Valence state of iron in the presence of ascorbic acid and ethylenediaminetetraacetic acid

The valence state of iron in solutions of pH 2.6-6.0 in the presence of ascorbic acid (AA) and ethylenediaminetetraacetic acid (EDTA) was investigated by reduction-oxidation titration. The results indicate that 2 mol of Fe(III) was reduced by 1 mol of AA in the pH range. The reduction rate of Fe(III) by AA decreased drastically as the pH value increased. Addition of EDTA prevented Fe(III) from being reduced by AA; that is, when a 1:1 Fe(III)-EDTA complex was formed, Fe(III) could not be reduced by AA, at pH 2.6-6.0. Ferric iron can be reduced by AA only when the pH is below a limit, somewhere between pH 6.0 and 6.8. Above that pH limit, AA is no longer an effective reducing agent for Fe(III). The results clarify some controversies reported in the literature and have important implications in the formulation of iron and AA additives in foods and pharmaceuticals.