Dependence on ionic strength of protonation enthalpies of polycarboxylic 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 less than or equal to I less than or equal to 5 mol kg(-1), in NaCl aqueous solutions. Some potentiometric measurements were also made at 15 and 55 degreesC. Both calorimetric and potentiometric data (including previously reported data) were used to obtain complete sets of DeltaG, DeltaH, and T DeltaS values for the protonation of carboxylate anions. The dependence of DeltaH on ionic strength was modeled by simple linear equations and using the Pitzer equations. Both DeltaH values and their dependence on ionic strength follow regular trends as a function of the protonation steps, and general empirical relationships are proposed.