Solubility, acid-base properties and thermodynamics of interaction between three NTA-phosphonate derivatives and the main cationic components (H+, Na+, Mg2+ and Ca2+) of natural fluids

Solution thermodynamics of three aminophosphonic acids are studied in this work. In particular, the protonation constants [ log(KiH/mol kg−1)] of N-phosphonomethyliminodiacetic acid (NTAP), N,N-Bis(phosphonomethyl)glycine (NTAP2) and Nitrilotris(methylenephosphonic acid) (NTAP3) and the solubility of NTAP were determined by potentiometric measurements (ISE-[H+] glass electrode) in three different ionic media, namely NaCl(aq), (CH3)4NCl(aq) and (C2H5)4NI(aq) at different ionic strengths (in the range 0 < I/mol kg−1 ≤ 1.7) and at T = 298.15. In NaCl(aq), measurements were performed also at T = 283.15 K and 318.15 K. The dependence of the logKiH on ionic strength and temperature was studied by Debye-Hückel type equation and by Specific ion Interaction Theory (SIT). At infinite dilution, the refined values of the first protonation constants are: log(KiH/mol kg−1) = 11.4 ± 0.1, 12.4 ± 0.1 and 13.3 ± 0.1 for NTAP, NTAP2 and NTAP3, respectively and the enthalpy changes relative to the first protonation step are: −15.7 ± 0.3, −20.5 ± 0.4 and -33.5 ± 0.2 kJ mol−1 (same order). The basicity trend: NTAP3 > NTAP2 > NTAP is observed in all the ionic media. The differences in the values of the protonation constants obtained in the three ionic media were also interpreted in terms of formation of weak complex and eleven Na+/L species are obtained using the so called ΔpK method. Solubility investigations performed on NTAP allowed us to determine the Setschenow and the activity coefficients of the neutral species. The values of the total and specific solubility in pure water are: log (S0T/mol kg−1) = −1.431 ± 0.003 and log (S00/mol kg−1) = −2.293 ± 0.006, respectively. The formation constants of the three ligands with Ca2+ and Mg2+ cations were determined at T = 298.15 K in NaCl(aq) at I = 0.151 mol kg−1, finding the presence of the ML and MHL species. The stability of the Ca2+/L is slightly higher than that of the Mg2+/L ones.