The acid-base properties of concentrated phytic acid were studied in self-medium and in NaCI,q (0.5 < I/ mol " L-1 4.0) by ISE-H* potentiometry and by direct calorimetry, at T= 298.15 K. At ligand concentrations grhy) > 0.012 mol . L-1, the formation of several binuclear Hi(Phy)2 (2 < i < 10) species was observed, in addition to the mononuclear HiFhy (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 S 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. 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 selfmedium of the binuclear Hi(Phy)2 species, at T = 298.15 K. It was observed that the AN/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: AH22 =(-23.6 0.6) kJ mo1-1, and (-23.7 0.2) kJ. mo1-1 at I = 0.50 and 2.0 mol " L-1, respectively. 2010 Elsevier Ltd. All rights reserved.