Electrochemical studies with radioactive indicators; electrolyses in extremely diluted solution

Abstract

The ordinary methods of determining electrochemical potentials cease to be applicable in extreme dilution. The energy conditions of the formation of very thin electrolytic deposits (less than a monoatomic layer) can then be explored by means of radioactive indicators, following particular polarisation curves. These curves give the rate of deposition or the equilibria of distribution as a function of the potential applied to an inert electrode. A discussion is given of the relationship between the critical potential defined by such curves and the theoretical potential computed from the Nernst equation for the given concentration. Experimental results are collected for the critical potentials of the deposition of Po, Bi, Pb, Ag, etc., from extremely dilute solutions on to various metals. It is shown that the heterogeneity of the electrode surface plays a considerable rôle in these electrolyses. It explains the overvoltage and the undervoltage observed in certain cases, and the exponential form of the polarisation curves. In particular, the repartition of bismuth between a nitric acid solution of the ions of this metal and a silver cathode is represented by two Freundlich isotherms: — one of these with an exponent 1/n<1 corresponds to very small coverages and is explained by the heterogeneity of the surface; the other one with 1/n>1 is valid for coverages greater than 1% and is due to the intervention of attraction forces between the deposited atoms. Finally, it is concluded, from electrolytic experiments on Bi+++ 3.10−16 N solutions, that even at this extreme dilution (6.104 ions/cm3) the thermodynamic activity of these ions in solution is quite normal.