Heterogeneous catalysis in solution. Part 26.—Electrochemical and kinetic studies of the reaction between N,N-dimethyl-p-phenylenediamine and chloropentaamminocobalt(III) at platinum, gold and silver surfaces

Abstract

The rate of the heterogeneously catalysed reaction between N,N-dimethyl-p-phenylenediamine (DMPPD) and Co(NH₃)₅Cl²⁺ in phosphate buffer solutions at 25 °C has been studied at large rotating discs of platinum, gold and silver. Only at the silver surface did these rates agree with the values calculated from the mixture currents obtained from combining the anodic current–potential curve of DMPPD and the cathodic curve of Co(NH₃)₅Cl²⁺. At the platinum and gold surfaces it was found that addition of small amounts of DMPPD to the solution of Co(NH₃)₅Cl²⁺ lowered its rate of reduction and shifted the current–potential curve to lower potentials. Combination of these DMPPD-modified curves then produced mixture currents which were in good accord with the catalytic rates. These results confirm the need to reformulate the Wagner and Traud additivity principle, and show that addition of current–potential curves is valid only if these curves have been determined in circumstances which correspond to those of the mixture.