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 View the MathML source[AuCl4-] and a fractional order in [oxalic acid], the reactive entities being AuCl3(OH)− and View the MathML sourceHC2O4- ions. The pseudo first-order rate, k obs, with respect to [Au(III)], is retarded by increasing [H+] and [Cl−]. The retardation by H+ ion is caused by the dissociation equilibrium View the MathML source(COOH)2⇌HC2O4-+H+. A mechanism in which a substitution complex, View the MathML sourceAuCl3C2O42- is formed from AuCl3(OH)− and View the MathML sourceHC2O4- 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 K 1 for the formation of the substitution complex View the MathML sourceAuCl3C2O42- and its thermodynamic parameters are also reported.