Heterogeneous catalysis in solution. Part 27.—Reaction between titanium(III) and triiodide ions catalysed by platinum

Abstract

The rate of the reaction between titanium(III) chloride and triiodide ions in an acid chloride medium has been studied in the absence and presence of a rotating platinum disk. At low concentrations of triiodide the catalytic rate at platinum was found to be first order in I₃^–, zero order with respect to Ti^{I I I} and H⁺, and proportional to the square root of the rotation speed of the disk. These results pointed to diffusion-controlled catalysis, as did the low activation energy of 19 kJ mol^–1 as compared with 40 kJ mol^–1 for the uncatalysed reaction.

All these findings are consistent with an electrochemical interpretation of the catalytic mechanism, as was shown by the current–potential curves determined for the two reactants at the same reduced platinum surface. Provided that the voltammograms for Ti^{I I I} had been carried out in the presence of the same concentration of Kl as in the reaction mixtures, the rates and mixture potentials determined electrochemically agreed well with the catalytic rates and potentials measured experimentally. The fact that quite different results were obtained when no Kl was present in the TiCl₃ solution provides further confirmation for the modified form of the additivity principle.