Note: Descriptions are shown in the official language in which they were submitted.
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D23-014
Field of the Invention
The invention relates to ion-sensitive electrodes
and comprises a flat surface electrode formed from a
portion of a bulb of ion-selec~ive membrane material
fused to the end of a tube through the use of radiation.
It is particularly useful in forming low resistance
electrodes from high resistivity material.
Background of the Invention
Ion-sensitive electrodes measure the activity of
ions in solution (both aqueous and non-aqueous)and are
well known in the art of analytical chemistry. One
example of such a measurement is pH, which is a measure
of the activi~y of hydrogen ions in solution, and is an
important parameter for many chemical processes.
Another example is the measurement of sodium ions in
foods or biological fluids.
Ion-sensitive electrodes are commonly formed from
a tubular shell having one end sealed with an ion-
selective membrane. The membrane is selectivelypermeable to ions of one type, while excluding others
present in the sample solution. Inside the tube there is
a means for providing a fixed potential, either a
solution of fixed composition or a solid conduc~or in
contact with the membrane. The potential across the
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membrane, measured from the internal contact, through the
sample to a second reference contact provides a measure
of the sample ion activity.
Ion-selective membranes are most commonly formed
with either a bulbous or a flat shape. For membranes
formed in the glassy state, bulbous-shaped electrodes are
more readily formed than flat-membrane electrodes, and
are suitable for measurements of liquid samples where
there is a significant quantity of liquid available for
measurement. Flat-membrane electrodes, in contrast, are
desirable, or even required, for measuring samples where
there is a limited quantity of material available, and
for measuring ~oist solids where the membrane must be
pressed against the sample without immersion in it.
The membranes used for ion-sensitive electrodes
typically present a high input impedence to the measuring
instrument, e.g., up to 1000-20000 mego~ms. This
impeoence limits the accuracy of measurements because of
noise pickup in the electrode. In particular, the ion-
selective membranes for pH-sensing electrodes are
commonly formed from glass. In common pH-sensing
glasses, high selectivity for a hydrogen ion is ~ypically
also accompanied by high resi~tivity, and thus the
improved sensitivity otherwise obtainable from the
material is masked by the increased noise pickup caused
by the higher resistivity. This can be particularly a
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problem with flat surface membranes in which conventional
manufacturing techniques place stringent limits on the
extent to which ~he membrane thickness (and thus, it
resistance for a material of given resistivity) may be
controlled.
Flat-membrance surface ion-sensitive electrodes
are commonly constructed by a dipping process in which a
tubular section of glass is immersed in a molten bath of
membrane material. A bead of molten ma~erial typically
adheres to the end of the tub~lar section, and is
fabricated into a flat membrane on cooliny. The molten
glass must have a coefficient of expansion closely
matching that of the tube. If the coefficients of
expansion of the tube and the molten glass differ
greatly, either the tube or the membrane material will
frequently crack upon cooling, due to differing rates of
contraction. Further, the seal between.~he tube and the
membrane glass is often irregularly formed and prone to
failure. In addition, dipping processes are difficult to
control for uniformity and repeatability of membrane
thickness. Sample to sample thickness variations may
lead to large variations in strength or electrical
resistance.
Once the dipped tube has cooled, the pH glass may be
~round to a desired thickness for the flat membrane
required. Grinding is a time consuming process and
z~
results in a high percentage of defective electrode bodies due to
accidental breaking of the thinned membrane material. Further
the grinding process introduces micro-grooves and stresses into
the membrane. Impurities from the grinding material may also embed
themselves into the areas that are ground and thereby distort
membrane properties. Finally, there is a physical limit to the
thickness to which one can grind a material, without breaking that
material. The limitation is due to the impact nature of the grind-
ing process and the brittle nature of membrane material. This
limitation has prevented the use, in flat or substantially flat
membranes, of low-sodium interference high-resistivity glass.
A need therefore exists for a new method of manufacturing
electrode bodies which will allow for the development of improved
electrodes utilizing improved materials and having none of the
drawbacks of conventional electrodes.
- Surnmary of the Invention
According to one aspect of the invention there is pro-
vided Eor manufacturing an ion-sensitive-electrode body having a
portion formed of an ion-selective membrane material, a method
comprising the steps of:
A. inserting an end of a tube, which tube consists essen-
tially of material that absorbs radiation of a predetermined wave-
length, into a bulb consisting essentially of an ion-selective
membrane material that is substantially transparent to radiation
of the predetermined wavelength so that the end of said tube con-
tacts an inner surface of said bulb; and
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B. shining radiation of the predetermined wavelength throuyh
said bulb onto the end of said tube in contact with said bulb to
fuse said end of said tube to the inner surface of said bulb.
According to another aspect of the invention there is
provided an ion-sensitive electrode comprising a tubular body
bonded at oné end to a flat thin membrane of a material whose
resistivity is greater than 105 ohm-centimeters and which is
selectively permeable to an ion whose concentration is to be mea-
sured.
According to a further aspect of the invention there is
provided an ion-sensitive electrode of low electrical resistance
comprising a body of an infrared absorbent material bonded by
infrared radiation to an ion-selective membrane having a resis-
tivity greater than 105 ohm-centimeters, having limited thickness,
and comprising material transparent to infrared radiation.
According to yet anther aspect of the invention there
is provided an ion-sensitive electrode formed according to a method
comprising the steps of:
a. forming a bulb of ion-sensitive membrane material which
is transparent to radiation;
b. resting said bulb upon a tube of radiation absorbtive
material; and
c. irradiating said bulb and said tube in order to form a
bond therebetween.
Brief Description of the Drawings
The foregoing and other features and ad~antages of the
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invention will be apparent from the following more particular
description of the preferred embodiment of the invention, as illus-
trated in the accompanying drawings, in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
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D23-014
Figure 1 is an expanded cross sectional view of a
portion of an electrode formed by a dipping process. It
depicts the prior art.
Figure 2 is a partial cross-section view of an ion-
sensitive electrode.
Figure 3 is a view of the electrode body prior to
membrane bonding in which an infrared light source is
shown schematically.
Figure 3A is an enlarged view of a portion of Figure
3A.
- Figure 4 is an expanded cross-section of the
operational end of the ion-sensitive electrode of Figures
2 and 3.
Detailed Description of the Invention
'
In figure 1, a typical flat membrane electrode
formed by the usual dipping process characteristic of the
prior art is shown. The membrane material 5 which is
bonded to the tube 8 during the dipping process has an
irregular contour on its inner surface 6. This irregular
contour cannot be corrected by grinding and as a result,
varies with each electrode manufactured. This results in
a membrane of variable and high resistance and thus of
adverse noise pickup characteristics.
An improved ion sensitive electrode 10 is shown in
Figure 2. The electrode 10 has an electrode body 12 of
,
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D23-014
generally tubular shape having one end thereof sealed by
a substantially flat membrane 14. The electrode body is
formed from an energy absorbent (preferably infrared-
absorbent) glass tube and a bulbous membrane preform as
detailed below. ~se of the bulbous preform for theelectroae body permits the manufacture of flat surface
electrodes with high resistivity membrane materials. As
is common in ion sensitive electrodes, an internal
filling solution 16 provides an electrically conductive
path between the membrane and an electrode element 18
which measures a potential difference caused by a change
in the ion concentration in the sample filling solution.
The membrane 1~ is preferably formed from a pH or
other ion selective glass. Such glasses are typically a
mixture of several oxides incl~ding Li2o, Cs2O, La2O3,
; CaO, and Na2o. A variety of other similar consti~uents
have also been used. Further, the membrane 14 is formed
of a thin, substantially flat material, preferably less
than 0.025 inches thick, and as thin as .005 inches.
This is a f~ar thinner membrane section than previously
could be used on flat membrane ion exchange electrodes,
ana therefore can be formea of low-interference materials
such ac low-sodium interference glasses having
resistivities greater than 105 ohm-centimeter~. Although
such materials have high resistivities, preferably about
D23-014
2.5 X 10~ ohm-centimeters, the reduced membrane thickness
offsets the increased resistivity, and results in a
membrane with overall moderate resistance. Accordingly,
electrical noise pick up is significantly reduced and a
more accurate measurement of an increased pH range is
obtained. For example, ~lat-membrane electrodes capable
of measuring pH over the range of 0 to 14 can produced by
the present process.
In order to protect the glass electrode body 12
from accidental breakage during use, an outer protective
tube 20 is placea around it. This outer tube is
preferably constructed of resilient plastic and is
attached to the membrane end of the inner tube 12 by
means of a shock absorbing rubber gasket 22. A cap 24
and lead wires 26 are attached at the remote end of the
electrode boay to complete the structure.
~ he electrode of Figure 2 is manufactured as
follows: Referring to Figure 3, a preform 30 is formed
in the shape of a cylindrical tube having a bulbous head
32 with a diameter substantially larger than that of the
ena of body 12 on which the membrane is to be formed. As
mentioned above, for pH electrodes, the preform 30 may
advantageously be made from a low sodium-interference,
high resistivity material which is transparent to
infrared radiation. Further the bulb 32 is formed to a
reduced wall thickness, e.g. on the order of 0.005
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D23-014
inches. The wall thickness of the bulb is easily
controlled by varying the bulb radius (R) for a given
amount of glass.
The flatness of the bulb is controlled by selection of
the ratio of bulb diameter to ~ube diameter. In
particular, with reference to Figure 3A, the departure
'h' of the bulb membrane from perfect flatness (h=0) can
readily be computed as h=~-~l-cos(sin~la)]/a, where r is
the tube radius and a is the ratio, r/R, of tube radius
to bulb raaius. For a ratio of a=0.5, h=0.268r, that
is,the subtended portion of the membrane sealing, the
tube departs from flatness by less than fourteen per cent
of the tube diameter. For a=0.33, the departure is less
than nine per cent.
Despite its limited thickness, the bulb is
structurally quite strong and thus is relatively stable
sh~pc
and easy to handle. Further, the slightly arched ~
is beleved to contribute to the strengh, since glass is
stronger in compression than tension. A flat plate of
similar thickness would be extremely fragile and quite
dif~icult to handle. Further, the bulb has a relatively
constant wall thickness so that membrane thickness, and
therefore resistance, may be closely controlled.
The preform 30 is placed over one end of the
electrode body 12, with the bulb resting directly on the
edge of the tubular body 12. As mentioned above, the
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body 12 preferably comprises an infrared absorbent glass.
Infrared absorbent glass is commonly called ~green
glass"; examples include 'SRI' glass and 'STI' glass
manufactured by the Nippon Electric Glass Co., Ltd., 1-1
KAKUDA-CHO, KITADU, Osaka, Japan, as well as certain
glasses manufactura by the Schoot Company, e.y. Schott
~o. 4840E glass.
The next step in the ~anufacture of the electrode
body is to focus a beam of radiation, such as from an
infrarea source 15, slightly above the interface between
the bulb 32 and the end 12a of body 12. Tne light
passes through the infrared transparent bulb 32 with
little absorption and thus little heating, and evenly
heats the 'lip' of the infrared absorptive glass tube 12
at its area of contact with the membrane. The tube is
rotated at this time in order to keep the heating
uniform. The radiation is then brought to a focus at the
interface so as to melt the lip of the tube to thereby
enable fusing of the tube to the membrane. The infrared
energy is then removed (e.9O/ the source is turned off).
It should be noted that the melting point of the glass of
the tube is lower than that of the membranous b~lb. If
this were not the case, the thin bulb might soften and
collapse during the fusing process.
As the assembly begins to cool, it is useful to
slightly pressurize the air space within the tubular
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12
section 12 in oraer to promote formation of a uniform
seal joint 34 (Figure 4) between the glass electrode body
and the membrane 14. Ihis slight pressurization of the
air space also eliminates internal stresses created by
the fusing process in the membrane 14 and the joint 34.
As the fused assembly cools, the remainder of the
membrane material tends to crack and fall away. The
fusea assembly then need only be polished at any ragged
edges about the periphery of the membrane 14 before it is
ready for use. The main section of the membrane
material, which is thin and unsupported, does not need to
be polished or ground. The electrode body 12, with the
fused membrane material 14 is then ready for final
assembly and the electrode element 18 can be inserted
into the boay and terminated at the opposite end of the
housing.
The improved ion-sensitive electrode made by the
above process is capable of superior operation when
compared with previous flat surface, ion-sensitive pH
~o electrodes. The method of manufacture, as described
above, permits virtually flat membrane wall thicknesses
as low as .005 inches and therefore allows the use of
much higher resistivity materials for flat membranes than
those available previously. Thus, high performance
materials, desirable for the reduced sodium interference
effects which characterize them but hithereto
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13
contraindicated by their high resistivity which led to
increased electerical noise pickup, can now
advantageously be used to form pH electrodes operable
~ over a wide pH range.
This process of manufacture also makes advantageous
use of the highly uniform wall thicknesses that can be
achieved in blowing glass bulbs. The bulb 32 of membrane
material is blown to a uniform desired wall thickness; as
a result the membrane formed on the tube 12 also
possesses a uniform wall thickness. This avoids the
unoesirable electroae resistance variations discussed in
reference to figure 1.
The membrane is also structurally im~roved by the
use of this process. The uniform joint between the tube
boay 12 and membrane 14 is quite strong and less likely
to separate than the joints formed by previous methods.
Further, microscratches and stresses which are induced by
conventional grinding of a membrane surface to the proper
thicknesses for flat membranes are completely eliminated
by this process. The membraneoùs bulb needs no further
processing after its fusing to the ~ubular body of the
electrode probe. ~his results in an improved membrane
surface with less likelihood of electrode cracking.
Finally, it should be noted that the process used in
the manufacture of this improved ion sensitive electrode
substantially reduces the cost of manufacture~ Since
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hand grinding and polishing is largely eliminated, the most time
consuminy and delicate operation in the construction of flat sur-
face ion sensitive electrodes has been eliminated. Further, waste
caused by membrane breakage during grinding and polishing is also
eliminated.
While the invention has been particularly shown and
described with reference to the preferred embodiment thereof, it
will be understood by those skilled in the art, that various
changes in form and de-tail may be made therein without departing
from the spirit and scope of the invention as defined by the append-
ed claims. It is possible, for example, to utilize other electxo-
magnetic energy sources such as ultraviolet light to fuse the
material of the body to the membrane material. Further, the
materials used to construct the probe need not be llmited to glass-
es: ceramic and epoxy materials have also been used in ion
sensitive electrode devices with good results. In appropriate
cases, an intermediate meltable bonding material, compatible with
both the tubular wall material and the membrane material, may be
used to effectuate the desired bond in cases where the membrane
material and the tubular wall material may not themselves be
sufficiently compatible.
