Thermodynamic study on the protonation and complexation of GLDA with Ca2+ and Mg2+ at different ionic strengths and ionic media at 298.15 K

The protonation constants (logKiH) of l-glutamic acid N,N-diacetic acid tetrasodium salt (GLDA) were determined at T = 298.15 K and various ionic strengths in NaClaq, 0.18 ≤ I ≤ 3.20 mol·kg-1, (CH3) 4 NClaq 0.17 ≤ I ≤ 4.00 mol·kg-1, and (C2 H5) 4 NIaq 0.13 ≤ I ≤ 1.10 molkg-1 by using the potentiometric method. The calculation of ionic strength dependence parameters was performed by Debye-Hückel type equation, Specific ion Interaction Theory (SIT). and Pitzer equations (only for NaClaq). These equations make it feasible to calculate and predict the protonation constants of GLDA in the desired range of ionic strength theoretically. The values of pure water protonation constants are logK1H 10.558 ± 0.028, logK2H 5.348 ± 0.011, logK3H 3.088 ± 0.015 and logK4H 1.918 ± 0.009, the first protonation refers to the nitrogen group and the other three to carboxylate groups. The different values of logKiH obtained in NaClaq, (CH3) 4NClaq, and (C2 H5) 4 NIaq were interpreted in terms of formation of weak complexes between GLDA and the cations of the supporting electrolytes. Binding constants referring to the weak species formed by GLDA with sodium and tetramethylammonium cations were calculated. For example, the stability of the NaL and (CH3) 4NL species, at infinite dilution, are log K = 2.5 ± 0.2 and 1.6 ± 0.3, respectively. The complexing ability of GLDA versus calcium and magnesium was also determined in NaClaq at various ionic strength values. The pure water formation constants are for calcium logKCaL0 = 8.181 ± 0.024 and for magnesium logKMgL0 = 7.263 ± 0.028.