Modeling ATP protonation and activity coefficients in NaClaq and KClaq by SIT and Pitzer equations

The acid-base properties of Adenosine 5′-triphosphate (ATP) in NaCl and KCl aqueous solutions at different ionic strengths (0 < I / mol L- 1 ≤ 5 for NaClaq, 0 < I / mol L- 1 ≤ 3 for KClaq) and at t = 25 °C were investigated. A selection of literature data on ATP protonation constants and on activity isopiestic coefficients was performed, together with new potentiometric measurements (by ISE-H+, glass electrode). Both literature and new experimental data were used to model the dependence on ionic strength and ionic medium of ATP protonation by SIT (Specific ion Interaction Theory) and Pitzer equations. In addition to values of first and second ATP protonation constants in NaClaq and KClaq at different ionic strengths, stability constants of NaATP3- and KATP3- complexes, SIT interaction coefficients and Pitzer parameters were calculated, together with protonation constants at infinite dilution: log TK1H = pTKa2 = 7.656 ± 0.010 and log TK2H = pTKa1 = 4.561 ± 0.006 (in the molar concentration scale, ± 95% confidence interval). Both SIT and Pitzer approaches give satisfactory results.