Complex stability of ferrous ascorbate in aqueous solution and its significance for iron absorption

The greater absorption of iron in vivo from ferrous ascorbate [Fe(HL)2] as compared with ferrous sulfate has been ascribed both to retardation or prevention of Fe(II) oxidation by ascorbate and to the existence of Fe(II) as a chelate with ascorbate. The available literature and our own results demonstrate that Fe(HL)2 dissociates in aqueous solution into a monomeric cationic species Fe(HL)1+, Fe2+ and HL-. The HL anion acts as a monodentate. The low stability constant KFe(HL)1, about 20 l.mol-1 at mu = 0 and 25 degrees C, results in the conclusion that Fe(HL)2 is almost completely dissociated into Fe2+ and HL- at about pH = 5, so (chelate) complex formation does not contribute significantly to the increased iron absorption. Between pH = 6 and pH = 8 a solubility enhancing effect of ascorbate is observed which may be of relevance for the iron absorption from ferrous ascorbate.