Enthalpy of oxidation-reduction for the ethylenediaminetetraacetatocobaltate(III/II) couple relative to the hydrogen ion formation.

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Oxidation of ethylenediaminetetraacetatocobaltate(II) with permanganate ion has been studied spectrophotometrically. The first step in the oxidation is the rapid formation of [CoIII(edta)(H2O)]−, followed by the ring closure reaction of the complex to [CoIII(edta)]−. On the basis of the spectrophotometric result, the enthalpies of the reactions, [CoII(edta)]2−+1/5 MnO−4+8/5 H+→[CoIII(edta)(H2O)]−+1/5 Mn2++4/5 H2O and [CoIII(edta)(H2O)]−→[CoIII(edta)]−+H2O, have been determined at 25 °C and at an ionic strength of 0.1 by a direct thermometric titration procedure. The enthalpy of the reaction, [CoIII(edta)]−+1/2 H2\rightleftharpoons[CoII(edta)]2−+H+, has been calculated by combining them with some other calorimetric data. The thermodynamic functions for formation of the [CoIII(edta)]− complex are estimated from the values so far reported. At 25 °C and at an ionic strength of 0.1 ΔG=−242 kJ mol−1, ΔH=−101 kJ mol−1, and ΔS=474 J mol−1 K−1.
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