Kinetics and mechanism of the oxidation of oxalic acid by tetrachloroaurate(III) ion

The oxidation of oxalic acid by tetrachloroaurate(III) ion in 0.005 ⩽ [HClO4] ⩽ 0.5 mol dm−3 is first order in [AuCl4-] and a fractional order in [oxalic acid], the reactive entities being AuCl3(OH)− and HC2O4- ions. The pseudo first-order rate, kobs, with respect to [Au(III)], is retarded by increasing [H+] and [Cl−]. The retardation by H+ ion is caused by the dissociation equilibrium (COOH)2⇌HC2O4-+H+. A mechanism in which a substitution complex, AuCl3C2O42- is formed from AuCl3(OH)− and HC2O4- ions prior to its rate limiting disproportionation into products is suggested. The rate limiting constant, k, has been evaluated and its activation parameters are reported. The equilibrium constant K1 for the formation of the substitution complex AuCl3C2O42- and its thermodynamic parameters are also reported.