Raman spectroscopic investigations on the chromium(VI) equilibria part 2-species present, influence of ionic strength and CrO42−Cr2O72− equilibrium constant

Abstract In addition to CrO 4 2− and Cr 2 O 7 2− , species such as H 2 CrO 4 , HCrO 4 − and HCr 2 O 7 2− have been considered to exist in Cr(VI) equilibria in aqueous solution. The Raman spectra of dilute solutions of Cr(VI) at different pH have confirmed the non‐existence of the last three species in a Cr(VI) concentration range between 0.1 M and the lower detection limit of concentration ( ca 0.003 M). The proposed equilibrium, 2CrO 4 2− +2H + ⇌Cr 2 O 7 2− +H 2 O, seems to be the only one existing between pH 1 and 11. A quantitative Raman study of the influence of ionic strength on the corresponding apparent formation constant β′ 22 led us to the determination of the equilibrium constant at zero ionic strength, namely log β 22 =14.85±0.04 at 25°C. In more acidic aqueous Cr(VI) solutions, up to 12 M in HNO 3 , preliminary investigations corroborated the presence of the trichromate and tetrachromate species.