Iron(III)-Complexes Engaged in the Biochemical Processes in Seawater. II. Voltammetry of Fe(III)-Malate Complexes in Model Aqueous Solution

Characteristics of iron(III) complexes with malic acid in 0.55 mol L(-1) NaCl were investigated by voltammetric techniques. Three iron(III)-malate redox processes were detected in the pH range from 4.5 to 11: first one at -0.11 V, second at -0.35 V and third at -0.60 V. First process was reversible, so stability constants of iron(III) and iron(II) complexes were calculated: log K(1)(Fe(III)(mal)) = 12.66 +/- 0.33, log beta(2)(Fe(III)(mal)(2)) = 15.21 +/- 0.25, log K(1)(Fe(II)(mal)) = 2.25 +/- 0.36, and log beta(2)(Fe(II)(mal)(2)) = 3.18 +/- 0.32. In the case of second and third reduction process, conditional cumulative stability constants of the involved complexes were determined using the competition method: log beta(Fe(mal)(2)(OH)(x)) = 15.28 +/- 0.10 and log beta(Fe(mal)(2)(OH)(y)) = 27.20 +/- 0.09.