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 degrees 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 (sic) 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 Cr(2)O(7)(2)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) speciatio n. (C) 2017 Elsevier B.V. All rights reserved.