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 β 2 (Fe III (mal) 2 )=15.21±0.25, log K 1 (Fe II (mal))=2.25±0.36, and log β 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 β (Fe(mal) 2 (OH) x )=15.28±0.10 and log β (Fe(mal) 2 (OH) y )=27.20±0.09.