Possible Discovery of Ion-Pairs by Interferometry

Abstract

IN the course of an investigation on electro-deposition to determine the cause of what Ibl and Muller¹ called an “anomalous maximum” near the cathode, and which O'Brien and Axon² independently noted and named the “second wave” in a complete, electrode-to-electrode interference pattern, it was found that this second wave does not develop in electrolyte solutions in which ion-pairs do not form. Such a solution is nickel sulphate³, in which it has been shown that, presumably because of the strength of the bonds and lack of lability in the octahedral nickel aquo-complex, the closest approach of the sulphate ion cannot be less than the diameter of one water molecule (see Figs. 1 and 2). Electrolysis of a silver perchlorate solution gave the faintest traces of a second wave, which may be considered to be additional supporting evidence. Since Fajans and Luhdemann⁴ have shown that free ions in solution may be expected to give higher refractive indices than ion pairs, and indeed a plot of concentrations versus refractive index of values taken from Landolt-Bornstein shows no deviation from linearity for nickel sulphate (which deviation is usually attributed to ion-pairs) until concentrations in the neighbourhood of 1 M, it follows that the cathode maximum may be explained by the dissociation of ion-pairs.