The kinetics of some reactions involving chromium (VI)

Abstract

The thesis is concerned with the elucidation of the kinetics and mechanisms for the two analytically important chromic acid oxidations, those of iodide and of ferrous iron.The introduction reviews past work in this field. Once a satisfactory procedure for following thereactions had been decided upon, the order of theinitial stages of each reaction was determined. Concerning the chromic acid oxidation of iodide, which was examined by titrimetry, the effect of altering the experimental conditions e.g. by changing the ionic strength, dieletric constant and solvent was studied.By measuring rates at various temperatures it became possible to find the activation energy for the reaction. Experiments were also carried out in the attempt to ascertain the intermediate oxidation stage of chromium involved in the reaction, both by studies of induced oxidation and by analysis of the products formed when hydrobenzoin was added to the reaction mixture.A similar investigation was made of the reaction of Cr(VI) with ferrous iron, using a spectrophotometic technique. The reactive Cr(VI) species is at least closely allied to H Cr O4. It was therefore important to haveinformation about the equilibriumunder the various experimental conditions. This equilibrium constant was measured spectrophotometrically, and the results were considered in relation to the determinations of other workers.In the last section, following a brief discussion of the equilibria existing in acid chromate solutions, the proposed rate expressions and the rate constants for the expressions are given. For the Cr(VI)/I reaction a fairly detailed mechanism is proposed and tested against the kinetic data, but the present work enables one to decide only certain limited aspects of the mechanism of the Cr(VI) oxidation of Fe(ll) aq.