Double potential step chronoamperometry for reversible follow-up chemical reactions. Application to the aquation kinetics of bis(ethylenediaminemonoacetato)cobalt(II).

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A theory of double potential step chronoamperometry (DPSCA) is developed for the reversible follow-up reactions represented as Ox+ne\rightleftarrowsRed, Red\oversetk\undersetkK\rightleftarrowsZ, where k and K are the rate and the equilibrium constants of the follow-up chemical reaction, respectively. DPSCA is applied to investigate the kinetics of aquation reaction of [Co(edma)2]0 (edma: ethylenediamine-N-acetate), which is produced by electrolytic reduction of [Co(edma)2]+ in the range of pH 1.5 to 10.5. At pH<4.5, the aquation reaction of [Co(edma)2]0 is irreversible. The rate of aquation reaction is accelerated by hydrogen ions and approaches an upper limiting value (2.1×103s−1). At pH>4.5,on the other hand, the hydrogen ions do not substantially contribute to the aquation and thus the rate approaches a lower limiting value (4.7×102s−1), which is in good agreement with that obtained in neutral region (4.5<pH<7), where the reverse (complex formation) reaction must be taken in to consideration. Above pH 7, the[Co(edma)2]0 species exists as main species in the solution and thus the kinetic behavior of the aquation is not observed. It is demonstrated that DPSCA is a useful method for kinetic studies of the dissociation-association reactions of labile complexes.
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