Thermochimica Acta, Vol.423, No.1-2, 63-69, 2004
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 < l/mol L-1 < 1) and at t = 25 degreesC. 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 K-i(H) less than or equal to 1, 8 less than or equal to i less than or equal to 12). The analysis of results evidenced slight variations among enthalpy and entropy values, for the same protonation step (i), at different ionic strengths. TDeltaS(i) 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 K-i(H) and DeltaH(i) was taken into account by Specific ion Interaction Theory (SIT). Useful predictive relationships for the dependence of TDeltaS on DeltaG and on protonation step 'i' were proposed. (C) 2004 Elsevier B.V. All rights reserved.