Dependence on Ionic Strength of Protonation Enthalpies of Polycarboxylate Anions in NaCl Aqueous Solution

The protonation enthalpies of some polycarboxylate anions (malonate, citrate, 1,2,3-propanetricarboxylate, and 1,2,3,4-butanetetracarboxylate) were studied calorimetrically at different ionic strengths, 1 ≤ I ≤ 5 mol kg -1, in NaCl aqueous solutions. Some potentiometric measurements were also made at 15 and 55 °C. Both calorimetric and potentiometric data (including previously reported data) were used to obtain complete sets of Δ G, Δ H, and T Δ S values for the protonation of carboxylate anions. The dependence of Δ H on ionic strength was modeled by simple linear equations and using the Pitzer equations. Both Δ H values and their dependence on ionic strength follow regular trends as a function of the protonation steps, and general empirical relationships are proposed.