Speciation of phytate ion in aqueous solution: Thermodynamic parameters for protonation in NaCl

Phytate protonation enthalpies and entropies were determined by direct calorimetric titrations, in sodium chloride aqueous media at different ionic strengths (0.1≤I/mol L−1≤1) and at t=25 °C. Only the first seven protonation steps were taken into account, since the remaining five protons of phytic acid could be considered as “strongly acidic” (log KiH≤1, 8≤i≤12). The analysis of results evidenced slight variations among enthalpy and entropy values, for the same protonation step (i), at different ionic strengths. TΔSi values for each protonation step are significantly higher than the corresponding enthalpy changes, revealing the entropic nature of the bond involved in phytate protonation. The enthalpy changes for phytate protonation generally increase for each protonation step, as typical for phosphate ligands. Dependence on ionic strength for both log KiH and ΔHi was taken into account by Specific ion Interaction Theory (SIT). Useful predictive relationships for the dependence of TΔS on ΔG and on protonation step “i” were proposed.