Electrochemical behaviour of iron-complexes in presence of competitive ligands: A strategy for optimization of current density

Triethanolamine and D-gluconate both form alkali stable complexes with iron(III)- and iron(II)-ions. The redox behaviour of solutions containing one type of ligand is compared with solutions containing both ligands in excess compared with the iron(III)-concentration. Spectrophotometric investigations and experiments with cyclic voltammetry yield information about follow-up reactions proceeding during the cathodic reduction of the iron(III)-triethanolamine complex when D-gluconate is present in addition. The results indicate that the Fe(III)-triethanolamine complex is the predominant species as long as c(triethanolamine) exceeds c(Fe(III)) and thus the cathodic current density is mainly determined by the concentration of this complex. When the concentration of triethanolamine is lowered to 20% of the iron concentration then iron(III)–D-gluconate is the major species present in the solution. Then, besides ligand exchange reactions, an increase in cathodic current density indicates a redox reaction between Fe(II)-triethanolamine and Fe(III)–D-gluconate similar to the E_cat-type in cyclic voltammetry. Such conditions are of particular interest for the optimization of mediator systems with regard to the current density attainable at given iron(III)-concentration. The results show that a combination of different ligands can lead to an addition of the specific advantages of each metal-complex mediator.