Ca2+–Fe3+–D-gluconate-complexes in alkaline solution. Complex stabilities and electrochemical properties

The complexation of Fe3+ and Ca2+ ions by D-gluconate (DGL) in alkaline solutions was studied by potentiometric titration. Stability constants for the complexes were determined by mathematical treatment of the data with the computer program BEST. Depending on the concentration of ligand, complexes of Fe3+ ∶ DGL stoichiometry 1 ∶ 1, 1 ∶ 2 and 2 ∶ 2 were identified. In the presence of Ca2+ ions mixed binuclear complexes are formed with the Ca2+ ∶ Fe3+ ∶ DGL stoichiometries 1 ∶ 1 ∶ 1 and 1 ∶ 1 ∶ 2. The complex system was studied with cyclic voltammetry and with constant current electrolysis experiments. The uptake of Ca2+ into the FeIIIDGL complexes results in a distinct change of the electrochemical properties of the iron(II/III)-redox couple. A shift of the formal redox potential towards more negative values is observed. The Ca2+ ∶ Fe3+ ∶ DGL complexes can be regarded as models for an iron(II/III)-redox system whose electrochemical behaviour is determined by the concentration of an electrochemically inactive ion.