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

Abstract

The greater absorption of ironin 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 K_Fc(HL)., about 20 l.mol^-s-1 at Μ = 0 and 25‡C, results in the conclusion that Fe(HL)₂ is almost completely dissociated into Fe²⁺ and HL^/s- 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.