On the interaction of phytate with proton and monocharged inorganic cations in different ionic media, and modeling of acid-base properties at low ionic strength

In this paper, new data are reported on the protonation of phytate at T = 298.15 K in different ionic media and ionic strengths, namely NH4Cl(aq) (0.1 6 I/mol kg1 6 1.9) and NaNO3(aq) (0.1 6 I/mol kg1 6 5.4). A complete set of phytate protonation constants, with general formula HiPhy (with 1 6 i 6 7) was proposed. The data were modeled by a logarithmic empiric equation, as well as an Extended Debye–Hückel (EDH) and specific ion interaction theory (SIT) models. A strong similarity was found between protonation data in NH4Cl(aq), NaNO3(aq) and those previously reported in NaCl(aq), KCl(aq) and KNO3(aq), so that an unique set of thermodynamic protonation data has been proposed for phytate protonation at I 6 0.15 mol kg1. Thermodynamic parameters obtained in these conditions can be useful to model the behavior of phytate in real aqueous systems at low ionic strengths. An empirical relationship has also been proposed for all the protonation data at 0 < I/mol kg1 6 0.15: TDS (±1.5) = 11.7 0.72 DG. Using literature data for the phytate protonation in (C2H5)4NI(aq), the weak complexes between Na+ or NHþ 4 and phytic acid have been calculated by the DpK method (i.e., by the comparison of the protonation constants determined in an interacting medium, such as NH4Cl(aq), and those obtained in a non-interacting supporting electrolytes, like tetraalkylammonium salts). In terms of weak complex formation constants, the results are similar for NaNO3(aq) and NH4Cl(aq), and twelve HiMjPhy species are reported together with their stability. For example, for the formation of the M6Phy species, it is: log b = 25.9 and 25.6 (±0.2) for M = Na+ and NHþ 4 , respectively, at I = 0.1 mol dm3. This paper represents an advance in the understanding of the acid-base behavior of phytic acid in a wide number of ionic media at different ionic strengths.