Thermodynamic Study on the Protonation and Na + , Ca 2+ , Mg 2+ -Complexation of a Biodegradable Chelant (HEIDA) at Different Ionic Strengths and Temperatures

potentiometric method has been used for the determination of the protonation constants of N -(2-hydroxyethyl)iminodiacetic acid (HEIDA or L) at various temperatures 283.15 ≤ T /K ≤ 383.15 and different ionic strengths of NaCl (aq) , 0.12 ≤ I /mol·kg −1 ≤ 4.84. Ionic strength dependence parameters were calculated using a Debye–Hückel type equation, Specific Ion Interaction Theory and Pitzer equations. Protonation constants at infinite dilution calculated by the SIT model are log_10( ^TK_1^H) = 8.998 ± 0.008 (amino group), log_10( ^TK_2^H) = 2.515 ± 0.009 and log_10( ^TK_3^H) = 1.06 ± 0.002 (carboxylic groups). The formation constants of HEIDA complexes with sodium, calcium and magnesium were determined. In the first case, the formation of a weak complex species, NaL, was found and the stability constant value at infinite dilution is log 10 K NaL = 0.78 ± 0.23. For Ca 2+ and Mg 2+ , the CaL, CaHL, CaL 2 and MgL species were found, respectively. The calculated stability constants for the calcium complexes at T = 298.15 K and I = 0.150 mol·dm −3 are: log 10 β CaL = 4.92 ± 0.01, log 10 β CaHL = 11.11 ± 0.02 and log_10β_CaL_2 = 7.84 ± 0.03, while for the magnesium complex (at I = 0.176 mol·dm −3 ): log 10 β MgL = 2.928 ± 0.006. Protonation thermodynamic functions have also been calculated and interpreted.