Construction and Evaluation of an Inexpensive Reference Electrode with Internal Electrolyte in Agar Matrix Leandm Victoria, M. Gloria Ortega, and Jose A. Ibaiiez Universidad de Murcia, 30071 Murcia, Spain The use of reference and ion selective electrodes is of great importance in many branches of chemistry (1, 2), and experiments using these electrodes are found in most undergraduate programs. Thus the introduction of studenta to the construction of these electrodes is a useful exercise in developing and understanding of electrode design (3-7). In this paper, we show how to construct a reference electrode of Ag/ AgCl with an internal electrolyte in agar matrix. This electrode is less expensive and, in many cases, more stable than the commercially available models. The Electrode The electrode has the configuration Agar gel-AgCI (saturated),KC1 (3M)lAgIl and is formed of the following parts: an external tuhe, an internal tuhe concentric with the former, an active tip, an upper stopper, and an inner reference electrolyte supported in a agar gel matrix (Fig. 1). The external glass tuhe has a hole (1-mm diameter, approximately) in its lower part, through which the liquid junction between the internal electrolyte and the solution being studied is accomplished. This junction has a very low electric resistance and, consequently, a very high capacity to transport the electric current. This tube contains the gel, and concentric with it there is another glass tuhe with the active tip. This tip is ashort length of platinum wire (10mm) fused t o the glass tube hy means of an oxygen-methane torch or by epoxy adhesion. This tip is coated with silver hy electrolysis. Platinum wire of 0.4-mm diameter was used. Inside this central tuhe there is a small quantity of mercury for contact between the active tip and a constantan wire that is welded to a copper wire for the lead. Both tubes are supported by a nylon housing; this housing is joined to a BNC connector. The three parts of this piece are shown in Figure 1: part A is internally threaded to house the outer tube of the electrode; two ruhher 0 rings in A facilitate the fitting of the central tuhe. Part B is a washer lodged in part A, acting as a butt to prevent the upward movement of the central tube (this part is stuck to A). Part C is a stopper to couple part A with the BNC connector. Preparation of Active Tlp The platinum wire of the active tip is coated with Ag by a procedure that is a modification of Brown's method (8).The silver plating solution was prepared using Basset and Corbet's method (9) hut AgN03 was used instead of AgCN, with a secondary reaction with KCN in excess. This is possible because of the following complexation process: AgNO, + KCN ~t AgCN + KNO, AgCN + KCN e K[Ag(CN)], AgNO, + 2KCN e K[Ag(CN),] + KNO,
Figure 1. Dlagrarn of a completed reference electrode.
Preparation of Internal Electrolyte When a polymer is swollen with a liquid, a gel is formed through which the liquid component may move by a comhination of diffusion and convection. Substances dissolved i n t h e liquid may also be transported through the gel. In our.electrode, the internal matrix is a gel prepared with an aqueous solution of potassium chloride at concentration 3 M, previously saturated with AgCl, and with agar as solute a t 1%. The prepared solution is stirred and heated to the hoiling point, this state being maintained until the turbid appearance of the solution clears. Afterwards, the solution is slowly cooled to a point near gel formation; a t this point i t is poured into the external tuhe of the electrode and gels there; the process is accelerated by immersion in a vessel containing the remaining solution of KC1 and AgCl without agar a t ambient temperature. I t is necessary for the lower hole of the electrode tuhe to remain filled with gel, the formation of a gel drop outside the tuhe being convenient to prevent the appearance of air bubbles in the liquid junction.
Electrode Evaluation The experimental evaluation of our electrode was accomplished comparing its hehavior using an indicator (ion selective) electrode (ISE) for CI- (Orion, model 94-17), with the hehavior of a commercial reference electrode (Orion, model 90-02) also using an ISE for C1-, measuring the electric response of both electrodes by means an ion analyzer (Orion RA
mni
The assembly for measuring this response is Reference electrodelASE C1-
(1)
In this cell, the emf, E , is given hy
The reagent, K[Ag(CN)2],must have a concentration of 1%,and, using this, the platinum wire is coated with silver by electrolysis, which takes place over 6 h with a direct current of 0.5 mA. The tip, when coated, acts as cathode.
where Eind, E,F, and Ej are the potential of the indicator electrode, of the reference electrode, and of the liquid junction, respectively. If E,,fis constant and Ej is either constant Volume 67 Number 2
February 1990
179
gel-AgCI, KCI/Ag AgC1, KCVAg
+
E = -53.438 log C 100.358 E = -54.918 log C - 14.927
(4) (5)
with correlation coefficients equal to 0.9974 in both cases. The expected linearity in the Nernst equation is found and the slope values are near the theoretical one. We have worked using conditions of constant temperature, since reproducibility of measurements is limited by this factor. It has been observed that within the ooeratina range of our experiments (10-3-10-1 M), and carrying out several calibrations, the measurements obtained are within an uncertainty o f f 2%. When the electrode system (reference-ISE) is removed from one solution and nlaced in another of different concentration, the response t h e (the time required to reach 990i of the stable ootential reading) is not affected. This time is of the order G a fewseconds, always less than 30 s, and depends on the magnitude and direction of concentration chance; its value is smaller when the change is from dilute soluti& to concentrated solutions. The advantages of the described electrode are: (1) It is much cheaper. (The total cost is $20; the price of a similar
reference electrode is over $150.) Figure 2. Emf obtained wilh a chloride ion selective electrode vs. our reference electrode (a) and a commercial reference electrode (0) for five standard solutions. Experimenml points and fined straight lines.
or negligible, and using the Nernst equation to express Ejnd eq 2 becomes
RT F
E = E:' --In
Ci = E!'
- 2.3-RTF log Ci
(3)
where Et' = Ei- E,er - (RTIF)In (y;), with yian appropriate activity coefficient, and Ci the molar concentration of ion C1-, such as Ci = C, C being the salt concentration. Equation 3 establishes alinear dependence between E and log C with a slope that is a function of the temperature, having a theoretical value of 59.1 mV at 25 ' C . Figure 2 shows the experimental points (log C, E ) obtained for our electrode ( 0 )and the commercial reference electrode (0). All experiments were conducted in a thermostated vessel at 25 OC. The fitting of two straight lines to these points leads to the following equations:
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Journal of Chemical Education
(2) The liquid junction is favored by the absence of a separating
phase between the inner electrolyte and the problem solution. (3) The possible contamination in the analyte solution because of
the inner electrolyte is minimal, due to the interaction between this electrolyte and the agar matrix. (4) The electrode does not have to he refilled, which means maintenance is easy. Acknowledgment
The authors acknowledge financial support from the C.I.C.Y.T. of Spanish Government under grant No. PB850240-CO2-02. Literature Clted 1. Skom.D. A : West.D. M.P~inci~isso/Instrum~nfolAnolysis:Saundcrr: Philadelphia. 19&: Chapter3 18and 19. 2. Bailey, P. L. Anaiyiris uith Ion-Sd~ctimEieefrodes: Hayden: London, 1976. 3. Lamb.R.E.:Natush,D. E.S.:O'Roiliy.J.E.; Watkins,N. J C h r m E d u r . 1973.50.42-
~
~~~~. .
8. B ~ ~ ~ ' A . JCham. . A ~Snc. : 1934,56.64664;. 9. Ban3et,A.;Corbel,J. J. Chem. Soc. 19Z9,125, ,660-1663.
