Thermodynamics for proton binding of phytate in KNO3(aq) at different temperatures and ionic strengths

Potentiometric measurements were performed in KNO3(aq), to determine the apparent protonation constants of phytate at different temperatures (278.15 ≤ T (K) ≤ 323.15) and ionic strengths (0.25 ≤ I (mol) dm-3 ≤ 3.0) values. In general, the protonation constants decrease with increasing both temperature and ionic strength. The data reported were critically compared with previous results obtained in KCl and the values are in a good agreement, considering the experimental errors and slight differences between the activity coefficients of the various species in KCl and KNO3. Experimental data were then modeled as a function of temperature and ionic strength using, with comparable results, two approaches: the extended Debye.Huckel equation and the specific ion interaction theory (SIT). The single specific ion interaction coefficients, ε, were also determined. The corresponding values are higher than those in Na+ media. The protonation constants were also analyzed considering a simplified weak interaction model using an empirical equation that contains an additional term which takes into account the formation of weak complexes. The results obtained for the modeling of the protonation constants are in agreement with the literature findings. Thermodynamic protonation parameters were also obtained at different temperatures and ionic strengths. The proton association process is slightly exothermic and the enthalpic contribution is less negative than that in NaCl solution. As observed in other cases for phytate anion, the major contribution for the proton association is entropic in nature. A rough value of δcp was also determined. Finally, a complete set of suggested data is proposed at different temperatures and ionic strengths.