Abstract

kinetic study of the co-ordination to trivalent chromium ions of a simple monovalent anion, the acetate radical (of acid strength similar to the acid residues of collagen), has been undertaken following reports in the literature that this is a first order reaction1.At moderate mole ratios a different rate law was found to operate, reaction rates being dependent upon the concentrations of both reactant species and experi mental rate data for the initial reaction conforming to a second order mechanism. Examination of the initial gradients of the second order reaction plots enabled a general rate constant to be determined, corre sponding to the initial reaction step. Observed deviation of the plots from initial linearity reflecting a change in kinetic characteristics, is attributed to the effects of olation over the latter reaction period. Band patterns obtained from electrophoretic studies on the equilibrium re action solutions are in general agreement with the nature and number of chromium species indicated by solvent extraction. Colour changes accompanying acetate co-ordination in a similar reaction series, were subjected to spectrophotometric examination. Penetration of acetate ions into the co-ordination sphere was found to be accompanied by a decrease in height of the 420 mi absorption peak and an increase in height of the 570 mg absorption peak. These modifications appear to be due to the formation of acetato-chromium complexes having lower molar extinction coefficients at the 420 mg peak and higher molar extinction coefficients at the 570 m/L peak, during the course of re action. Variation in the initial solution absorption values can be shown to be due to the protolysis equilibrium controlling the relative propor tions of hexaquochromium and hydroxopentaquochromium ions.