Oxidation of oxalato complexes of Chromium (III) by Cerium (IV) in aqueous sulfuric acid

Abstract

In aqueous sulfuric acid media cerium(IV) reacts smoothly with each of the oxalate complex ions, trisoxalatochromate(III), cis-bisoxalatodiaquochromate(III), trans-bisoxalatodiaquochromate(III) and mono-oxalatotetraaquochromium(III). The observations are consistent with the view that the reactions proceed, at least initially, according to the stoichiometries: Cr(C2O4) --2Ce(IV)--> cis-Cr(H2O)2(C2O4)2 + 2CO2 (1) cis-Cr(H2O)2(C2O4)2 -- 2Ce(IV)--> Cr(H2O)4C2O4 + 2CO2 (2) trans-Cr(H2O)2(C2O4)2 --2Ce(IV)--> Cr(H2O)4C2O4 + 2CO2 (3) Cr(H2O)4C2O4 --2Ce(IV)--> Cr(H2O)6-2n(SO4)n 3 - 2n + 2CO2 (4) The various stoichiometries have been established by independent analyses and/or by examination of kinetic results. Kinetic measurements were obtained by spectrophotometry, and in some cases also by titrimetry. Analysis of products from reaction (4) revealed also the presence of chromium(VI), thus pointing to the importance of the further reaction: Cr(H2O)6-2n(SO4)n 3 - 2n --3Ce(IV)--> 3Ce(III) + Cr(VI) (5) In treatment of the kinetic data for the Ce(IV)trans-Cr(H2O)2(C2O4)2 reaction the isomerization of trans-Cr(H2O)2(C2O4)2 to the cis form has been taken into consideration. Also, kinetic analysis of reaction (1) requires consideration of the competing aquation of Cr(C2O4)3 -3, the Ce(IV) consumption by the resulting free oxalate, and reaction (2), which, like the aquation, proceeds at a rate comparable to reaction (1). The Ce(IV)---cis-Cr(H2O)2(C2O4)2 and Ce(IV)---Cr(C2O4)3 reactions exhibit second order behavior in 1.83M H2SO4; while deviations from linearity in the second-order rate plots, caused by Ce(III) inhibition, are observed for the Ce(IV)--trans-Cr(H2O)2(C2O4)2 reaction in 0.95M H2SO4, the Ce(IV)cis-Cr(H20)2(C204)2 reaction in 0.95M - 0.43M H2SO4 and the Ce(IV)---Cr(H2O)4C2O4 reaction in 1.83M - 0.43M H2SO4. The experimental results for the more general set of conditions were found to be consistent with a rate law of the form: -d(Ce(IV)) / dt = k'(oxalato complex)(Ce(IV))2 / k"(Ce(III)) + k"'(Ce(IV)) (6) where the values of the constants are functions of the reaction media. For conditions where second order behavior is observed, various plausible mechanisms are considered, and it is concluded that the mechanism below is more likely than the other possibilities. [TRUNCATED]