Speciation of chromium aqua and chloro complexes in hydrochloric acid solutions at 298 K

Abstract

The distribution of metal aqua and chloro complexes is fundamental information for analysis of a chemical reaction involving these complexes. The present study investigated the speciation and structures of chromium aqua and chloro complexes using the thermodynamic model fitting analysis of UV-vis/X-ray absorption spectra. The existence of a negatively charged species was examined by adsorbability of chromium species on a strong base anion exchanger. In the results, a planar square [Cr^III(H₂O)₄]³⁺, a planar square or distorted tetrahedral [Cr^IIICl(H₂O)₃]²⁺, a trigonal bipyramidal [Cr^IIICl₃(H₂O)₂]⁰, and a distorted octahedral [Cr^IIICl₄(H₂O)₂]⁻ were confirmed and the thermodynamic parameters of complexation reactions were quantitatively determined. Investigation of the evolution of speciation of chromium aqua and chloro complexes in a pH 1 solution suggested the existence of [Cr^IIICl₂(H₂O)_m]⁺ (m = 2 or 3) during the hydration process, which diminished in the equilibrium state. The kinetic analysis deserves further investigation to understand the speciation process quantitatively.