SURFACE COMPLEXATION DESCRIPTION OF THE DISSOLUTION OF CHROMIUM(III) HYDROUS OXIDES BY OXALIC-ACID

Aqueous oxalic acid forms surface Cr-III-oxalato complexes with suspen ded chromium(III) oxide particles; the FTIR spectra demonstrate that b oth carboxylate groups of the ligand are bound to surface Cr-III. Surf ace complexation is followed by changes in the surface redox potential toward more negative values and by the dissolution of the oxide. The dissolving steady state potential is in the range -60 to -210 mV again st SHE. During surface conditioning, traces of oxidants at the interfa ce are reduced, and some reduced metal ions accumulate. Minor amounts of dissolved Cr-II are generated and can be collected at a vicinal rin g electrode set at -60 mV. In agreement, dissolution kinetics suggest that generation of Cr-II by ligand-to-metal charge transfer within the surface complexes produces a large increase in the rate of phase tran sfer, as expected from the properties of Cr-III and Cr-II. Added chrom ous salts also catalyze the dissolution through intervalence charge tr ansfer within an oxalato-bridged Cr-III-L-Cr-II surface dimeric comple x. The rate of dissolution at 65 degrees C follows a Langmuir-Hinshelw ood dependence on oxalic acid concentration, a power law (order 0.31) dependence on proton concentration, and an a + b[Cr-II](0.64) dependen ce on [Cr(II)]. The Langmuir-Hinshelwood parameters are interpreted in terms of the stability constant of the surface Cr-III-oxalato complex .