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

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 [CrIII(H2O)4]3+, a planar square or distorted tetrahedral [CrIIICl(H2O)3]2+, a trigonal bipyramidal [CrIIICl3(H2O)2]0, and a distorted octahedral [CrIIICl4(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 [CrIIICl2(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.