Equilibria and kinetics of chromium(VI) speciation in aqueous solution – A comprehensive study from pH 2 to 11

A coherent and comprehensive speciation study was conducted in the aqueous Cr(VI) system in order to clarify several discrepancies in the literature. Using pH potentiometric UV–vis titrations and stopped-flow kinetic measurements, it was shown that the dominant Cr(VI) species are Cr2O72−, HCrO4− and CrO42− in an aqueous solution of Cr(VI) under ambient conditions ([Cr(VI)] = 7.7 mM–155 mM, pH = 2–11, I = 1.00 M NaClO4, T = 25.0 °C). This paper provides a comprehensive interpretation of the kinetic features of the dimerization equilibria of Cr(VI) in the acidic – alkaline pH range from pH 2 to 11. Under acidic conditions, the conversion of HCrO4− and Cr2O72− is catalyzed by H+. This observation is consistent with the formation of a protonated intermediate, which is presumably HCr2O7−. The forward and backward rate constants for the 2 HCrO4− ⇌ Cr2O72− + H2O reaction were determined at various pH values in concentration jump experiments. In the absence of catalysts, the slowest dimerization reaction was observed at pH 7 with a lifetime of a few minutes. At basic pH, OH− reacts with Cr2O72− to give CrO42− on stopped-flow timescale. The basic hydrolysis of Cr2O72− is irreversible, and the reaction is first order with respect to both Cr2O72− and OH−. The results presented here are consistent with some of the previously published experimental observations, and resolve the inconsistencies in the literature of Cr(VI) speciation.