Effect of temperature on the protonation of N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid in aqueous solutions: Potentiometric and calorimetric studies

The first two steps of protonation of HEDTA occur on the diamine nitrogen atoms and are exothermic, while the third step occurs on the oxygen of one carboxylate group and is slightly endothermic. This trend is in good agreement with the enthalpies of protonation directly determined by calorimetry. The energetics of the protonation can be well explained by the difference in the degree of hydration between the amine nitrogen and carboxylate oxygen sites.