Abstract
Experiment shows that under the same conditions the rate of decomposition of permanganate proceeds more rapidly in KOD than in KOH. The concentration of D 2O was measured by means of the refractive index: the immersion refractometer. The inverse isotope effect [ α = ( k D/ k H) > 1] is largest in alkaline solutions of low concentration and decreases with rising hydroxide content. This supports the mechanism of reaction proposed in former communications: MnO − 4 + OH − ⇌ MnO 2- 4 + ·OH OH − + ·OH →·O − + H 2O The resultant isotope effect is a product of a kinetic and a thermodynamic isotope effect which counteract one another. The predominating thermodynamic effect is a result of the stability of the OD radicals which is higher than that of the OH radicals. The thermodynamic isotope effect of the preceding equilibrium and the kinetic isotope effect relating to the subsequent reaction were theoretically estimated making simplifying assumptions. A comparison with experiment was made. The non linear dependence of the isotope effect on the deuterium content of the water is due to the equilibria H 2 O + OD − = HDO + OH −, HDO + OD − = D 2 O + OH − the constants of which could be calculated from the deviation from linearity.
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