Complexation Equilibria of Indium in Aqueous Chloride,Sulfate and Nitrate Solutions: An Electrochemical Investigation

Electrochemical methods have been used to determine the speciation and stability constants of various aqueous indium complexes. Qualitative behavior is observed using UV–Vis spectroscopy and cyclic voltammetry. Equilibrium constants are determined using differential pulse voltammetry. In a titration where the titrant and sample contain equal concentrations of acid and In³⁺ ions and equivalent concentrations of ligand and supporting electrolyte anions, respectively, small changes in ligand concentration can be made quickly and accurately while maintaining the overall ionic strength. From the change in the half-wave reduction potential as a function of ligand concentration, the coordination number and the stability constants of sulfate, chloride and nitrate complexes were determined. We also highlight the difficulties finding a supporting electrolyte that does not interact with the In³⁺ ion. On the one hand, it was not possible to prevent the slow formation of chloride in perchlorate electrolytes containing indium. On the other hand, we show that, at concentrations of nitrate anions commonly used in such experiments, nitrate complexes form. In the light of these new findings, previously published stability constants of indium using nitrate-based supporting electrolytes should be used cautiously.