Thermodynamic Study on the Protonation and Na+, Ca2+, Mg2+-Complexation of a Biodegradable Chelant (HEIDA) at Different Ionic Strengths and Temperatures

Abstract

A 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–Huckel type equation, Specific Ion Interaction Theory and Pitzer equations. Protonation constants at infinite dilution calculated by the SIT model are $$ \log_{10} \left( {{}^{T}K_{1}^{\text{H}} } \right) = 8.998 \pm 0.008 $$ (amino group), $$ \log_{10} \left( {{}^{T}K_{2}^{\text{H}} } \right) = 2.515 \pm 0.009 $$ and $$ \log_{10} \left( {{}^{T}K_{3}^{\text{H}} } \right) = 1.06 \pm 0.002 $$ (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 log10KNaL = 0.78 ± 0.23. For Ca2+ and Mg2+, the CaL, CaHL, CaL2 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: log10βCaL = 4.92 ± 0.01, log10βCaHL = 11.11 ± 0.02 and $$ \log_{10} \beta_{\text{Ca{L}}_{2}} $$ = 7.84 ± 0.03, while for the magnesium complex (at I = 0.176 mol·dm−3): log10βMgL = 2.928 ± 0.006. Protonation thermodynamic functions have also been calculated and interpreted.