Note: Descriptions are shown in the official language in which they were submitted.
~051~4~0
Method of Making an Ion-Selective Elec-trode
This invention relates to a method of making`an
ion-selective electrode, and more particularly to a method of
making an ion-selective electrode for measuring ion activities
such as Cl-, Br~ or I in solution
Recently there have been developed some devices for
measuring the ion activities using an ion-selective electrode,
for example as disclosed in U.S. Patent No. 3,563,874.
Such an ion-selective electrode comprises a solid
state electrode membrane. In the conventional art, the solid
state electrode membrane is made by the co-precipitation method.
For example, for the ion-selective electrode for
measuring the ion activities of Cl , Br or I , solution of
Na2S 9H2O and halide of Na or K selected from the group of NaCl,
~Br and KI are mixed in the desired ratio, and the mixed solution
is added to AgNO3 solution. Then there is formed a precipitated
mixture of Ag2S and AgX (X is Cl, Br or I). The resultant mixed
~powder is compressed to form a tablet under high pressure, and
the solid state membrane is provided. It is required, for these
membranes to operate as means for measuring the ion activities
in solution, that the granules of Ag2S and AgX are to be as
fine as possible and to react with each other, to improve the
characteristics of the ion-selective electrode. Also, presence
of free ion and metal in the membrane should be avoided because
it causes degradation of the characteristics of the ion-selective
electrode, i.e. decrease of detecting sensitivity and delay of
response time.
The conventional membrane of an ion-selective
electrode is not completely desirable for this requirement.
For example, in the co-precipitation method, although the
granules are very fine, all the granules are not reacted with
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each other because the probability that Ag2S is formed in-
dependently is not zero.
Therefore, an object of the present invention is
to provide a novel and improved method of making an ion-
selective electrode membrane which is free from the con-
ventional defects as described above.
A further object of the invention is to provide
a method of making an ion-selective electrode having
improved characteristics such as higher detecting sensitivity
and faster response time.
A further object of the invention is to provide an
improved method of making an ion-selective electrode easily
by simple means and with low cost.
These objects are achieved by providing a method
of making an ion-selective electrode according to the in-
vention comprising the steps of: adding an AgNO3 solution
to a solution of a chloride,bromide or iodide of Na or K which
is kept agitated strongly so as to provide finer granules
of a silver halide selected from the group consisting
of AgC1, AgBr and AgI; coating the surface of said granules
with a layer of Ag2S by chemical reaction in Na2S solution;
and compressing the resultant granules to form a membrane.
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These and other objects and the features of the
invention will be apparent upon consideration of the following
detailed description taken together with accompanying drawings,
in which:
Fig. 1 is a schematic side-elevational, cross-
sectional simplified view of the electrode made by the method
of the invention; and
Fig. 2 is a schematic side-elevational view of
a cell employing the electrode of Fig. 1 for measurement
of ion-activities in a sample solution.
Referring now to Fig. 1, the ion-selective elec-
trode made by the method of the invention contains a solid state
membrane designated by reference numeral 1. One surface of
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the membrane 1 is coated with a conductive film 2 such as gold,
silver, copper or nickel by the vacuum deposition method. A
metal plate 3, such as copper, is adhered on the conduction
film 2 with conductive adhesive 4, and further the usual eoa~ial
cable 5 is soldered to the copper plate 3 by solder 6. A
combination of the membrane 1 and the coaxial cable 5 is en-
closed in an elogated hollow tubular eontainer 7, which is made
by epoxy resin and open at both sides, at the lower open end
thereof and adhered thereto with epoxy resin 8. An annular
cap 10 is fitted with the other open end of the eontainer 7.
The cap 10 has an aperture through which the coaxial cable 5
is mounted. The outer surfaee 9 of the membrane 1, which contacts
the sample solution, is polished with alumina powder of neàrly
0.05~ in particle size and washed by an ultrasonic cleaner in
aleohol solution.
The membrane 1 of the eleetrode aecording to the
present invention eonsist-of a powder of a silver halide seleeted
from the group consisting of AgCl, AgBr and AgI according to
the desired response of the eleetrode, the surfaee of the powder
being eoated with layer of Ag2S whieh is formed in Na2S solution.
The powder eonsisting of Ag2S and silver halide is
pressed so as to form the membrane 1. Further, the eharaeter-
isties of the electrode ean be improved by etehing the surfaee
of Ag2S layer inthe etehing solution sueh as ~iNO3 so as to
aetivate the powder and then by erushin,gthe powder into finer
granules.
Then the layer of Ag2S is formed on the surfaces of
the granules by substitution reaetion in the solution of Na2S,
and the resultant granules are pressed to form the membrane 1.
In the method of the present invention, silver halide
is formed by putting AgNO3 solution into a solution of a halide of
K or Na, and it is important that the solution of halide of
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K or Na is kept agitated strongly so as to provide finer
granules of silver halide.
The characteristics of the electrode membrane made
by the method of the invention are dependent upon various factors
such as the weight percentage ratio of Ag2S to the silver
halide, etching condition of the powder, concentration and
temperature of the solutions of I~NO3, AgNO3 and halide of K
or Na and coating time of the layer of Ag2S in Na2S solution.
The following are examples of the preparation of the
membranes according to the invention, and the response of
electrodes using such membranes.
Where the response is noted as being Nerns~an, it is
intended to indicate that the ion-selective membrane responds
substantially in accordance with well-known Nernst equation in
a stable and reproducible manner.
The ion-selective electrode according to the invention
responds stably as being Nernstian to lower ion activity in
sample solution, and the response time is superior in the
point of reproducibility.
Example 1.
0.1 mol/liter AgNO3 solution was dropped into a
stoichiometric excess of 0.1 mol/liter NaCl solution which was
strongly agitated by a stirrer, and the fine granules of AgCl
of 10 gram were formed by chemical reaction.
Next, 0.1 mol/liter Na2S 9H2O solution was dropped
into the solution containing these fine granules of AgCl which
was agitated by a stirrer, and the surfaces of these granules
of AgCl were coated with Ag2S layer in the solution according
to the following chemical reaction:
2AgC14S) + Na2S(l) ~ Ag2S(S) + 2NaCl(l)
At this time Na2S solution was added to the solution containing
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the granules of AgCl so as to get a ratio of Ag2S : AgCl = 50:
50 in wei~ht percentage. The resultant AgCl powders coated with
~g2S layer were washed in fresh distilled water about twenty
times, and after washing they were dried ~L~r about one hour at
a temperature of 80C to 100C in inert gas. The dried powders
were crushed into fine granules by a usual crushing means.
Then the crushed powders weighin~ 10 grams were put
into solution of 0.1 mol/liter Na2S 9H2O which was agitated by
the stirrer, and the solution was kept agitated for ten minutes
so that the surface of the crushed powders were again coated
with an Ag2S layer. At this time the Na2S solution was used of
a volume sufficient to get the ratio of Ag2S:AgCl = 60:40 in
weight percentage. The resultant AgCl powders coated with Ag2S
layer were etched in 0.1 mol/liter Hi~03 solution for about
two minutes, and after being washed in fresh distilled water,
they were dried at a temperature of 80C to 100C in inert gas.
These final powders contained substantially no free
metal and no free metal ions such as Ag. Then the final
powder was pressed, for example under 10 tons/cm2 in a die
having a diameter of 10 mm, for several minutes at room tem-
perature in air to form a tablet or a membrane.
Using the membrane made as described above, the ion-
selective electrode as shown in Fig. 1 was constructed. Fig. 2
shows a schematic diagram of a device for measuring the ion-
activities in a solution by using the electrode 11 of Fi~. 1.
The electrode 11 is placed in a sample solution 13
under test so that the outer surface 9 of the membrane 1 contacts
the solution 13. A standard reference electrode, such as a
saturated calomel electrode, is also placed in contact with
30 the solution 13. The two electrodes 11 and 12 are connected
to a voltmeter 14, by which the potential developed by the
electrode 11 in the sample solution is measured, together with
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the reference electrode 12. The potential En is developed
according to the well known Nernst equation:
RT
En = Eo + ~ ln C
nF
where Eo, R, T and F are the usual well-known values, and
C represents the ion activities. Usually, the term RT/nF is
called Nernst's constant, and its value is 59.2 mV for a univalent
ion such as Cl-. The membrane is made according to the method
as described hereinbefore.
With the ion-selective electrode using the membrane, ~ -
ion activities of chlorine in a number of aqueous solutions
of KCl having different concentrations diluted precisely and
serially were measured, respectively.
Then, ion-strength in these solution were adgusted by
0.1 mol/liter KNO3 solution, respectively. The measured
potentials with this electrode areshown in the following Table
1 for each RCl solution of the different concentrations.
Table 1
, .~ . .. _ . _
; Conc. of Cl in ml/liter ~ 10 10 10 4 10 5 10 6
. . .
Potentials (mV) 15 74 133192 239 254
. . _ . ._
The dynamic response time of the electrode was as
shown in Table 2, where the concentration of solutions were
hastily changed from low concentration to high concentration,
and the solution was kept agitated for measuring the dynamic
response time.
Ta~le 2
¦Change of Conc. (mol~llter) ¦10-5 ~ 10-~¦10 4 ~ 10
. ~ _ _
Dynamic Response time (Sec) 1 ~ 0.5
. . _ ._ _ J
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Examplc 2
A membrane was made by a slmilar method as described
in Example 1, except that KBr solution was used instead of
the NaCl solution, and that the powders of AgBr coated with
Ag2S layer at the first coating process were put into 0.05 mol~
liter Na2S solution at the second coating process. Similarly
to Example 1, the ion-activities of bromine in a number of aqueous
solution of KBr of different concentrations were measured.
Table 3 shows the measured potentials developed by
the electrodes containing the membrane made as described above,
which consist of the granules of AgBr having surface coated
with Ag2S.
Table 3
_ ~
Conc. of Br in /liter 10 10 10 10 10 10 10
Potentials (mV) 2 61 121 180 239 291 310
, ,,, ,,~
Example 3
; A membrane was made by a similar method as described
in Example 2, except that KI solution was used instead of the
KBr solution.
Similarly to ~xample 1, tlle ion-activities o~ iodine
in a number of aqueous solution of KI of different concentrations
were measured.
` Table 4 shows the measured potentials developed by the
electrodes containing the membrane made as described above,
which consist of the granules of AgI having surfaces coated
with Ag2S
Table 4
. ._ ._
Conc. of I in mVlite _ _ _ _
Potentials (mV) -48 11 70 129 139 247 307 353
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