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

Abstract

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 Fe^IIIDGL 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.