Solubility and acid–base properties of concentrated phytate in self-medium and in NaClaq at T = 298.15 K

The acid–base properties of concentrated phytic acid were studied in self-medium and in NaClaq (0.5 ⩽ I/mol · L−1 ⩽ 4.0) by ISE-H+ potentiometry and by direct calorimetry, at T = 298.15 K. At ligand concentrations c(Phy) > 0.012 mol · L−1, the formation of several binuclear Hi(Phy)2 (2 ⩽ i ⩽ 10) species was observed, in addition to the mononuclear HiPhy (1 ⩽ i ⩽ 7) ones. The solubility of phytate dodecasodium salt was studied in pure water and in NaClaq at different ionic strengths; the total solubility in pure water is S0T=(0.300±0.004)mol·L-1 and it decreases markedly with increasing ionic strength; for example the total solubility of Na12Phy at I = 3.0 mol · L−1 is 0.008 mol · L−1. By the dependence on ionic strength (salt concentration) of the solubility, it was possible to calculate the activity coefficients of phytate as a function of medium concentration. Direct calorimetric titrations were also carried out on Na12Phy aqueous solutions at different phytate concentrations (0.025 ⩽ c(Phy)/mol · L−1 ⩽ 0.100) and without addition of supporting electrolyte, in order to calculate the enthalpy changes for the protonation equilibria in self-medium of the binuclear Hi(Phy)2 species, at T = 298.15 K. It was observed that the ΔH/kJ · mol−1 of the binuclear species are, within the experimental error, independent of the ionic strength; for example for the H2(Phy)2 species we obtained: ΔH22 = (−23.6 ± 0.6) kJ · mol−1, and (−23.7 ± 0.2) kJ · mol−1 at I = 0.50 and 2.0 mol · L−1, respectively.