Dielectric constant dependence of formation constants: complexation of dioxovanadium(V) with cysteine

Abstract

The complexation of dioxovanadium(V) with L-cysteine was investigated in mixed solvent systems of methanol + water and dioxane + water at 25°C and 0.1 mol dm−3 ionic media, using a combination of potentiometric and spectrophotometric techniques. Sodium perchlorate solution was used to maintain the ionic strength. The compositions of the formed complexes were determined, and it was shown that dioxovanadium(V), VO, forms two mononuclear species with the ligand. The stability constants of the complexes formed are given in the above mixed solvent media and interpreted. Linear relationships are observed when plotted log β versus 1/D, where β and D represent formation and dielectric constants, respectively. The results are discussed in terms of the effect on mole fraction of the solvent both for protonation and complexation of cysteine. An attempt has been made to ascertain the nature of the correction factor in determining the pH in the mixed solvent solutions and the effect of increasing organic solvent on log β is also considered.