Dissolution of chromium hydroxides monitored by turbidimetry

A turbidimetric method was used to study the dissolution kinetics of different chromium (hydr)oxides in aqueous HClO4. There was a very good agreement between the results obtained with this method and those obtained with a standard one (quantifying the amount of chromium released to solution at different reaction times, after the separation of the solid and liquid phases by centrifugation or filtration), revealing that turbidimetry can be used to monitor the dissolution behavior of the studied materials. Theoretical support for the use of turbidimetry in dissolution studies can be obtained from light scattering theory. Chromium (hydr)oxide particles were composed of a mixture of monomeric and low oligomeric Cr(III) species that dissolves almost instantaneously in HClO4 (rapidly dissolving material) and a more polymerized material that undergoes dissolution at a lower and measurable rate (slowly dissolving material). The dissolution product was also a mixture of monomers and more polymerized Cr(III) species. Apparent activation energies (40-70 kJ/mol) for the dissolution of the slowly dissolving material were high enough to discard diffusion in aqueous solution as the rate-determining step and considerably lower than those corresponding to ligand exchange in Cr(III) species. This suggests that diffusion into the solid could be controlling the dissolution rate, although more direct evidence is needed.