Gordon H. Fricke and Martha J. Kuntzl lndiana-Purdue University at Indianapolis Indianapolis. 46205
1
I
Inexpensive Solid-state IO~-selective Electrodes for Student Use
Ion-selective electrodes are widely used in industry and research. A recent survey places them second only to liquid chromatography in growth in usage of all types of analytical instruments and equipment ( I ). Results of another recent survey show that many college and medical school teachers have placed ion-selective electrodes in the VERY IMPORTANT class of topics to he taught in undergraduate programs 12). ~-, ~
Brief discussions of the several types of ion-selective electrodes may he found in recent textbooks in analytical chemistry (3-5). A few articles have been published, which are intended for student experiments, which describe the preparation andlor usage of several types of ion-selective electrodes (6-10). The present article describes the class of electrodes which are based on a solid-state sensing membrane. Background and Theory The most familiar ion-selective electrode is the glass electrode for the measurement of the activity of H+ which was introduced in 1909. Glass electrodes which respond to other ions were first introduced in 1957. At the present time glass electrodes have been prepared to detect activities of H+, Li+, Na+, K+, Rh+, Cs+, Ag+, and NH4+. Glass electrodes respond onlv to monovalent cations. The ions to he selected muHt be ableto exchange with the ions associated with the elass matrix (anions can not exchange) and the ions in an aqueous solution must he able to migrate into the outer hydrated glass layer of the electrode in order to undergo an ion-exchange process (polyvalent cations are not sufficiently mobile). Therefore, to prepare an electrode which would respond to anions and polyvalent cations the sensing memhrane had to he replaced by a different type of material which is not hydrated to any appreciable extent in aqueous solutions and one which allows the anions of the matrix to undergo exchange. The first type of non-glass ion-selective electrode was reported in 1965 by Pungor e t al. (11) who formed a pellet sensing membrane from a 1:l mixture of silver halide and silicone rubber. The pellet was sealed into the end of a nonreactive support tube and the pellet became the sensing membrane. These electrodes are com~letelvanalagous in construction to the glass memhrane elec~trodesndrespond to horh Ag7 and the halide lon. Presently, electrodes are available with internal filling solutionssimilarro the glasselectrode and with the internal reference wire attached directly to the internal side of the precipitate hased sensing membrane
a pellet a t a high pressure (-105 psi) and sometimes a t a high temperature (15).Electrodes of this type are made from single inorganic precipitates or mixtures of inorganic precipitates. ~helndividualelectrodes are responsive to; variety ofcations and anions (Ap+, . Ph2+, Cuz+, Cd2+,La3+, F-,C1-, Br-, I-, CN-, SCN-, and Sod2-). Consider a few of the situations which were investigated hy the authors to explain a simple model (Table 1). The solidstate ion-selective electrodes which are now commercially available (with the exception of the fluoride electrode) can he considered to respond to the silver ion by the reduction Depending on the complexity of the material which is used as the sensing memhrane, the response of the electrode may involve any of the following sequence of steps 1) Reduction (or Activity Differences)
2) Solubility Equilibria 3) Competing Solubility Equilibria (or Ion-Exchange)
For example, the silver wire electrode will respond by step (1) only. When a silver wire is used as the indicator electrode and a small portion of AgCl is introduced into the system, a solubility equilibrium is set up. This introduces the second step in the model given by Electrode System (3) in Tahle 1. T h e C1- in solution establishes an equilibrium with AgCl which is dependent upon the solubility product constant of AgCl. Theactivity of Ag+ is fixed and the silver wire responds by the reduction of Ag+ given hyeqn. (1).It isthermodynamically impossible for the silver wire to respond properly to changes in the chloride ion activity without the AgCl precipitate in the system, as evidenced by the standard potential for the following reduction: Clz 2e- e 2C1-. Electrode Systems (4) and (5)are used to introduce the third step into the model. It is seen that SZ- is the common ion between two competing equilibria reactions. Since the solubility product constant for PhS (or CuS) must remain constant, the changes in the activity of Pb2+ (or Cu2+)must he accompanied by changes in the activity of S2-. Since S2- is the
+
Table 1. SomeSyrtemr Studied to Explain a Model for Electrode Resoonse -
Electrode System
E g u i l i b r i u m Reactions
AgCI or Aq/AgCI
AgCl
,.-,
IlZl.
A single crystal sensing membrane of LaF3, which responds t o F- and La3+, was reported in 1966 (13). Then, it was reasoned that it should be possible to press a precipitate into a pellet to prepare systems for which it was difficult to grow a sinele crvstal. This was done successfullv with AelS in 1968. In 1969 &archers began to report thai ptlletskade from metal sulfidrs mixed u.ith silver sulfide would resoond to the second metal ion in the membrane. The idea was extended to a three compuncnt precipitate mixture in 1972 (141. T h e non-glass solid-state ion-selrcti\.e electrode consists of n fair!), insoluble inorganic precipitate whirh is pressed into
3
~ -
4
e Ag+ + CI-
++e- Ft A4
-
4
A%S/PbS
AQXSc' 2A9+ + 5'PbS t' Pbai + 5'AQ+ + e- P A9
5
AgiSlCuS
A g 2 S t' 2Ag' + 5'CUS a cu2++ 5'~ ( l + + e- t ' Aq
'NSF-URP participant, Summer 1915.
Volume 54, Number 8, August 1977 1 517
same S2-associated with the A n 8 equilibrium and since the . sduhility product constant d A g ? S must remain constnnt. an activitv d.4e' must lie established which is del~endenton the activity of 6h2+ (or Cu2+). The electrode detects Pb2+ (or Cu2+)indirectly by the reduction of Ag+ as given by eqn. (1). The activity of Ag+ may he calculated from the solubility (activity) product constants of Ag2S as follows = 10WO Kar2s = 02~,+as*-
(2)
(3) Kpbs = apytas1- = = K*~~sapb2+ = 10-11apb2+1/2 (4) Kpbs Equation (4) indicates that the activity of Ag+ is very low for any activity of Pb2+. An activity of less than one sulfide ion in 50 ml is also calculated. This would be impossihle unless the equilibria which involve S2- are considered as dynamic equilibria. The competing equilibria step can be extended to include a third component as given by Electrode System (6)in the table. This system, reported in the literature (14, IG), has two common ions, Ph2+ and S2-, and the electrode responds to a divalent anion. The electrode detects the activity of S042indirectly by setting up the competing equilibria and establishing a certain activity of Ag+. Again, the electrode responds by the reduction of Ag+. If the model is valid, the steps should be separable. The precipitates are necessary only to establish the competing equilibria andlor the solubility equilibria. As explained earlier, AgCl precipitate may be used in conjunction with a silver wire to determine the activity of C1-. The idea may be extended to include two precipitates which have an ion in common. I t can be shown that a silver wire will r e s ~ o n dto the activitv of Pb2+when Ag2S and PbS are present in the system ( ~ l e c t ~ o d e Svstem 4. Table 1).Since the silver wire will not resoond properly t o the activity of Pb2+ without the precipitate present. because i t is thermodvnamicallv im~ossible,it is assumed that the precipitates provide the means f o r t h e adjustment of the Aa+ - activitv which can be detected bv the silver wire. The model may he used to explain and predict selectivity. If it is assumed that all ions have equal access to the sites on
Table 2. Suggested Laboratory Experiments imate ~recipitare
G r o u ~ Electrode
1 2
Wire Ag2S/ ~9
Mixfuren none none
silicone 3
Ag2S pellet
1
Ag
2
A%S/ silicone
wire
A9CI/ silicone AgCI/ silicone
A9,S
1
Ag
2
Ag&/cus/ Silicone
3
A%S
Ag2S/CuS/ ~ilicone
1
Ag Wire
2
~g?s/PbS/ s111cone
Ag,S/PbS/ rillcone
3
0
the
pellet
Ag2S
oellet
A P D I Yvery little of electrode surface.
Potential of 0.1 M
Selec~
solution,
tivity
mv
Terr
1485
0.1pbl+ M
+25
0.1 M Br.
lo-'M Agt
A9CII
3
Wire
Activities
10.' none
Illicone
&et
Telf solution
Ag,S/CuS/ silicone none
none
Ag,S/PbS/ rilicone
lo-'10-~
M CI-
lo-'lo-' M cu'+
lo-'lo-=
+255
t220
0.1Mn%+ M
0.1 M
Mn"
M pb2+
the precipitate mixture to a rmali portion
518 1 Journal of Chemical Education
the surface of the electrode, selectivity may he directly associated with the ion-exchange process. For example, consider the selectivity of Br- in the presence of C1- using a AglAgCI electrode. AaC1 Br- s ApBr C1-
+
+
This shows that Br- will interfere with this electrode. That is. the electrode is not selective to C1- in the Dresence of Br-. he same treatment for the selectivity of Pb2+ by an Ag2S electrode leads to K..I = 10-22. This indicates that in the presence of Ag+ the Pb2+will not interfere. However, when there is no An+ initiallv Dresent, the A& electrode will rebecause certain activity of Ag+ will be esspond to tablished as shown by eqn. (4).
;
Approx
Sfudent
-log a,,. Figure 1. Student response of two silver wire electrodes to silver ion. A meter with a nonexpanded scale was used. The separate line gives the theoretical Nernst shoe.
of
Design of Student Experiments
Table 2 presents a series of experiments which may be performed by one, two, or three groups of students. It has been found that a student can prepare the silver wire electrode and test for Agi in one 3-hr laboratory period. The student can test for CI-and either Cu2+or Ph2+in a second 3-hr lahoratorv period. There appear to be fewer problems with the copper ion-seiective electrode. A maximum of 10 min was necessary to establish equilibrium for any of the systems tried. It is suggested that measurements be made in styrofoam cups. Serial dilutions can be made from 5 ml of the 0.1 M solutions and 45 mlO.l M KN03 (far monovalent ions) or 45 ml 0.1 M Ca(N0:h (for divalent cations) or 45 ml of deionized water (activitiescalculated). Refer to Figures 1 3 for typical student responses with n meter without an expanded scale. Half of the 0.1 M solution should be saved far a selectivity test. Add an equal amount of the ion to be used for the selectivity test. Only the Br- tested with the CI- electrode will give a dramatic change in potential as explained by eqn. (6). Preparation of the Precipitates Siluer chloride is prepared by slowly adding0.1 M AgNO3 toasolutian which has aslight excess of NaC1. Wash thoroughly, filter, and dry. Frequently, students prepare AgCl when they determine chloride gravimetrically. They may save the precipitate until later in the semester far this experiment. Photoreduction of silver ion does not interfere significantly. Siluer sulfide is prepared hy slowly adding 0.1 M AgNOa to a solution which has a slight excess of Na2S.9H20. Wash thoroughly with
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