following sections were therefore prepared from membranes
pressed from materials produced by this procedure. The
method involves mixing the metal ion solutions and adding
this mixture to a slight excess (v 10%) of sulphide solu-
tion with constant stirring to assure that an excess of
sulphide ions is present throughout the precipitation pro-
cedure - an important requirement. To obtain finely div-
ided powders, rapid mixing of relatively concentrated
(1M) solutions was used, the process being carried out
below 20°C. The precipitate was then thoroughly washed
several times with hot and cold deionized water, dilute
nitric acid, carbon disulphide and acetone before being
dried at 80°C in air. Proper washing and drying was
eritical since any contamination with metal ions or

other salts caused insensitivity in the resulting elec-
trodes. For lead sulphide - silver sulphide coprecipi-
tates, drying in vacuum is recommended since, in the pre-
sence of air and moisture, lead sulphide is rather easily
converted to lead sulphate. The presence of this com-
pound in some batches of precipitates was confirmed by
x-ray diffraction analysis and the resulting membranes
gave unsatisfactory electrodes.

In all cases a 1:1 mole ratio of silver sulphide to met-
al sulphide was found to give best results though other
ratios between 1:4 and 7:3 produced fairly good sensors.

Of the many methods of preparing membranes based on heavy
metal sulphides the most useful in this case is the pres-
sure forming of polycrystalline pellets. Such pellets
are rigid and, therefore, easy to handle but are soft
enough to be conveniently drilled and machined with nor-
mal tools.

While pressures of 100,000 p.s.i. maintained for periods
of a few minutes were sufficient to produce good cupric
ion electrodes, such cold pressings of cadmium and lead
coprecipitates gave pellets without any response to the
ion in question. Increase of pressing time and pressure
to 155,000 p.s.i. did not improve this situation. Cold
pressed Ag2S/PbS pellets, in fact, did not even hold to-
gether very well and readily split into thin circular
plates. Hot pressing of these materials, on the other
hand, had a pronounced effect on the performance of the
resulting electrodes. Temperatures above 100°C during
pressing produced much more robust pellets which did re-
spond to concentration changes. Optimum pressing times
and temperatures for both cadmium and lead pellets were
4-8 hours at 150°-250°C. Pressures greater than 100,000
p.s.i. showed no beneficial effect nor did times in ex-
cess of eight hours. When temperatures higher than 250°C
were used, layers of silver metal were formed on the sur-

-14-

se LEER


a jewélers saw and mounted in a 4 jaw lathe chuck. The
pellet is shaped into a cylinder of 1/8" diameter using
a very sharp cutting tool. It is then placed in a col-
let, reduced to a 1/16" diameter, remounted in a 1/16"
collet and sharpened to a pointed tip using a shallow
compound angle on the lathe. The micro sensing ele-
ment was then put in a glass sleeve which was half fill-
ed with the conducting epoxy. A length of wire was in-
serted into the epoxy from the other end of the sleeve
and the assembly was set aside to allow the epoxy to
harden. Next, the entire assembly is cemented with an
insulating epoxy into a 12 gauge stainless steel tube.
A 14 gauge hypodermic needle is cut to a length of
about 3 cm. and cemented into the other end of the
Stainless steel tube. The needle hub is threaded with
a 10-32 tap and a microadaptor soldered to the wire
leading from the membrane and screwed into the hub of
the needle. The stainless steel tube acts as a shield-
ing and is connected through the microadaptor to the
shielding of the miniature coaxial cable leading to the
PH meter.

A reference micro-salt bridge was prepared by drawing
a glass capillary tube to a fine point (tip opening
about 20 microns) and the tip bent so that the salt
bridge capillary could be fastened to the ion-selec-
tive microelectrode to bring the two tips very close
together. The micro-salt bridge was filled with 10%
potassium nitrate solution using vacuum. Polyethyl-
ene tubing was attached to the glass capillary, fill-
ed with 10% potassium nitrate, and inserted into a
small container which held the reference electrode
and a few milliliters of the electrolyte solution.
The position of the reference container had to be ad-
justed so that the hydrostatic pressure at the tip

of the salt bridge would be small. However, it was
important to maintain a positive pressure at the tip
so that the very small sample drop was not siphoned
up into the salt bridge.

Contamination, evaporation and the delicate nature of
the electrode tips must be effectively dealt with when
making ion measurements in dilute solutions in sub-
microliter volumes. Various types of simple sample
holder designs have been successfully employed. One
arrangement involved delivery of the sample using a
very fine-drawn glass disposable pipette upon a fine
nylon screen stretched over a small beaker. The micro-
electrode assembly was mounted vertically in a micro-
manipulator (Hacker Model M-1, Hacker Instruments, W.
Caldwell, N. J.) with a precision three directional

-8-



be monitored by casting a number of different membranes
into a single plastic block; if several idential pellets
were arranged in this manner, the progress of a reaction
could be followed at various stages in a single stream.

We also developed a novel split crystal membrane electrode
which permits almost perfect matching of electrode char-
acteristics and allows "double beam" measurements to be
made by simultaneously operating one side of the split
electrode as a reference and the other as the indicating
electrode. Aside from the convenience of having matched
electrodes, this new system has the important advantage
of minimizing the need for restandardization with changes
in conditions or in prolonged use. This desirable fea-
ture results from the fact that the split sensor is form-
ed from a single membrane which undergoes uniform aging
with time, specific use or temperature changes.

Although the general principle of the split membrane ap-
proach should be applicable to many membrane electrodes,
we selected the Ags S/AgI type iodide sensor for detailed
study because of oar previous experience with this
electrode and its known reliability. When used in the
differential mode, the split membrane sensor eliminates
the need for an external reference electrode and should
be useful for continuous monitoring, automated analysis,
and process control.

Coprecipitated AgI/Ag2S powder was prepared and pressed
into a wafer thin disc membrane (13mm diameter by 0.7mm
thickness). The disc membrane was then split to give
two semi-circular membranes which were embedded together
in one Bio-Plastic mold. The two half membranes were em-
bedded side by side but not in contact. A flow channel
was drilled through each split membrane and connnected
to polyethylene tubing of appropriate diameter. The
flow channels were polished with diamond powder to im-
prove the response of the electrodes. Internal con-
tact to the membrane was made by drilling a hole at the
edge of the plastic body to the membrane and attaching

a piece of plexiglass tubing. AgNO3 (0.1M) was placed
in the side arm to provide liquid contact with the mem-
brane. A piece of silver wire attached to a BNC connec-
tor completed the circuit for each arm of the electrode.

Each split electrode membrane was first calibrated in
a flowing stream against an external reference elec-
trode using a series of KI solutions of ionic strength
adjusted to 0.3M with KNO The two electrode halves
were calibrated simultaneéusly at room temperature

(21 + 1°C).

-18-



Abstract

Under this project, new ion-selective electrodes were
developed for several ions not previously accessible to
electrode measurement. In addition, new electrode con-
figurations were constructed and evaluated in terms of
suitability for monitoring purposes.

Specifically, a liquid membrane electrode for carbonate
and a solid membrane electrode for sulfate were devised.
The properties of these electrodes were evaluated and
found to be useful for measurements in water systems.

Both micro and flow-through electrodes for a number of
ions were constructed and tested. Particular success
in this connection was achieved for sensors responsive
te the -halide and heavy metal ions. Electrodes were
applied to the measurement of NTA in waters and the
study of ion association.

This report was submitted in fulfillment of Grant Number
R-800991 by the State University of New York at Buffalo
under the sponsorship of the Environmental Protection
Agency. Work was completed as of August 1974.

iii




faces of the pellet indicating perhaps some decomposition
of the silver sulphide. These membranes made unsatisfac-
tory electrodes though their performance could be slight-
ly improved by heavy sanding of the surface.

For all 13mm pellets, between two and three grams of mat-
erial were pressed to give pellets approximately 3mm
thick. In the case of 5mm membranes proportionately less
material was used. Dies were evacuated throughout the
pressing period using a Duo-Seal Model 1400 vacuum pump
(W. M. Welch Co., Chicago, I1l.). The different condi-
tions required to produce good lead and cadmium membranes
can be rationalized with the fact that, while copper and
silver sulphides readily form solid solutions, the other
sulphides do not. Silver sulphide is known to undergo

a phase transition at 178°C; perhaps temperatures in
this region are required to form a suitable lead or cad-
mium sulphide - silver sulphide mixture. Simultaneous
heating and pressing are necessary, however, as cold
pressing followed by heating at 200°C for four hours in
air or nitrogen failed to produce responsive membranes.
Pellets treated in this manner blistered and could not

be sanded down to give a smooth surface.

We have developed two methods of achieving suitable dir-
ect internal contacts. The first uses a silver filled
epoxy (E-Solder R, Epoxy Products Co., New Haven, Conn.)
to join a silver or copper wire directly to the pellet.
On curing this epoxy forms a strong conductive bond bet~
ween the silver sulphide of the pellet and the metal
wire. Best silver particle to silver sulphide contact
is achieved if the membrane surface is lightly sanded
and well cleaned before application of the epoxy. The
second method involves pressing a thin layer of silver
powder followed by a layer of copper powder (M.C.B.
Manufacturing Chemists, Norwood, Ohio) on top of the
coprecipitate during pellet preparation. A copper
connecting wire can then be soldered to the copper lay-
er or affixed with conductive epoxy. The latter means

is perhaps more satisfactory since the heat induced ex-
pansion involved in soldering can cause the metal lay-
ers to separate from the pellet. When the membranes are
mounted as discussed in the following section, the inter-
nal contacts are well protected from the sample solutions.

A number of factors are involved in the choice of the best
method of mounting the pellet. If, as was the case in
our initial work, performance figures for the electrode
used in the dipping and flow-through modes are to be
compared, the membrane cannot be totally enclosed in the
electrode body. The pellet, with the connecting wire
attached, is sealed with Araldite R epoxy resin into one

-15-


KGBERT L. FEDER
VI. Applications Methodology

Because of concern over water pollution problems related
to the growth of aquatic vegetation, materials such as
nitrilotriacetic acid (NTA) are being substituted for phos-
phates in detergents. As a result there is a great need
for new analytical methods to determine NTA in the presence
of common detergent constituents. Previously, NTA has been
determined by thermometric titration and by titration with
copper(II) using azurol S as indicator after removal of
phosphates by precipitation. We developed an electrode
method, which permits the direct determination of NTA

by copper(II) potentiometric titration without prior sep-
aration of phosphates, sulfates, or sulfonates.

A weighed amount of NTA was diluted with distilled-deminer-
alized water in a 100 ml volumetric flask. An aliquot

was added to the thermostatted cell, 5 ml of 1 M NH3~NH,NO3
buffer was added and brought to 50 ml with water so that
the NTA was 0.001 M. The solution was stirred constantly
by means of a Teflon coated magnetic stirring bar. The
electrodes were inserted into the solution and the sample
titrated with 0.01 M copper(II) nitrate. Potentials

were recorded on a Beckman pH recorder. Investigation of
interfering materials was made by weighing them directly
into the original NTA solution.

Typical titration curves for the titration of NTA in
NH3-NH4NO3 buffer (pH v 9.6) at various concentration
levels show that NTA may be titrated directly at concen-
trations as low as 5 x 10-5 M under these solution condi-
tions.

Since NTA is commonly used in detergents in combination
with triphosphates, the effect of this constituent on the
NTA titration was investigated. While the increasing con-
centrations of triphosphate decrease the total potential
change of the titration, the position of the end point is
not changed over the range studied. Thus, NTA can be
titrated in the presence of triphosphate without prior
separation.

Comparable experiments carried out in the presence of sul-
fate or sulfonate gave similar results and showed that
sulfate shifts the position of the titration curve but
does not alter the location of the endpoint.

Additional experiments carried out on commercial detergent
samples known to contain phosphate and sulfonates also
showed that NTA could be determined in such mixtures using
the proposed method. Thus, it appears that the potentio-
Metric titration of NTA with copper(II) as monitored by the
copper ion-selective electrode should be useful for analyti-
cal purposes as the use of NTA in household products con~
tinues to increase.

-21-


the membrane surface.

It had been noted initially that fine polishing of mem-
brane surfaces greatly improved the response times and
sensitivities of dip-type electrodes. Such treatment
proved to be even more important in the case of flow-
through electrodes as will be detailed below. Effective
polishing of the narrow channels was achieved using a
fine cotton-covered wire or needle soaked in water and
impregnated with diamond dust. Oil based diamond pastes,
while more efficient, contaminated the channel and
virtually destroyed response.

When used in the dipping mode, all electrodes exhibited
Nernstian response in the 107 1m to 1075M concentration
range. The copper electrode remained Nernstian to 10-6
while the cadmium and lead models seldom gave better

than 16-18 mv change between 107°M and 10-6M. In the flow-
ing mode each electrode closely reproduced its dipping
behavior provided surface pretreatments were performed.
Absolute potential values were found to vary as much as

30 mv from pellet to pellet. Perhaps part of this could
be attributed to minor variations in the internal con-
tacts but the major portion seemed to be due to surface
treatment differences and inherent properties of the
pellets. Evidence that the latter was most important

was gained when two copper electrodes were made from

the halves of a single pellet. These displayed identi-

cal potentials (+ 0.2 mv or better) throughout the 10-5M
to 10-1m range when directly compared in the same solu-
tions.

Polishing to a bright mirror finish proved to be the
single most effective method of producing a substantial
improvement in electrode behavior. For cadmium and lead
sensors the difference in both response time and flow
rate dependence was very marked. Not only is the equili-
brium potential reached much more rapidly (less than 5
sec. compared to almost one minute) but the potential
change is much more nearly Nernstian. In addition, the
at rest potential is almost the same as that of the_flow-
ing stream. Flow rates in each case were 1 ml sec7l.

We have, thus, shown that a number of solid membrane
flow-through electrodes can be made in the laboratory.
Silver sulphide based membranes for chloride, bromide and
iodide ions have also been successfully prepared and we
expect the range of ions which can be sensed in this man-
ner to be quite large. Applications of such sensors
could include continuous process or auto-analyser efflu-
ent monitoring and fast flow kinetic studies. In the
former case, it is quite feasible that several ions could

-17-


RD-674
UNITED STATES
ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460

Official Business

hn ‘

8
rn mo

Ny

POSTAGE AND FEES PAID
ENVIRONMENTAL PROTECTION AGENCY

EPA-335

ECIAL FOURTH-CLASS RATE
sP BOOK

baoeeperk O°)
OR . «
USA-COE-Ohio Riv. Div.

P.O. Box 1159
Cincinnati, OH 45201

If your address is incorrect, please change on the above label;
tear off; and return to the above address,

If you do not desire to continue receiving this technical report
series, CHECK HERE (C] ; tear off label, and retum it to the
above address,



at lower concentrations the slope begins to decrease in
the same fashion as for the commercial macroelectrode.
It is of interest to note that another 50% silver chlor-
ide/silver sulfide coprecipitate, prepared with excess
sulfide and chloride, is responsive to silver (I) but
not to chloride.

Considerable difficulty was encountered during attempts
to prepare iodide sensitive microelectrodes using the
procedures just detailed. Electrodes obtained display-
ed sub-Nernstian response or poor stability during pro-
longed use. These problems were ultimately traced to
incompatibility of the membrane material with the sil-
ver epoxy used to make electrical contact and to the
need for proper conditioning of the electrode tip.

Successful iodide microelectrodes were finally realiz-
ed by pressing 50% silver iodide/silver sulfide copre-
cipitates, prepared in the presence of excess iodide,
at room temperature and an applied pressure of 115,000
p-s.i. for 1-1/2 hour periods. After machining, the
microelectrode was mounted in a glass tube equipped
with a permanently sealed silver-silver (I) liquid
internal reference. With conditioning in 0.1 M iod-
ide solutions and gentle polishing of the tip before
use, this electrode yields excellent Nernstian be-
havior over wide iodide concentration ranges and has
excellent reproducibility. The iodide electrode is
the only microelectrode of this set which is not
completely solid state, although the sealed internal
reference solution requires no attention during normal
use. It seems likely that further investigation of con-
ductive cementing materials would result in a suitable
procedure for constructing a completely solid state
version.

The general properties of the heavy metal and halide
electrodes described are quite similar to those of com-
Mercial macro electrodes in terms of range, selectivity,
and dynamic characteristics. In view of the low cost
and relative ease of preparation of these electrodes,

it is hoped that such microprobes could find use in
analytical and environmental measurements where sample
sizes are the restrictive limitation.

The importance of. ion selective electrodes in many as-
pects of chemistry has been demonstrated by the rapid
increase in the volume of literature on the subject
over the last few years. As a result, the characteris-
tics of the many electrodes available for a large num-
ber of ions have been well documented in this period.
The applications which have been proposed cover a range

-12-



or plastic electrode bodies and internally connected to
a silver contact wire. Great care was taken to prevent
surface contamination of the membranes.

Although a number of compositions gave membranes with
sulfate response and greater or lesser sulfate selectiv-
ities, optimum results were obtained with membranes con-
sisting of 32 mole % Ag2S, 31 mole % PbS, 32 mole %
PbSO4, and 5 mole % CugS. The most successful membranes
were formed by pressing with an applied pressure of 102000
pounds per square inch for 18 hours at a temperature of
300°C. The elevated temperature appears to be the criti-
cal variable and may be related to the fact that Ag2S
undergoes a phase transition at 178°C. The small amount
of Cu2S in the pellet appears to improve the response
time of the electrode membrane and may be related to the
fact that Cu2S has semiconductor properties.

Conditioning of the electrode surface appears to be im-
portant in giving optimum performance. We alternately
soaked the electrode in dilute solutions of Na2S0O4 and
in mixtures of AgNO3 and Pb(NO3)2 for periods of several
hours. Occasionally, after prolonged experimentation
under severe conditions, the shiny surface of the elec-
trode became dull and performance was improved by gentle
polishing with a wiping tissue or, in extreme cases,
emery paper.

The principal results obtained with the sulfate membrane
electrode indicate that the electrode yields a nearly
Nernstian slope of 29 millivolts per decade (29.6
theoretical) over a fairly wide range of sulfate activity
and, furthermore, displays appreciable selectivity for
sulfate over a wide variety of common univalent anions.
The electrode employed reached stable potential values in
less than one minute in the sulfate solutions. Good
electrode response is also obtained at sulfate concentra-
tions greater than 10-2M. For practical purposes, where
empirical calibration curves are constructed or experiments
are carried out at constant ionic strength, the electrode
should be useful at least over the 1071 to 1074M sulfate
range.

Although the interference of the three halide ions test-
ed appears to be related to the solubilities of the cor-
responding silver salts, we feel that it is not meaning-
ful to compute numerical selectivity ratios for the ions
tested on the basis of the present data. In our view,

the small potential changes observed with large variations
in concentrations of the univalent anions indicate that
the electrode is essentially insensitive to these ions

-4-





vil.

a.)

b.)

c.)

d.)

e.)

£.)

h.)

i.)

j.)

k.)

Publications

H. B. Herman, G. A. Rechnitz, "Carbonate Ion Selective
Membrane Electrode", Science, 184, 1074 (1974).

R. Wawro, G. A. Rechnitz, “Split Crystal Ion Selective
Membrane Electrode", Anal. Chem., 46, 806 (1974).

J. Czaban, G. A. Rechnitz, "Solid State Ion Selective
Microelectrodes", Anal. Chem., 45, 471 (1973).

M. S. Mohan, G. A. Rechnitz, "Preparation and Proper-
ties of the Sulfate Ion Selective Membrane Electrode",
Anal. Chem., 45, 1323 (1973).

H. Thompson, G. A. Rechnitz, “Ion Selective Flow
through Electrodes", Chem. Instr., 4, 239 (1972).

G. A. Rechnitz, G. H. Fricke, M. S. Mohan "Sulfate
Ion Selective Membrane Electrode", Anal. Chem., 44,
1098 (1972).

L. F. Heerman, G. A. Rechnitz, "Ion-Selective Electrode
Study of Copper(I) Complexes in Acetonitrile", Anal.
Chem., 44, 1655 (1972).

H. I. Thompson, G. A. Rechnitz, "Fast Reaction Flow
System Using Crystal Membrane Ion Selective Elec-
trodes", Anal. Chem., 44, 300 (1972).

G. A. Rechnitz, E. Eyal, "Selectivity of Cyclic
Polyether Type Liquid Membrane Electrodes", Anal.
Chem., 44, 370 (1972).

E. Eyal, G. A. Rechnitz, "Mechanistic Studies on the
Valinomycin based Potassium Electrode", Anal. Chem.,
43, 1090 (1971).

G. A. Rechnitz, N. C. Kenny, "Determination of NTA

with Ion Selective Membrane Electrodes", Anal. Letters,
3, 509 (1970).

-24-



Conclusions

1. The development of new membrane electrodes for ions
not previously accessible to such measurements has been
shown to be entirely feasible as exemplified by the de-
sign of carbonate and sulfate ion selective electrodes.

2. Micro and flow-through membrane electrodes can read-

ily be constructed for both anions and cations by utiliz-
ing various configurations. These designs are eminently

suited for continuous monitoring applications or measure-
ments on restricted sample volumes.

3. By combining electrode potentiometry with chemical
manipulation, the measurement capabilities of membrane
sensors can be extended to non-electroactive materials
by virtue of direct reaction or ion binding.


Section

II

Tit

Iv

VI

VII

Contents

EPA Review Notice
Abstract
Conclusions

New Electrodes and Potential
Electrode Systems

Micro and Flow-through Electrode
Sensors

Applications Methodology

Publications

Iv

21
24



from enzyme assays to pollution analysis. In the vast
majority of such applications, electrodes are used in

the dipping mode with relatively large sample volumes.
Relatively few sensors have been developed for use in
small volume flowing streams and so far these have been
only of the liquid membrane or glass capillary type.
Electrodes of this type for Nat, K+, Cat+, and Mgt+ are
commercially available and the utility of such electrodes
for measuring a number of rate constants using a fast

flow system has been demonstrated. A method by which
solid membrane electrodes could be incorporated into such
a system using a cap device greatly reduces the necess-—
ary flowing volume while using a regular dip-type sensor.
There is no doubt that where sample volume is restricted
and automated determinations of a large number of samples
is desired such a system is both convenient and reliable.
We also dealt with the construction, treatment and char-
acteristics of solid membrane flow through sensors where
the test solution is channeled through the sensing membrane
and not merely across its surface. The advantages of

this technique are substantial reduction in sample volume
and the possibility of determining several ions by a
series of sensors placed close together, the flow being
led directly from one to the next with a common reference
electrode. We have succeeded in preparing and evaluating
flow-through sensors for several heavy metal ions. Opti-
mum results are shown to depend upon careful attention

to the physical construction of the electrodes, the choice
of preparative procedures, as well as the polishing and
conditioning of the membrane surfaces. Specific procedur-
es and methods are proposed to facilitate the construction
of such flow-through sensors in other laboratories and

the results of our evaluations are presented.

Many methods of preparing the metal sulphide membrane

materials have been suggested, from coprecipitation by
various techniques to direct reactions between metals

and sulphur.

Individual precipitation of sulphides from sodium sulphide
and metal nitrate solutions, followed by thorough mechan-
ical mixing in the desired ratio proved quite successful
in some cases but reproducibility was not sufficient for
our purposes. Using the direct reaction technique prepar-
ations of single and mixed sulphides in our trials led to
highly unsatisfactory membrane materials giving very limit-
ed or no response to the ion in question. This could have
been due to the difficulty in maintaining the proper ex-
cess of sulphur during the high temperature reaction be-
cause of the volatility of sulfur. Overall, the most re-
liable technique was bulk coprecipitation as recommended
in the ORION patent. All electrodes discussed in the

-13-


Measurements carried out in mixtures of carbonate and
chloride show negligible chloride interference even in
samples containing 10-1 molar chloride and 1075 molar car-
bonate. We calculate the carbonate to chloride selectiv-
ity to be 5.4 x 103. This value should be regarded as

a lower limit because of possible residual carbonate
levels in the chloride reagent used. Similarly, we have
determined the carbonate to sulfate selectivity of the
electrode as 6.7 x 103 and the carbonate to phosphate
(taken as HPOG) selectivity as 3.8 x 103.

The overall potentiometric behavior of the new electrode
is typical of liquid membrane electrodes in general. We
observe response times ranging from 30 seconds to 2 min-
utes in carbonate solutions, the slowest response being
found at the lowest concentration levels. Electrical
noise in carbonate solutions does not exceed 0.2 milli-
volts and reproducibility of + 0.5 millivolts from
sample to sample is readily achieved. Electrode lifetime
in routine use will depend on factors such as the specif-
ic support membrane used and solution conditions but
appears to be consistent with that reported for liquid
membrane electrodes in general. In view of the dynamic
range, selectivity, and other properties of this elec-
trode we expect it to be useful for carbonate measure-
ments in environmental systems.

Efforts to prepare useful sulfate ion-selective membrane
electrodes have been largely unsuccessful. The work of
Hirsch-Ayalon on BaSO4-cellophane membranes, of Pungor
and co-workers on precipitate impregnated silicone rub-
ber membranes, and our own work on the Pungor electrodes
showed that it is relatively easy to achieve potentio-
metric response to the sulfate ion but extremely diffi-
cult to obtain potentiometric selectivity for sulfate.

Recent progress on crystal membrane electrodes encour-
aged us to investigate pressed membranes on various cry-
stal mixtures and we are now able to report on a new
electrode with good response and selectivity for the
sulfate ion. The electrode is relatively easy to prepare
in the laboratory and has properties which suggest that
it should be useful for analytical purposes.

Various inorganic salt mixtures, all based upon the use
of Ag2S as a membrane matrix, were pressed into membranes
and evaluated for sulfate response. Typically, finely
divided powders of the constituents were carefully mixed
in known ratios and then pressed into membranes with a
laboratory pellet press under controlled conditions of
applied pressure, temperature, and pressing time. The
resulting crystal membranes were then sealed into glass

-3-


In the differential mode, a solution of known iodide con-
centration was passed through one of the membrane halves
which served as the reference electrode. Solutions of vary-
ing iodide concentrations were passed through the other mem-
brane (sample electrode), and the potential differences
between the two electrodes were measured with the differen-
tial amplifier. The recorder was used to record the poten-
tial differences as well as to measure response times.

The readings at each iodide concentration agree to within
one millivolt. The critical factor in determining matched
potentials does not depend so much on the electrode mem-
brane but rather on the internal electrical contact, i.e.
the filling solution, silver wire contact and B.N.C. con-
nections must be identical. The silver wire leads had to
be lightly sanded to remove any coatings and good tight
fits of the silver wires into the B.N.C. connections were
essential for matched readings.

The potential readings varied from day to day with the
indicated difference being the non-cumulative drift that
normally results from such external factors as temperature
changes, junction potentials and membrane conditioning.
Although the potential readings varied, the response of

the two electrodes remained matched and the response slope
remained constant (57.3 + 0.4mV) for each calibration.

This is an important factor for possible use of the split
electrode system as a continuous monitor. The system may
be used over a long period of time since the response of
the matched membrane electrodes will not be affected by
normal changes in external factors. The electrodes change
in exactly the same manner and, thus, the need for frequent
standardization is eliminated. We have used the electrodes
over a period of three to four weeks with no deterioration
in performance.

In the dual flow differential experiments, the same iodide
solution was initially passed through both electrode mem-
branes. Because the two membranes are matched to < Imv,
the resultant differential reading was always found to be
less than 1 mV. Then, one side of the electrode (I) was
used to measure a series of iodide solutions while the
original solution continued to pass through the other
electrode (II) serving as the reference. The potential
differences between the two electrodes were recorded for
each iodide solution and were found to give a straight
line calibration plot. The electrodes were then operated
so that electrode I was now the reference and electrode II
was the sample electrode with very similar results.

The concentration of the referenece solution (1075 to
10-1m KI) was found to have no effect upon the response
or performance of the sample electrode. In a series of
consecutive runs, using electrode I as the sample side,
the replicate calibration slopes were 57.6 + 0.6mV while

-19-


74-079

EPA 660/2
August 1974

ion Technology Ser

ironmental Protect

Envi



comparable to that of biological membranes and it can be
shown that some compounds with large anti-microbial activ-
ity are those for which complex formation constants with
cations are high.

Various theories have been proposed to explain the role
of valinomycin in ion transport through membranes.
These mechanisms involve some complex formation between
valinomycin and potassium ion. According to one model,
the valinomycin molecule acts only at the phase inter-
face to enable the ions to pass into the membrane where-
in they move as free ions. A second model explains

the enhanced permeability on the basis of channel form-
ation involving the passage of ions through a channel
of ordered valinomycin molecules which span the thick-
ness of the membrane. The third mechanism, which we
also favor, involves initial complex formation between
the valinomycin and the cation followed by the transport
of the ion through the membrane in the cavity of the
valinomycin ligand, which acts as a "carrier" or trans-
port catalyst. This model is also supported by the
work of Eisenman et.al. on the actins. By measuring
the potentiometric selectivities of the valinomycin
electrode and the formation constants of the valinomy-
cin-cation complexes, we showed that the selectivity
coefficient is approximately equal to the formation
constant quotient for various pairs of alkali metal
ions; in thick electrode membranes similar results
were also obtained when cyclic polyethers were used as
carriers. Furthermore, we showed that ion selectivity
is lost when the membranes are "frozen" even though
some residual conductivity persists; this is taken to
support the carrier model because we feel that freezing
the membrane greatly reduces the mobility of the bulky
carrier molecule and largely negates its transport
catalyzing action in electrode membranes.

-6-



and is primarily reflecting secondary activity, liquid
junction, and membrane solubility effects.

Calibration curves were also constructed using sulfate
salts with cations of various types and charges. The fact
that these calibration curves were identical in slope and
potential, indicates that the electrode is not responsive
to cations. Preliminary experiments carried out as a
function of solution pH showed little pH effect in the pH
3-10 range. At pH values below 2.5 there is a sharp
potential change in the direction of apparent less sul-
fate; this is probably due to the formation of HSO,TM in
acid solution. Above pH 10.5 electrode response also
deteriorates; we tentatively attribute this to hydroxide
film formation involving the metal constituents of the
membrane.

The mechanism of operation of this electrode is still ob-
scure although the observation that best performance is
obtained with membranes where the three principal con-
stituents are present in a 1:1:1 ratio is provocative.

It appears that the present electrodes possess sensitiv-
ity, selectivity, and response time properties suited to
analytical purposes.

Neutral carrier type liquid membrane electrodes with
selective response to monovalent cations have gained
considerable acceptance in recent years because of their
high selectivity coefficients. Neutral carriers include
polyethers, polyesters, and certain antibiotics.

Typically, valinomycin is dissolved in an appropriate
solvent such as hexane, octanol, or phenyl ether, and
placed in a liquid membrane electrode assembly giving
rise to an electrode with selectivity ratios of the

order of 5000:1 for potassium over sodium; such a
selectivity ratio is considerably higher than that of
about 30:1 for cation sensitive glasses. Thus, an under-
standing of the electrode mechanism is important not only
to make better electrodes but to improve understanding
of ion transport in biological membranes.

In the latter connection, it must be recognized that
the electrode consists of a solution of valinomycin in
an organic solvent that is immiscible with water, inter-
posed between two aqueous solutions containing the ions
to be measured, just as the membrane of a living cell
or of mitochondria separates the internal from the ex-
ternal solution of physiological character. In both
cases valinomycin has been shown to greatly enhance

the permeability of the membrane to potassium ions.
Indeed, the selectivity of the valinomycin electrode is

-5-




SELECTED WATER
RESOURCES ABSTRACTS

INPUT TRANSACTION FORM

4, Title
ION SELECTIVE MEMBRANE ELECTRODES FOR WATER
POLLUTION MONITORING

7. Author(s)

Rechnitz, Garry A.
9. Organization

2] Grant No.

Department of Chemistry ~
State University of New York R-800991
Buffalo, New York 14214

15, Supplement. Notes

Environmental Protection Agency report number
EPA-660/2-74-079, August 1974

16. Abstract’ Under this project, new ion-selective electrodes were
developed for several ions not previously accessible to electrode
Measurement. In addition, new electrode configurations were con~
structed and evaluated in terms of suitability for monitoring
purposes.

Specifically, a liquid membrane electrode for carbonate and a
solid membrane electrode for sulfate were devised. The properties
of these electrodes were evaluated and found to be useful for
measurements in water systems.

Both micro and flow-through electrodes for a number of ions were
constructed and tested. Particular success in this connection was
achieved for sensors responsive to the halide and heavy metal
ions, Electrodes were applied to the measurement of NTA in

waters and the study of ion association.

17a, Descriptors *carbonates, *chemical analysis, cadmium, copper,
*electrodes, halides, *ion-exchange, lead, monitoring

17h, Identifiers *ion-selective, *microelectrodes, association constants,
complexes, silver, NTA, nitrilotriacetate, continuous flow,
flow-through

Ic. COWRR Field & Group

18, Availability Send To:

WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON, 0. C. 20240

Abstractor Garry A. Rechnitz State University of New York

WRSIC 10,



Iv. New Electrodes and Potential Electrode Systems

Previous attempts to devise bicarbonate or carbonate ion
selective electrodes have resulted in electrodes with poor
selectivity for the desired ion with respect to such
common interferences as chloride. We have now succeeded
in preparing a liquid membrane electrode with selectivity
for carbonate with respect to chloride, sulfate, and phos-
phate of at least 10° and negligible response to bicar-
bonate. The electrode yields Nernstian response to car-
bonate levels in the 10-2 to 10-6 molar range and has a
response time of 30 sec. to 2 min. depending on the con-
centration levels employed.

The liquid membrane electrode is constructed in the con-
ventional manner using plastic electrode bodies (Orion)
and cellulose acetate support membranes with 0.lum aver-
age pore diameter (Millipore Corp.) or Orion 92-20 mem-
branes. The active electrode material is formed using
a liquid phase consisting of 1% (by volume) of tri-
caprylyl methylammonium chloride (General Mills Chemi-
cals, Inc., Aliquat 336) dissolved in trifluoroacetyl-
p-butylbenzene. The latter is prepared by a Friedel-
Crafts acetylation of butylbenzene with trifluoroace-
tic anhydride using anhydrous aluminum chloride as a
catalyst, purified by fractional distillation, and iden-
tified by mass spectrometry (m/e 230), infrared spec-
troscopy, and nuclear magnetic resonance. The quater-
nary ammonium salt is used in the chloride form, as
received, or converted to the bicarbonate form. An
aqueous solution, 0.1 molar in both sodium chloride

and sodium bicarbonate, is used as the internal elec-
trode reference solution. All measurements are taken
against a double junction Orion reference electrode.

Since carbonate coexists with bicarbonate in pH depen-
dent equilibrium, the carbonate levels must be calcul-
ated Oey, the relevant equilibrium constants (Kj =

4.5 x 107 Ko = 4.8 x 10-11) and the modified bavies
equation. "our data indicate the absence of any response
to the bicarbonate ion. Indeed, experiments carried out
in mildly acidic media (pH < 5.5) where bicarbonate is
vastly favored over carbonate, still yield potentials
appropriate for a pure carbonate response even_fhough
the concentration of carbonate is less than 10 ~ molar.
In practical terms, however, the interconversion of car-
bonate with bicarbonate restricts the lower pH limit of
the electrode to approximately pH 5.5. The electrode has
an upper pH limit of approximately 8.5 at 4 x 1073 molar
carbonate and 9.5 at 5 x 10-2 molar carbonate.

-2-



end of a Plexiglas tube. Contact surfaces should be sand~
ed before sealing. Alternatively, the electrode body can

be filled with Bio-Plastic R polymer (Ward's Natural Science
Est., Rochester, N. Y.) which, in addition to sealing the
membrane to the body, makes the final electrode more ro-
bust. At the other end of the Plexiglas tube the connect-
ing wire terminates in a B.N.C. connector. There is no
necessity to shield this wire.

For an electrode to be used only in the flow-through man-
ner a second mounting technique is recommended. The
entire pellet and connecting wire are embedded in Bio-
Plastic using a circular mold. In this way, contamina-
tion of the pellet and wire are avoided. In this case,
the flow channel is drilled in the longitudinal axis of
the pellet and the internal connection is made to its
circumference. Obviously, pellets with silver and copper
layers are not suitable for mounting in this manner.
Again, sanding of all pellet surfaces before embedding
improves the seal of the plastic to the membrane. This
technique allows several pellets to be mounted in the
same embedment so that a series of sensors can be made

in one continuous block.

After final cure, the polymer can be machined to a con-
venient size and shape and any protruding connnecting
wire protected by a Plexiglas tube. This configuration
also lends itself to the use of an internal filling solu-
tion since the seal is good enough to keep internal and
flowing solutions separated even under the high pressures
of fast flow conditions.

The flow channel in the membranes was made by relatively
slow speed drilling with a regular steel drill bit of
suitable diameter. Depending on the application envis-
aged and the thickness of the pellet, holes of 0.025
inches to 0.075 inches in diameter were drilled. For
laterally channeled pellets the sensing path is relative-
ly long, up to 13mm, and hence smaller holes are prefer-
able. If, however, the electrode must measure activity
at more precise points in the flowing stream the second
configuration is more suitable. This gives a short
sensing path which can be less than one millimeter de-
pending only on the thickness of the pellet. Hence,
larger holes and faster flow rates are possible without
making the dead space within the membrane very large.
During drilling great care must be exercised so that the
seal between the body and the pellet is not fractured.
Even the slightest separation will permit solution to
enter by capillary action and such contamination is very
difficult to remove. In addition, no lubricant oil was
used as it was found that this irreversibly contaminates

~16-



Copper (I) ions are stable in a number of organic solvents,
e.g. acetonitrile and nitromethane. The stability of cop-
per(I) ions in such solvents has been explained by an in-

crease of the solvation energy of copper(I) ions and/or

a decrease of the solvation energy of copper(II) ions with
respect to the solvation energies of these ions in water.

In this work the formation constants of copper(I) complex-
es with the halides and with thiourea in acetonitrile have
been measured using a novel cuprous sulfide-membrane ion-
selective electrode as the indicator electrode.

The membrane (diameter: 13mm) was prepared by heating fine-
ly powdered cuprous sulfide (Alfa Inorganics) (1.7-2.0g) at
300°C for 24 hr. under a pressure of about 7 ton/cm*.

After fastening a copper leading wire directly to the mem-
brane, the pellet was imbedded in a glass tube with arald-
ite (which proved to be very resistant towards acetonit-
rile) and the electrode surface was polished with very

fine abrasive cloth. The electrode was then conditioned
overnight in a 1072M copper(I) perchlorate solution in
acetonitrile before the measurements were started.

The replacement of the internal electrode and filling
solution, normally used in glass electrodes and ion-selec~
tive electrodes, by a direct leading wire contact to the
membrane proved to be useful for the preparation of several
sulfide-based solid state ion-selective electrodes. Leak-
age of the solution through the side-wall of the membrane
and the electrode body, causing electrical contact between
the solution and the leading wire and resulting in break-
down of the electrode, is almost totally excluded and
facilitates use of such electrodes in non-aqueous solvents.

All experiments were carried out in a double-walled boro-
silicate glass vessel thermostated at 25°C. Electrodes

and burettes were inserted through a rubber stopper in the
cell. No special precautions were taken to avoid the entry
of oxygen in the cell since the solutions were found to be
stable towards.air oxidation during the time needed for

the measurements.

Potentiometric measurements (+ 0.1mV) were carried out with
a Corning Model 12 research pH meter. In solutions contain-
ing tetraethylammonium perchlorate as the supporting elec-
trolyte both a Beckman quartz fiber type SCE filled with
sat. KCl-methanol electrolyte or an Orion Model 90-02

double junction electrode with the supporting electrolyte
solution in the outer chamber were used as a reference.

In solutions containing sodium perchlorate as the support-
ing electrolyte however, stable potentials could be obtain-
ed only with the double junction electrode as a reference.

-22-

TSOTSI ann
ceemiaammanaanaaal


EPA Review Notice

This report has been reviewed by the Environmental Pro-
tection Agency and approved for publication. Approval
does not signify that the contents necessarily reflect
views and policies of the Environmental Protection
Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for
use.

ii

— Loeotesernn at tcc TT aT ete


mounting stage and maneuvered until the electrode tips
just penetrated the surface of the sample drop. The
nylon support is flexible enough so that no harm comes
to the electrode tips if penetration is too deep. A
number of sample drops can be placed in a small area
on the nylon support and the electrodes moved conven-
iently from one drop to the next.

Evaporation of the sample was not a significant prob-
lem as long as measurements were taken within 3-4 min-
utes. Generally, all the electrodes reached a stable
potential within 2 minutes but measurements can be
recorded prior to this provided all readings in a par-
ticular series are taken at approximately the same
time after insertion of the electrodes into the sample.

Evaluation of the electrodes was made with solutions
prepared by successive dilutions of 0.1 M stock solu-
tions. For the metal ion electrodes these stock solu-
tions were prepared with the corresponding nitrate

salts and for the halide electrodes the corresponding
potassium salts were used. Potassium nitrate was

used to adjust the ionic strength to 0.5 M and 0.2 M for
the heavy metal and halide ion test solutions respect-
ively.

Micro electrodes for silver (I) and copper (II) were
relatively easy to produce. The silver (I) microelec-
trode was prepared from silver sulfide produced by the
precipitation technique outlined above, whereas, the
copper (II) microelectrode was prepared from a 50%

mole ratio of copper sulfide/silver sulfide coprecipi-
tate. The exact composition of the coprecipitate was
not found to be a critical factor in electrode per-
formance. Both the silver and copper electrodes were
pressed at room temperature under approximately 100,000
p-s.i. for about 30 minutes. Variations in the pre-
sure and duration of the pressing did not significant-
ly affect the response characteristics of the electrodes.
Pressures as low as 70,000 p.s.i. and pressing time

as short as 1 minute produced electrode membranes with
Nernstian response. The response of these microelectrod-
es is reasonably fast. Instant potentials were usually
within 5-10 millivolts of the final values reached with-
in 30 seconds in sub microliter sample volumes.

Satisfactory lead (II) and cadmium (II) microelectrodes
proved to be significantly more difficult to produce
than the silver and copper electrodes. The composition
of the lead electrode membranes was varied from 20-70%
mole ratio lead sulfide/silver sulfide and the membranes

~9-


A 50% mole ratio lead sulfide/silver sulfide coprecipit-
ate prepared according to this method was hot pressed at
approximately 115,000 p.s.i. for about 17 hours. This
membrane had no cracks and, after calibration as a macro-
electrode, was shpaed into a microelectrode which respond-
ed well to lead (II) at levels as low as 10-5M in sample
volumes of less than 1 microliter. The electrode re-
sponded within 10 seconds and stable potentials were
reached in 2~3 minutes.

Similar preparative problems were encountered with the
cadmium (II) electrode. However, using the revised
washing procedure and the hot pressing technique ( v
150°C, 15-20 hours, and 115,000 p.s.i.), a 50% mole
ratio cadmium sulfide/silver sulfide membrane was pre-
pared which responded in a Nernstian fashion to cad-
mium test solutions. Membranes of this composition
were very difficult to machine into micromembranes and
the composition was finally altered to 25% cadmium sul-
fide/silver sulfide in order to improve the machining
characteristics. A membrane of this composition was
hot pressed as above and machined into a micromembrane
which responded to cadmium solutions down to a concen-
tration of 107-5M and reached a steady potential within
1 minute.

For the preparation of bromide ion responsive micro-
electrodes, membrane material composed of 50% mole ratio
silver bromide/silver sulfide was prepared as outlined.
A membrane was pressed at room temperature under

100,000 p.s.i. for about 1 hour. The membrane showed
no evidence of cracking and, after testing its response
to bromide solution, in the macro form, it was machined
and mounted as a micromembrane. The bromide micro-
electrode responded well in 0.5 microliter samples and
reached a stable potential within 1-2 minutes.

The chloride electrode was produced using a 50% mole
ratio silver chloride/silver sulfide coprecipitate
prepared by the general procedure in the presence of
slight excess of chloride. A membrane with Nernstian
behavior was prepared by pressing under the same condi-
tions as used for the bromide electrode for about 1
hour. It should be noted that during the machining of
the pellet, prolonged contact between the membrane and
the metal collet of the lathe tends to cause corrosion;
this problem can be minimized by coating the metal parts
in contact with the membrane with a teflon spray.

After machining, the micromembrane was incorporated into
an electrode body and found to respond with a theoretic-
al slope to chloride levels above 5 x 10°°M chloride;

-1ll-


EPA-660/2=74-079
ROBERT L. FEDSE August 1974

Ion-Selective Membrane Electrodes for Water

Pollution Monitoring
by

G. A. Rechnitz

Grant No. R-800991
Roap/Task 16ADN 17
Program Element 1BA027
Project Officer
Dr. Thomas B. Hoover

Southeast Environmental Research Laboratory
Athens, Georgia 30601

PREPARED FOR
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D. C. 20460

For sale by the Superintendent of Documents, U.8. Government Printing Office, Washington, D.C. 20402 - Price 80 cents


electrode II gave a slope of 57.3 + 0.6mV. When consecu-
tive runs were performed using electrode I as the sample and,
then with electrode II as the sample, the slope values were
found to agree within 0.3mV per decade.

It is clear that the results obtained are consistent with
the degree of electrode matching achieved. At the present
time, such matching of the two electrode halves includes
agreement of E° values to <lmV, agreement of calibration
slopes to <0.3mV, and comparable response times. Most im-
portant, this degree of matching appears to be unchanged
with time over the four week period studied. Thus, the
novel electrode configuration described should be useful
for differential potentiometry under a variety of condi-
tions.

~20-



RESEARCH REPORTING SERIES

Research reports of the Office of Research and
Monitoring, Environmental Protection Agency, have
been grouped into five series. These five broad
categories were established to facilitate further
development and application of environmental
technology. Elimination of traditional grouping
was consciously planned to foster technology
transfer and a maximum interface in related
fields. The five series are:

1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research

4. Environmental Monitoring

5. Socioeconomic Environmental Studies

This report has been assigned to the ENVIRONMENTAL

PROTECTION TECHNOLOGY . series. This series
describes research performed to develop and
demonstrate instrumentation, equipment and

methodology to repair or prevent environmental
degradation from point and non-point sources of
pollution. This work provides the new or improved
technology required for the control and treatment
of pollution sources to meet environmental quality
standards.

EPA REVIEW NOTICE

The Office of Research and Development has reviewed this report
and approved its publication. Mention of trade names or
commercial products does not constitute endorsement or recommen—
dation for use.




Calibration curves were recorded by adding increments of a
10-2mM or a 2.5 x 10-2M copper(I) perchlorate solution to a
known volume of solution containing the supporting electro-
lyte. Micro-burettes used for calibration or titration
were graduated to 0.02 ml (for recording calibration curves
a microsyringe was used for the lower concentrations). In
complex formation studies the electrode was calibrated prior
to each titration run.

For complex formation studies with chloride the total cop-
per(I) concentration was held constant at v 5 x 10°°M

during the titration by adding the same amount of copper

to the titrant. For complex formation studies with bromide
or iodide the total copper(I) concentration was v 1073M

and thiourea titrations were performed at different total
copper(I) concentrations varying from 5 x 1074M to 5 x 1073M.
The ligand concentration was varied over the range zero to
10-2m for the halides (except chloride) and from zero to

2 x 10-2M for thiourea.

The electrode displayed an almost Nernstian response to
copper(I) ion concentrations over the range 1072M - 10-5M
with a slope of 55-56 mV/decade (theoretically 59.16 mV at
25°C). It was found that the slope decreased and the re-
sponse time increased when x in Cu,_,S was varied from 0.21
to zero, the best results thus beifig obtained for Cu, 79S.
The cuprous sulfide used in this work had an almost per-
fect stiochiometric composition (analytical data supplied
by the manufacturer), so that the small deviations of the
slope from the theoretical value can be accounted for by
the high metal to sulfur ratio of the membrane. No detail-
ed study was made of the response time of the electrode.

It was observed, however, that less than 60 seconds was re-
quired to obtain steady potentials for copper(I) concen-
trations > 5 x 1074M. Below this concentration level the
response became more sluggish and an interval of 5-10 min.
was allowed for equilibration. The E° values changed by
several millivolts from one day to another but remained
constant during the time needed for a titration run.

Our studies indicated that the electrode showed an almost
Nernstian response, i.e. slope of 55-56 mV, for copper(I)
concentration down to at least 3 x 10-/M in the presence
of complexing ligands.

The electrode was used for the measurement of the stepwise
formation constants of copper(I) complexes with the hal-
ides in acetonitrile. Log 8; and log By values were as
follows: C17, 4.9 and 10.7;  Br7, 3.8 and 7.8; and I~,
3.2 and 6.4. The electrode might be particularly useful
for the study of interactions of copper(I) ions with or-
ganic ligands in acetonitrile. As an example, complex
between copper(I) and thiourea has been investigated. The
experimental data suggest Cu[S=C (NH,) 9] 9* as the predomin-
ant complex species for which the formation constant has
been evaluated as log Bo = 6.3.

-23-


V. Micro and Flow-through Electrode Sensors

We have devoted a good deal of effort to the construction
and evaluation of a series of microelectrodes of the type
metal sulfide/silver sulfide, selective towards Agt, Cu2t,
Pb2+ and cd2+, as well as of the type silver halide/-
silver sulfide, selective towards S¢~-, Br, C17, I7.
Electrodes constructed in our laboratory have tip sizes
in the range of 50-100 microns and have been evaluated

in solution volumes of between 0.5 and 1 microliter, al-
though the dimension of the electrodes would allow use

of sample volumes as small as 0.05 microliters.

All membrane materials were prepared by coprecipitation
of silver sulfide and the corresponding metal sulfide
(Cu, Cd, Pb) or silver halide (I, Br, Cl). In the case
of the heavy metal electrodes appropriate volumes of 1
M silver nitrate and 1 M metal nitrate were mixed and
added to excess sodium sulfide. In the case of the hal-
ide electrodes the appropriate weight of potassium hal-
ide was added to 1 M sodium sulfide and the stoichio-
metric amount of silver nitrate was then added to the
mixture of sodium sulfide and potassium halide. The
precipitations were carried out in the presence of
excess sulfide and/or halide, and the metal nitrate solu-
tions were always added to the potassium halide and/or
sodium sulfide solutions.

The precipitate was very thoroughly washed with hot
water, then with acetone, filtered and dried in air at
about 100°C. A revised washing procedure used in spec-
ial cases is detailed below for the lead and cadmium
electrodes.

In general, membranes were pressed using a KBr die with
a 13 mm diameter plunger. Approximately 2 grams of co-
precipitate was pressed under vacuum at room temperature
for various lengths of time under a 20,000 pound load,
which corresponds to about 100,000 p.s.i. The lead and
cadmium electrodes required pressing at elevated tem-
peratures which were achieved by wrapping a heating

tape (Briscoe Mfg. Co., Columbus, Ohio) around the die.

After removing the membrane from the die a silver wire
was attached to one face using a silver-based conduct-
ing epoxy (E-Solder, Epoxy Prod. Co., New Haven, Conn.).
A coaxial cable was soldered to the silver wire and was
used as the input lead to the pH meter. After prelimin-
ary tests showed that the macro membrane functioned pro-
perly, the microelectrode tip was machined as desired.
The central membrane section is cut from the pellet with

-7-


vo

were pressed at 100,000 p.s.i. for different lengths

of time varying from a few hours to 1 day. All mem-
branes except the 20% pellet exhibited fracture planes
parallel to the surfaces, apparently due to poor press-
ing characteristics of lead sulfide. The presence of

a large percentage of silver sulfide, which has excell-
ent pressing characteristics, in the 20% lead sulfide/-
silver sulfide membrane: prevented cracks from develop-
ing; however, the membrane showed no response to lead
(II). This behavior was thought to be due to a lack of
availability of lead sulfide at the surface of the mem-
brane.

In order to incorporate a larger percentage of lead sul-
fide in the membrane and still prevent cracking of the
pellet, the die was heated to approximately 150°C during
the pressing operation. Silver sulfide exhibits a

phase transition from the rhombic to the cubic form at
175°C under 1 atmosphere pressure. At the very high
pressures used for pressing the pellets, e.g. about

6800 atmospheres, this transition should occur at a
temperature well below 175°C. This phase change

might allow for a more favorable packing of silver sul-
fide and lead sulfide to permit the use of higher lead
sulfide ratios. Thus, a 50% mole ratio lead sulfide/-
silver sulfide coprecipitate was hot pressed at 100,000
p.s.i. for about 3 hours. The membrane was not cracked
when removed from the die and it was found to respond

to lead ions. However, the slope of the calibration
curve decreased continuously from a near Nernstian value
in 1071-1072 M lead nitrate solutions to less than_15
millivolts per decade at a concentration below 10-3 M
lead nitrate.

X-ray diffraction studies did not yield much structural
information about the lead sulfide/silver sulfide mem-
branes, but did indicate the presence of as much as 3-5%
of lead sulfate in the pellets. Since lead sulfate has
a solubility product of about 10-8, which is much larger
than that of lead sulfide (K_. v 10728), its presence

in the membrane may be respoj Bible for the poor response
of the hot pressed 50% lead sulfide/silver sulfide mem-
brane. Lead sulfate can form from moist lead sulfide

at temperatures as low as 50°C and thus, might have been
produced during the drying procedure.

Therefore, a revised washing procedure was used in order
to eliminate the possibility of lead sulfate contamina-
tion. The coprecipitate was boiled for an hour, filter-
ed, washed with dilute nitric acid and washed many times
with distilled, deionized water and, then, dried in vacuum
at room temperature. The precipitate is mixed with car-
bon disulfide and filtered, then washed with acetone and
dried in air at 80-100°C.

-10-