Abstract
The redox reaction between cerium(IV) ions and malic acid (HL) proceeds via an inner-sphere mechanism and has been investigated at an ionic strength of 1·50M over the temperature range 10·6–25 °C using the stopped-flow method. Two intermediate complexes have been identified [equation (i)], the reactive species [CeL]3+[Ce(HL)]4+ [graphic omitted] H++[CeL]3+ [graphic omitted] CeIII+ L˙(i) being somewhat more stable; K3= 11 ± 4 mol l–1 at 25 °C. Formation constants for the complexes have been obtained both from initial optical-density changes and kinetic data. Activation parameters for the intramolecular electron transfer involving the complex [CeL]3+ have been derived; kβ= 0·52 ± 0·06 s–1 at 25 °C with ΔH‡= 20·6 ± 1·5 kcal mol–1 and ΔS‡= 9·5 ± 5 cal K–1 mol–1. The first hydrolysis constant for cerium(IV)[equation (ii)] has been redetermined; Kh= 0·2 ± 0·02 mol l–1 at 25 °C with ΔH= 5·2 ± 0·;7 kcal mol–1 and ΔS= Ce4+(aq) [graphic omitted] [Ce(OH)]3+(aq)+ H+(ii) 14·3 ± 4 cal K–1 mol–1, and compared with other values. Possible paths for the rapid decomposition of the radical formed in the redox step are discussed.
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