The kinetics of the Cr3+/Cr 2+ electrode system have been investigated by several workers. In 0.5-1 M perchlorates or alkali halides the apparent standard rate constant was found to be I of the order 10 5 cm s 1. Some of these investigations were oriented to the investigation of a potential-dependence of the transfer coefficienO 2. Several papers discussed the oxidation of chromium(II) catalyzed by adsorbed halides. Especially works of Kemula and Rakowska 3, Aikens and Ross 4 and Anson with coworkers 5-7 should be mentioned here.
The purpose of this study is the investigation of the influence of the water activity in the solutions investigated on kinetic parameters of the Cr 3 +/Cr 2 + reaction. Such a study, enabling the determination of the order of electrode reaction in respect to water, may give insight into the electrode processes of aquo-complexes which is very important in better understanding of the nature of electrode processes. Studies recently carried out in our laboratory revealed a large change in the kinetics of Cu 2 +/Cu(Hg) system with water activity 8.
In other systems investigated the kinetic parameters were practically independent of the water activity 9, however, a large change in formal potentials was found 9-1°. From that dependence the hydratation numbers could be estimated. In the present paper in addition to perchlorate solutions, the Cr 3 +/Cr 2+ system was also studied in concentrated solutions of calcium chloride. The electrode reduction of chromium(II) to metallic chromium at a mercury electrode in concentrated perchlorates and chlorides was also studied and some results are described here.
EXPERIMENTAL Reagents
Chromium(III) perchlorate was prepared by dissolving metallic chromium p.a. in 70°,~ HC10 4. The product obtained was purified by crystallization. Calcium perchlorate solution was obtained by neutralizing CaCO 3 with 70~o HC104. Solutions of sodium perehlorate were prepared by dissolution of solid compound in triply distilled water. CaC12 used was purified by crystallization. NaC1 and HC104 were used without additional purification. All these compounds were p.a. All solutions were prepared with triply distilled water. The second distillation was carried out from alkaline solution of permanganate and the third from an all-quartz still.