The kinetics of some redox reactions involving Cr (VI)

The investigations described in this thesis involve the study of two redox reactions of Cr(VI). The reductants being As (III) and SCN respectively.The results are presented in two sections, each of which commences with a review of previous work conducted in that particular field. Two procedures found to be satisfactory for following the reactions of Cr(VI), were a spectrophotometric method involving the observation of the change of absorbance due to HCrO4 at 28,300 cm, and a radiochemical method using Cr as a tracer. The order of the reaction with respect to the various reactants was determined, and from the effects on the reaction rates caused by variations in ionic strength, specific ions present, and solvent medium, an insight into the mechanisms of the reactions was obtained. Activation energies were calculated from observations of the effect of change of temperature on the reaction rate constants. A rate equation and mechanism are postulated for the reaction of Cr(VI) with As(III) in the pH range 1.3 to 4.0. The rate equation is shown to be in better agreement with experimental data than others previously proposed.For the reaction of Cr(VI) with SCN a two term rate equation is postulated, which fits the experimental results. The intermediates of this reaction have proved difficult to identify, so two possible reaction paths have been suggested involving different intermediates.