Dependence on ionic strength of protonation enthalpies of polycarboxylate anions in NaCl aqueous solution

The protonation enthalpies of some polycarboxylate anions (malonate, citrat e, 1,2,3-propanetricarboxylate, and 1,2,3,4-butanetetracarboxylate) were st udied calorimetrically at different ionic strengths, 1 less than or equal t o I less than or equal to 5 mol kg(-1), in NaCl aqueous solutions. Some pot entiometric measurements were also made at 15 and 55 degreesC. Both calorim etric and potentiometric data (including previously reported data) were use d to obtain complete sets of DeltaG, DeltaH, and T DeltaS values for the pr otonation of carboxylate anions. The dependence of DeltaH on ionic strength was modeled by simple linear equations and using the Pitzer equations. Bot h DeltaH values and their dependence on ionic strength follow regular trend s as a function of the protonation steps, and general empirical relationshi ps are proposed.