d Electroanal Chem, 72 (1976) 107--109 107 © Elsevier Sequoia S.A , Lausanne -- Printed in The Netherlands

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CALOMEL REFERENCE ELECTRODE IN PROPYLENE CARBONATE

NGUYEN HUU CUONG, A. MAIORNIKOFF and H.D HURWITZ

Facultg des Sciences, Universitd Libre de Bruxelles, 50, av F D Roosevelt, 1050 Bruxelles (Belgmm)

(Received 8th October 1975, in revised form 1st December 1975)

It follows from the study of Frmd and Barak [ 1] that from a practical point of view the saturated te t raethylammonium chloride (TEAC) calomel electrode seems to be most convenient and suitable for electrode vol tammetry in propylene carbonate (PC).

The present note reports some results concerning the temperature coeffi- cients of this kind of electrode.

Exper imen ta l

PC (Schuchardt) was distilled twice at 94°C under 4 Torr pressure atmo- sphere of nitrogen previously dried by passage through P20 5 (Granusic A. Baker Chemical).

Before each distillation, dry nitrogen has been bubbled through the sample for 10 h under 4 Tort at 60°C to remove volatile impurities.

Distilled PC contained less than 10 ppm H20 as measured by the K. Fischer method (KF4 Aquameter-Beckman).

TEAC (Aldrich) was dried for 3 days under vacuum at 60°C over PeOs. Hg2C12 (Merck) was mixed with Hg and solution in a mortar. All those manipulations (including collection of distilled solvent) were

carried out in a dry box under air atmosphere at 1 ppm H20 (dispensed by a Puregas Air Dryer). [

Chemically purified mercury was distilled twice under vacuum before use. The thermocell arrangement for EMF measurements is depicted in Fig. 1.

Preparation o f the e lec trode

The electrode section was ca. 4.5 cm 2. Reference calomel electrodes

Hg/Hg2C12/(CeHs)4NC1

saturated and 0.5 molal were prepared and kept stored in the dry box. After 1 day following their preparation, the measured potential difference

between any pair of saturated electrodes was less than 0.5 mV and decreased

108

Fig. 1 Arrangement of the thermoeell. (A) Jacket, (B) hall' cells, (C1, C2) thermometers, (D) calomel, (E) Hg, (F) Pt wire, (G) sintered glass, (H) TEAC solution meniscus.

to a mean deviation of -+ 0.3 mV after 3 days. No significant difference of po- tential was found between electrodes prepared at one month interval as far as they are all, at least, 3 days old.

These measurements indicate values of bias potential at short term and long term (for a period of at least five months) of our saturated calomel electrodes in TEAC which are much lower and more stable than the values reported by Murray and Aikens [2] for bias potential stability in the case of the Hg/Hg2C12 (s)/LiC1 {saturated) electrode system. Thus the assumption made by these authors that reversibility of the electrode is increased substantially by traces of water is contradicted in the present work by our results since water content is less than 10--20 ppm, whmh is the amount obtained in their study.

Stabilization of the 0.5 molal calomel electrode occurs faster than of the saturated one and reproductibility of measurements is better. It was found that the 0.5 molal electrode was stable after one day following the preparation and that the potential difference between any electrode pair was less than 0.2 mV. Accordingly, only 0.5 molal electrodes have been used for thermo- electric measurements.

Measurement of the micropolarization plot indicates no detectable polariza- tion resistance. (The difference between this slope of the micropolarization plot differs from the electrolyte resistance by less than 3%).

Measurement of temperature coefficients of electrode potential

The EMF of a 0.5 molal calomel electrode thermocell was obtained by keeping one electrode at 25.00 -+ 0.05°C and varying cyclically the temperature of the other one from 45 ° to 5°C by 5°C steps.

0 . 5 5

d~/mV'C -1

0 5 0

109

I I | , A | ; , i 0 t,5 5 15 25 5 45

T /°C Fig. 2. Variation of the temperature coefficients of the TEAC 0 5 mola] calomel electrode in PC vs. temperature.

The thermosta t was maintained at a constant temperature during an extended time for each measurement. The temperature was measured directly by im- mersing a precision the rmomete r in the solution of the half cell. The precision was -+ 0.05°C. The measmements include the difference of thermal diffusion potentials in TEAC solution between X and X1 of the thermocell arrangement and the thermoelectr ic tension of the Pt--Hg thermocouples. This thermo- electric tension was found to be less than 1 pV/degree.

Experimental data have been fi t ted to the equation

V - - V25 = 0.5303(t -- 25) + 0 .0012(t -- 25) 2 (1}

by a standard least square method. ( V is in mV and t in deg. C). The stationary thermal temperature coefficients of the electrode were plot ted

v.s. temperature in Fig. 2. The straight line is computed from eqn. (1). The scatter of the values corresponding to the standard deviations of mean observa- tion of data is indicated by vertical lines.

REFERENCES

1 I. Fried and H Barak, J. Electroanal. Chem., 27 (1970) 167. 2 R.C. Murray, Jr. and D A Aikens, Electrochim. Acta, 20 (1975) 259