Reaction kinetics at linearly increased temperature. V. DTA and UV study of the oxidation of malonic, tartronic, glyoxylic and monobromomalonic acids by ceric sulphate in dilute sulphuric acid

Abstract

DTA and non-isothermal UV measurements of the oxidation of malonic, tartronic, glyoxylic and monobromomalonic acids by ceric sulphate in dilute aqueous sulphuric acid were evaluated using the novel one-step strategies for the determination of quantities typical of mechanism. Due to the kinetic evaluation, experiments with malonic acid reveal the expected bimolecular reaction only in definite concentration ranges. For very small ceric ion concentrations, the large resulting reaction heats suggest that ceric ion catalyzes the oxidation of malonic acid and its oxidation products, via the corresponding organic radicals and intermediate addition of molecular oxygen. If the concentration is raised, different scenarios in a rather complex mechanism are passed, which include situations of first-order, zeroth-order, autocatalytic and consecutive character with crossing-over effects. For sufficient concentration of the cerium salt, the kinetics are influenced by repetitive oxidation of malonic acid to tartronic and glyoxylic acid by the metal salt. This interpretation is in accordance both with results of experiments with tartronic acid and glyoxylic acid as well as with attempts to simulate the experimental signals approximately using a model of seven coupled reactions.