Speciation of phytate ion in aqueous solution. Characterisation of Ca-phytate sparingly soluble species

The formation of sparingly soluble Ca(2+)-phytate (Phy) species was studied using several techniques (potentiometry, calorimetry, thermoanalysis, ICP, SEM). Species with empirical formula Ca(x)H((12-2x))Phy, (6 less than or equal to x less than or equal to 4.5) were formed in experiments performed at pH 5.0, 7.5 and 10.5; at pH = 10.5 and in excess of Na(+) the species Ca(5)Na(2)Phy was also found. Solubilities (s 1.0 x 10(-9) and 1.1 x 10(-10) mol L(-1) for the species Ca(6)Phy and Ca(5)Na(2)Phy respectively, at l = 0 mol L(-1)), thermogravimetric profiles (dehydratation, monohydrogen-phosphate and final pyrophosphate formation), and formation enthalpies (DeltaH = 7.6 +/- 1 and 3.7 +/- 0.8 kJ mol(-1) for the equilibria 6Ca(2+) + H(3)L(9-) = Ca(6)L(0) + 3H(+) and 5Ca(2+) + 2Na(+) + H(3)L(9-) = Ca(5)Na(2)L(0) + 3H+, respectively), were obtained.