herein, is related primarily to
-47-
the thickness and composition of the ion-selective membrane
which regulates the rate of water permeation of the electrode.
Thus, the composition and configuration (e.g., physical
dimensions such as thickness of the electrode) play a very
significant role in the trace defined by any specific electrode
or set of electrodes. Specific results attained by varying
such thicknesses are shown in Example 47. It should, therefore,
be apparent that if precise measurements are to be achieved
using a series of disposable, single-use electrodes, it is
important that the thickness and composition of the ion-
selective membrane be carefully controlled and maintained at
some predetermined uniform thickness from electrode to electrode
and within regions of a single electrode intended for contact
with the test sample. A lack of such predetermined controlled
thickness uniformity will manifest itself as random or erratic
drift which cannot be calibrated as described herein. Such
drift will render it difficult, if not impossible, to calibrate
a series of electrodes because variations in membrane thickness
from electrode to electrode result in calibration curves
having different shapes which cannot be related to ion activity
or concentration in any meaningful fashion.
A study of Figure 3 indicates that, after scme
appropriate interval, generally about 10 minutes, in electrode
configurations of the type described herein, the potentials
exhibited by the various electrodes begin to stabilize (i.e. the
slope becomes constant) thus indicating the attainment of the
initial stages of equilibrium within the electrode. It is in the
extremes of this stabilized portion of the potentiometric curve
after wet storage or preconditioning that measurements Or
-48-
, .. . ..... ... . . , .. . -- . -- - -- - -- -
6~i
potential were made with the electrodes of the prior art and
from which ion concentrations were calculated using the Nernst
equation. We have discovered that using the electrodes of the
present invention after storage at ambient conditions, the drift
can actually be calibrated and that, using "cal~brated drift,"
ionic concentration is reproducibly and accurately determinable
almost immediately after contact of the surface of the
electrode with the aqueous test solution. Such results are
achieved without according the electrode any specialized
storage treatment prior to use except to insure freedom from
contamination as would be done for conventional laboratory
glassware and equipment.
The depth and width of the trough will vary somewhat
depending upon the ambient condition of use (primarily the
relative humidity) and the thickness of the various layers
(principally the hydrophobic membrane); however, these
variations are easily compensated for by using either a
differential measurement which compares the ion concentration
of the unknown sample with that of a similar sample of known
ion concentration (i.e., a calibrator or standard) simultaneously
applied to an identical electrode, or by initially deriving
calibration curves for the electrode for given sets of ambient
conditions and subsequently relating the conditions of
individual measurements to such calibration curves.
As will be described in the examples below, when
electrodes prepared as described in U.S. Patent No. 3,856,649
were stored "dry," i.e., at relative humidity below about 65%,
and used as ~ust described, a somewhat similar drift was ob-
served; however, in these cases the drift was random and erratic,
varied substantially from electrode to electrode and generally
-49-
:
6C~
provided "uncalibratable" results, due most probably to the
nonuniform thickness of the layers of such electrodes and
the need for the reference electrode to be hydrated or
equilibrated before true and uniform contact between the
internal reference electrode and the hydrophobic membrane
occurred.
Quite clearly, it is difficult to manufacture an
electrode having layers of predetermined uniform thickness
using a dipping technique, although such an electrode could
conceivably be prepared using coating solutions of highly
controlled viscosity and rotating the dipped work piece in a
fashion which inhibits formation of a bulbous structure of
non-uniform thickness. In view of the difficulties in use
of such techniques, applicants prefer to prepare their elec-
trodes in a planar format which not only simplifies manu-
facturing techniques but allows use by simply depositing a
very small quantity (i.e., micro amounts on the order of less
than about 50 ~1~ onto the planar electrode and measuring
therefrom.
The following examples will serve to better demon-
strate the successful practice of the present invention.
Example 1: Ag/AgX Electrode
A sample of vacuum-deposited metallic silver on
polyethylene terephthalate support ~-10 mg Ag/dm2) was
prepared. A portion of this sample was treated for 5 min-
utes in the following solution:
glacial acetic acid 0.45 ml
sodium hydroxide 0.20 g
30 potassium ferricyanide 0.80 g
potassium bromide 2.50 g
distilled water to 1 liter
-50-
,
... . . ... .. . . .. .
1~16~
The sample was then washed for 5 minutes in running
distilled water.
Visual inspection revealed that partial conversion
to silver bromide had occurred, leaving a contiguous layer
of metallic silver adjacent the support. A narrow strip
along one edge was dipped briefly in a thiosulfate bath to
uncover the silver layer for purposes of making electrical
contact.
Measurements of the electrochemical response were
performed by applying small samples of aqueous solutions
varying in ~r activity to the silver bromide layer. A
linear response, with approximately theoretical slope (Nernst
equation), was observed.
Example 2:
A Ag/AgX half-cell was prepared as described in
Example 1 except that the conversion conditions were 30
seconds in a solution containing 8.45 g/l of potassium
chlorochromate.
Measurements of electrochemical response were
performed and showed linear potential response with varying
Cl and Ag+ activity.
Example 3: Laminated Ion-Selective Electrode
A silver-silver chloride film on polyethylene
terephthalate was prepared as described in Example 2, 7.6
g/m2 total silver with 15% conversion to AgCl (1.16 g/m2)
and then coated with a 5% polyvinyl alcohol (PVA) - 0.2 M
KCl solution (1.5 g KCl, ~.0 g PVA/m2). After the PVA layer
~ ~ ,
was dried by heating to 130F for 10 minutes, a precast ion-
selective membrane comprising 0 50 g/m2 of valinomycin (VA1),
40.4 g/m2 of polyvinyl chloride (PVC) and 100.2 g/m2 of
bromophenyl phenyl ether (BPPE) as carrier solvent was
manually laminated cn top of the film coating.
The resulting ion-sensitive electrode, represented
as Ag/AgCl/PVA-KCl/ion-selective membrane was tested by:
(1) connecting the silver-silver chloride film
to the high-impedence input of a volt meter,
and
(2) suspending a drop (25-50 ~1) of the KCl
solution to be measured from the tip of a
saturated NaN03 salt bridge which was
connected to an external reference electrode
(Hg/Hg2C12) which was in turn connected to
the reference input of the volt meter, and
contacting the drop to the surface of the
electrode. The complete potentiometric cell
is represented by:
Hg/HgC12/KCl(X M) test/ion-selective membrane,
PVA-KCl/AgCl/Ag.
A linear semilogarithmic response to potassium ion
was Dbserved with a siope of 57 mv/decade over the range pK+
1 to 4.
~ ExamPle 4: Coated Ion-Selective Electrode
An electrode was prepared as in Example 3 except
that the ion-selective membrane comprising o.58 g/m2 VAL,-
22.9 g/~2 polyvinyl chloride and 111.2 g/m2 BPPE was coated
directly onto the KCl-PVA layer rather than being laminated
as in Example 3.
-52-
. ... _ .. . ..
166q~
This integral electrode was tested as in Example 3
and exhibited a linear semilogarithmic potassium ion
response having a slope of 55 mv/decade.
Example 5: Reference Electrolyte Composition Yariaticns
A series of electrodes were prepared utilizing a
variety of surfactants and water-soluble polymers as binders
for XCl in the reference electrolyte solution. The polymers
included polyvinyl alcohol (PVA), deionized gelatin and poly-
acrylamide (PAM) (see Table 1). Unless otherwise noted, all
electrodes contained 1.5 g/m KCl. These electrodes were
then laminated with a precast ion-selective membrane of the
composition described in Example 3. The resulting electrodes
were then evaluated as described in Example 3, with the
results shown in Table 1 below.
-53-
616~6
++~++~
xxxx~
~ l l l l l
oooooo ~
7~
C oooooo Co)
oC~ ,,
J~a) ~ o
u~ td ~
~, .,,
o a~
U~
>tn~
~ ~ b~
h
a) ~
~a o
~:
m ,
~ * ~* * 5 o
~1 ~~a ~I t ~ P~
D Q~ a> EI I O ~
~d ~Q~ bO~I I ~1 u~ P~ ~ ~1 r1
E~ ~h 'C ~ ~ ~
C) ~D N N
~q . OO O
=r
t~ ~ ~o
a~
h r4 rl O
~) ~) J~ ~1
~; ~ ~ r~ E
¢¢¢¢~ o o
h
m ~
C~
td
~-r~ >~
rl N ~)~ II~ O
a) v~
E~ * **
- -54-
6C~i
The data of Table 1 demonstrate that the elec-
trodes prepared as described give a linear potassium ion
response with a slope between 51 and 57 mv/decade.
Examples 6-16: Ion-Selective Membrane Composition
A number of electrodes, both laminated and coated,
were prepared to examine the ePfect of variations in the
composition of the ion-selective membrane on the response of
the electrode.
The elements were evaluated as descrlbed in Exam-
ple 3 with the results shown in Table 2 below.
-55~
, . , ,. _ ... . ............ .
6~ 6
+~ r-l ~ ~ x~
~ ~ r~r-l ~I r-l
r~ cu~ o o O
~) O ~ r-l r-l r~l r-l
r b~
N~ ~: l l l (~
bD r; 1~ ~
co o ¢ r-l r.l ~I r-l
u~ o a~
o~ Q~ cn
c ~:1 ,~ c: c) ~) a) h S~ O
O O ¢O ~I) ~ C O O ~ F~ ~ C tl~ t--r-l Is~ .rl
rl ~ ~ ~ O O O O Ir~ ~ \ 03 u~U~3 ~1
bD C
h r-lE~ r O c
1~ ~ 1 L~
O ~ ~) r
~d Ll~~ U~OOOO C bD OU~Oo
~ ~ o .~m ~I r~ JJ ~ ¢ ~ 3 ~a:~
~1 ~ ~1 r ~ C`
t~ ~ O ¢ ~ ~0
C~J ~ ULr~u~OOu~ ~o 000000
l S~E~~ :~ ~ U) r-l 1~1 r~ J J
H O \~ ~4 ~ .--1
~ O r-l ~ Ir-l r-l r~~ Q. bD
O U r-l ¢. . . - - E~ ¢ ~
~ ~00000 0 000000
H ~ I U H
¢~ X ¢~ ~ ~0
a) cr~ o x E~ J ~J u~o
r~ ~1 ~ ~ ~ r~
~0
E~ , bD
t~- .
bD
~D
56 -
. .
96
The data in Table 2 lllustrate the followlng
effects as a functlon of variations o~ VAL, BPPE and PVC ln
the electrode format:
A. LRSS than 0.2 g/m2 of valinomycin in ~he membrane
results in either marginal or no potassium lon
response.
B. BPPE/PVC ratios of less than 1:1 give dry unrespon-
sive membranes. In general, carrier-solvent-to-
polymer ratios of between 1:1 to 5:2 provlde use-
ful membrane layers. -
Examples 17-23: Ion-Selective Membrane Composition
A number of electrodes, both laminated and coated,
were prepared to demonstrate the utllity Or other polymers
in the ion-selective membrane layer Or the electrode.
Polymers which were tested include Butvar B76 (a polyvinyl
butyral available from Monsanto Chemical Co.), Estane 5107Fl
(an aromatic polyurethane available from B. F. Goodrich),
VYNS (a PVC/PVAc*-9O/lO copolymer available from Union
Carbide) and Silastic~ 731RTV (a silicone rubber ~rom Dow
Corning, Midland, Michigan). After preparation, the elec-
trodes were evaluated as described in Example 3, wlth the
results shown in Table 3 below.
- .
~PVAC = polyvinyl acetate
-~ -57-
+~+~+~+~ ~ ~
V ~I r~ l r ~ ~
I I ~ I I I
00 00 0 0
N~ ,~ ,_l ~ ,1 ~1 ~1
00 00 0 0
~ a)
N~ ~:~1 ~\J oo co 11~ ~D
I
u~ o E L
~ U~
al¦ h C hO P~¦ 1~1~
E~l h . O N m ~ a~ , ~ O
~ K ~ ..
O L ~1 h ~E~
~ ~ a) u~ ~,
bO ~ ~ u~
Z ~
O P b~
P~ P~ ~~''~ ~. ~ ~D '
~- o o o o ~ ~ o,1 U~
E~ ~1 ~I
N ~11 11~1~1~ Ir~ 11~ ~1
~ :,1 oo oo c~ r~
~O
~ ~ O Q~ . '
t--CO ~ O ~1
~I r-l ~J N (~J ~1
58
,,
:
1669~
The data of Table 3 illustrate that all of the
polymers tested are useful in the present electrode configura-
tion.
Examples 24-38: Ion-Selective Membrane Composition
(Carrier-Solvents)
A series Or electrodes were prepared to compare
bromophenyl phenyl ether (BPPE) with other possible carrier
solvents for the membrane layer. The other solvents which
were tested include the following: 3-methoxyphenyl phenyl
ether (3 MPPE), 4-methoxyphenyl phenyl ether (4MPPE), dimeth-
ylphthalate (DMP), dibutylphthalate (DBP), dioctylphenyl-
phosphonate (DOPP) and bis(2-ethylhexyl)phthalate (BEHP) and
dibutyl sebacate (DBS).
The integral electrodes were evaluated in the
manner described in Example 3 with the results shown in
Table 4 below.
_59_
-- 1116~6
~ ~ X ~; K
l l l l l l l l l
O o o O O O o o O
7 7 ~ ~ , ~ ~ 7 ~
oooooo . oo o
C)
~J ~u Lt~ N t~ ~ I
D e u~ Lr~ ~ ~ ~ E ~3 U~
E ~~ bD
~ U~
U~
O J~
s s m
td ~ 3 3 3 3 t~ lrl N ~ a ~
J E ~ o ~ o O
~ o o
~ ~: NE ~ Lr` o o o o U~ o o o
E~ IO ~ ` ~ =r 3 =r o a~ 3
~:1 E ~ e u~
h:~ bD o O o o o o~o o o o
t~
,1
u~ ,i m o
.~ ~a.
o c~ hc~
x a~ ,~
o ~ ~O O ~1
~ E~ ~ E~
Q~
~~ ~ .
td . td 3
-60-
, ~,,
+++ ++
~ X~
o o C .o o
q
7 ~ ~
ooo ~ ~ oo
o
' E~
O ~O Lr~
Q. Lr~ J ~1
~ U~ Il~ O
E ~ ~1 ~I C
_~ ^S
,
~:~ W 1~1 ~ ~ X
O ~ ~ ~ E Ou~ ~
c~a) E~ P~ l:L. ~ S1~ ~1
~ ~,~ s ~ ~ m
J~o bO ~U~ J ~
~I) U~ ON t~J N CO ~I N ~1
~1 Il~
~d ~
E~ E ,1 ~I X
t~ C~o o o ~ o ~o o
b~ ~ bD~1 ~1 ~ bO ~ ~1 ~1
¢ ¢ ~ O
S C~
W
. . . J O -
:' ~1000 J--~00
,1 0 0 t~
I ~I E
u~ ~ ~ bO~ Co
a~ ~ ~ ~ ~r:
~a
O ~ h ~ ~ O
~rl h
~t: t~ O C) O h
Q) E~ Q~ E~
~1 3 lr~ ~1 0
~ltr) ~rl ~) 1~1 N ~I ~ ~ ~)
E~ ~ ~ ~ E~
a~ bO a~ bD bD ~/
td ~O ~ ~O ~/
O . O . .,
t-- ~ C.) ~D O
~ --61-
1~l66;~6
The data in Table 4 illustrate that the use Or the
phenyl ethers, phthalates and the sebacate as carrier-
solvents results in electrodes which give good potasslum lon
respon~e.
Example 39:
Electrodes were prepared and evaluated as descrlbed
in Example 3 using various combinations Or ion carrier and
coating solvents in the membrane layer. The results are
shown in Table 5.
BEHP = Bis ethylhexyl phthalate
THF = tetrahydrofuran
MEK = methyl ethyl ketone
DDP = didodecyl phthalate
The composition of the membranes were as follows:
.48 g/m2 valinomycin
9.76 g/m2 polyvinylchloride
.15 g/m carrier-solvent
.
Table 5
Slope (Range
Ion Coating of Multiple
Carrier Solvent Measurements)
~ .
BEHP THF 51.9 - 59.3
BEHP MEK 56.3 - 58.9
DDP THF 56.2 - 59.3
DDP MEK 53.5 - 58.6
'
Example 40: Electrode Sensitivity
A coated electrode was prepared as described in
Example 3 and tested for selectivity as described below.
Composition
3 6.9 g/m2 Total Ag
1.4 g/m2 AgCl
1.5 g/m2 KCl
5.0 g/m 2PVA
9.68 g/~ PVC
24.2 g/m 2DDP
0.48 g/m VAL
. ~ -62-
6~i~
Evaluation:
The normal level for potassium ion ln blood serum
is about 4 meq/liter while that for sodlum is 30 to 40 tlmes
hlgher It is lmportant, therefore, that sodium lon not
lnterfere with the potassium ion measurement to any sub-
stantial degree. To examine the extent to which sodium ion
interferes with the potassium lon response, the selectlvity
coefficient KK /Na+~ defined by the equation:
E=E + 2.303 RT log [ (aK+ + (KK~/Na+)aNa ) ]
was determined for the above-described coating. Measure-
ments on this coating, using the constant interrerent method
gave a value of 1 x 10 3 in 0.15 M NaCl. In a solution
containing 5 mM K+ and 150 mM Na+, the sodium response
exhibited by this coating represents about a 3% interfer-
ence. Thus, small variations in Na over the clinical
range, i.e., 0.12 M to 0.16 M Na+ result in less than 1%
variation in the interference.
Example 41:
A redox reference electrode having a double-layer
structure was prepared by coating poly(ethylene terephthal-
ate) film support with a conductive layer comprising deion-
lzed gelatin (9.7 g/m2), particulate carbon (15.5 g/m2) and
Triton X-100 (a polyethoxy ethanol commercially available
from Rohm & Haas Co.) (0.28 g/m ) and a redox layer com-
prlsing deionized gelatin (4.85 g/m2) as a binder, potassium
ferricyanide (5.4 meq/m2), and potassium ferrocyanide (5.4
meq/m ). The resulting reference electrode was manually
laminated to a precast ion-selective membrane comprising
: valinomycin (VAL) (0.49 g/m2), bis(2-ethylhexyl)phthalate
3 (BEHP) (14.5 g/m2) and polyvinyl chloride (PVC) (9.2 g/m2).
-63-
_ . .
-- - . . .
The resulting composlte ion-selectlve electrode
was tested in the following cell:
0.15M ¦5b~ drop of 0.15M¦ ion-selectlve
NaCl CEI lNoa~ltcolnta~niKncgl ¦ electrode
Table 6
Potasslum Ion Response
Fe(II)/Fe(III) Internal Reference
KCL
_ 2 Min. (mv)
10 4 -59.0
10 3 _3.7
-2 ~54.4
lo~l +108.2
The emf at 2 minutes shows a linear semiloga-
rithmic dependence on potassium ion concentration with a
slope of 57 mv/decade. The potential drifts with time after
spotting the element with 50 ml Or test solution. The
magnitude of the reproducible drift is about 0.1 mv/minute
between 2 and 10 minutes.
Example 42: Ion-Selective Electrode Utilizing Single-
Layer FetII)/Fe(III) Reference Electrode
A reference electrode having a single-layer struc-
ture was prepared by coating polyethylene terephthalate film
support with a layer comprising deionized gelatin as binder
(4.3 g/m2), particulate carbon (6.9 g/m2), octylphenoxy
polyethoxy ethanol (0.12 g/m2), potassium ferricyanide (7.5
meq/m2) and potassium ferrocyanide (7.5 meq/m2). The result-
ing reference electrode was then manually laminated to a
3 precast ion-selective membrane comprising valinomycin (0.49
g/m2), BEHP (14.5 g/m2) and PVC (9.2 g/m2).
The resulting integral electrode was evaluated in
the manner described in Example 40 with the following results:
-64-
.. . ... .
166~
. . .
Tabie 7
Potassium Ion Response of Integral Electrode
Having "Single-Layer" FeLII ~ e(III
_ . KCL
M 2 Min. (mv)
10 3 ~64.o
-2 ~5 ~ -
10- +49.6
lo~l+102.4
The emf at 2 minutes shows a linear semiloga-
rithmic dependence on potassium ion concentration with a
slope Or 55 mV/decade. The potential of this "single-
layer" format drifts at a rate of about 1.0 mv/minute between
2 and 10 minutes.
Example 43: Electrode Utilizing Double Layer Co(II)/Co(III)
Reference Electrode
A reference electrode having a double-layer struc-
ture was prepared by coating polyethylene terephthalate film
support with a conductive layer comprising deionized gelatin
20 as binder (9.8 g/m2), particulate carbon (15.6 g/m2), sapo-
nin (0.2 g/m2) and bis(vinylsulfonylmethyl)ether (0.1 g/m2)
followed by a redox layer comprising deionized gelatin (10.8
g/m2) octylphenoxy polyethoxy ethanol (0.22 g/m2), bis(vinyl-
sulfonylmethyl)ether (0.22 g/m2), Co(terpyridyl)2(BF4)2
(210 ~moles/m2). The resulting coating was then soaked for
30 minutes in 0.1 N KCl, dried in room air for 24 hours and
then manually laminated to a precast ion-selective membrane
comprising VAL (0.49 g/m ), BEHP (1405 g/m2) and PVC (9.2
g/m2 ) .
The bathing step in the preparation procedure was
: included in this example as a method of absorbing potassium
ion into the redox layer to poise the potential Or the
membrane. This step was not necessary in Examples 40 and 41
-65-
1~66~
because the ferro/ferrlcyanide buffer was prepared wlth
potassium salts.
The evaluation of the element was carrled out as
ln Ex~mple 40 with the following results:
Table 8
Potassium Ion Response of Integral
Electrode Having "Double-Layer"
Co(II)/Co(III) Internal Reference
KCL Cell Emf at
M2 Min. (mv)
10-4-237.4
10 3-183.2
lo-l-126.3
10-68.5
The emf at 2 minutes shows a linear semiloga-
rithmic dependence on potassium ion concentration with a
slope of 57 mv/decade. The potential drifts at a rate Or
about 0.1 mv/minute over 3 to 10 minutes.
Example 44:
Electrodes were prepared as described in U.S.
Patent No. 3,856,649. During and after preparation, the
electrodes were maintained at 100 F and 66% RH. Evaluation
of these electrodes immediately after preparation indicated
response with little drift and linear slopes of about 60 mv/decade
over the range of 10 to 10 1 M KCl. Storage of identical
electrodes at ambient laboratory conditions of about 35-40%
RH for 1 to 15 days with subsequent evaluation by dipping
the electrodes into solutions of known KCl concentration and
reading as described above in Example 3 resulted in
3 erratic drifts of from 2-4 mv/min. The rate of drift slowed
until after about 10-14 minutes the electrode demonstrated a
relatively stable positive drift of about 1 mv/min. Sub-
~ -66-
... ..... . " ~ . . ..
6t~
sequent uses of the same electrodes gave smaller drifts,
indicating that in use the electrode, as expected, tends to
equilibrate as the internal reference becomes hydrated and
thus provides more accurate determinations with continued
wetting.
Example 45:
Wire electrodes having a generally bulbous shape
were prepared using the dipping techniques suggested by
Genshaw et al except that the electrolyte layer was dried at
135 F for a period Or 10 minutes prior to applicat~on of the
ion-selective membrane, to simulate the preparation Or
electrodes according to the present invention wherein the
hydrophilic layer is dried prior to application Or the ion-
selective membrane, but without control of layer-thickness
uniformity. Storage Or these electrodes at ambient con-
ditions (i.e., 35-40% RH), and subsequent use yielded curves
which demonstrated large, random initial drifts of from 16
to 57 mv/decade for from 2 to 10 minutes. When allowed to
soak in 10 1 M KCl~ the blistering or bursting of the outer
membrane as described in the Genshaw publication was observed
only after nine days of soaking in such a solution. Before
bursting, linear Nernstian responses over the range 1-10 mM K
were observed after an initial soaking of several hours.
Example 46:
A wire electrode was prepared as described in
Example 45 except that the ion selective membrane layer was
dried at 85 F instead of 100 F. When used after storage
at preselected ambient conditions of relative humidity
below about 80~ and without preconditiong these electrodes
exhibited large random drift up to about 15 to 16 minutes
66~
when drift stabilized and linear Nernstian response was
observed.
From the foregoing it should be apparent that
electrodes prepared according to the techniques described
in Genshaw et al exhibit linear Nernstian response when
properly preconditioned to achieve a hydrated state; however,
without such preconditioning their behavior is random and
erratic and incapable of calibration under normal ambient
conditions of use without some appropriate induction period.
As the foregoing Examples 44-46 show, electrodes
prepared as describéd in the prior art which comprise an
ion-selective membrane of vzrying thickness in regions thereof
intended for contact with a sample for analysis demonstrate
erratic drift which cannot be calibrated.
Example 47:
Coated electrodes were prepared as described in
Example 4, but with the following compositions for the reference
electrolyte layer and the ion-selective membrane:
Reference Electrolyte Layer PVA 4-8 g/m2 2
KCl-2.4 g g/m
Ion-Selective Membrane PVC9.7 g/m2
DDP14.6 g/m2
VALINOMYCIN 0.5 g/m
Figure 3 shows the shape of E vs. Time curves
obtained by varying the thickness of the foregoing layers
by doubling the laydown of the respective compositions. As
is clear these curves have different shapes, however, each
is calibratable and can provide precise and accurate indications
of potential related to ion activity and concentration.
-68-
,1
i69~
Example 48:
Coated electrodes were prepared as described in
Example 4~ but with the following compositions for the
reference electrolyte layer and the ion-selective membrane:
Reference Electrolyte Layer
Gelatin 5 g/m2
NaCl 2.5 g/m2
Surfactant .09 g/m
lon-Selective Membrane
PVC (1.8~ carboxylated) 10 g/m22
Tris~2-ethylhexyl)phosphate 12.5 g/m2
Sodium Tetraphenyl Boron .6 g/m2
Surfactant .o6 g/m
When drop size samples of aqueous sodium ion solutions
were applied to this electrode a Nernstian slope of 57 mv/dec
was observed.
Example 49:
Coated electrodes were prepared as described in
Example 48, but with the following composition for the ion-
selective membrane:
Ion-Selective Membrane
PVC 10 g/m2
4-octyltrifluoroacetophenone 5 g/m2
Didodecylphthalate 10 g/m2
Trioctylpropylammonium Chloride .5 g/m
This electrode demonstrated a slope of 27 mv/dec when an
aqueous sample containing C03 was applied to the ion-
selective membrane.
Example 50:
3 Coated electrodes were prepared as described in
Example 48, but with the following composition for the ion-
selective membrane:
, -69-
1~1&~
Ion-Selective Membrane
PVC 10 g/m2
Didodecyldimethylammonium . 2
Chloride 15 ~m2
Didodecylphthalate 25 g/m2
Trioctylpropylammonium Chloride .25 g/m
This electrode demonstrated a Nernstian slope of 58 mv/dec
when contacted with aqueous solutions containing chloride
ion.
Although the multilayer electrode elements of the
present invention have been described primarily in connection
with the p~tentiometric quantitation of alkali metal and
alkaline earth ions, the structures, compositions and tech-
niques described herein are equally applicable to the assembly
of electrodes for the analysis of other cations such as NH
and anions such as S3 principally by the selection of
appropriate ion-specific carriers for the ion-selective
membrane, and such electrodes are clearly within the contem-
plated scope of the instant invention.
Furthermore, it is within the scope of the instant
application to incorporate protective overlayers for the
electrode which may serve merely to protect the surface
thereof, increase mechanical strength, or serve multiple
additional purposes such as permitting selective permeability
to a specific ion, or permeability only to a particular
gaseous component of a solution under test, for example,
oxygen or carbon dioxide.
It is also contemplated that electrodes of the
type described herein would be useful in combination with
overlayers containing enzymes which act upon a substrate
specifically and selectively to release ions which can be
quantified by the electrode.
-70-
. .
6~6
While the invention has been described in detail
with particular reference to preferred embodiments thereof,
it will be understood that variations and modifications can
be effected within the spirit and scope of the invention.
~`
.~ .
~ .
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:
