Mixing effects on the protonation of some polycarboxylates in NaClaq + KClaq at different ionic strengths

Protonation constants of succinic, 1,2,3-propanetricarboxylic and 1,2,3,4-butanetetracarboxylic anions were determined in NaClaq + KClaq mixtures, at three ionic strengths, I = 1.2, 3 and 4.5 mol L−1. Experimental evidences showed that the function log KH = f(y) (y = [Na+]/([Na+] + [K+])) is not linear, indicating mixing effects on the protonation constants. The Guggenheim zeroth approximation holds that the above function can be written as: log 10 ⁡ K Na – K H = y   log 10 ⁡ K Na H + ( 1 − y ) log 10 ⁡ K K H + 4 Δ y ( 1 − y ) where K Na H   and   K K H represent protonation constants in pure salt solutions and Δ is a parameter that accounts for the mixing effect. Fitting of protonation constants to the above function gives excellent results. The Δ values can be treated in terms of mixing free energy. The behaviour of protonation constants in mixed salt solution can be interpreted by considering the formation of simple and mixed weak complexes; the protonation constants in mixed NaKCl electrolytes can be fitted to the equation: log 10 ⁡ K Na – K H = log 10 ⁡ K K H – log 10 ( 1 + A 1 C Na + A 2 C Na C K ) , where A1 is a measure of the interaction of Na+ with the carboxylic anion and A2 is proportional to the triple interaction Na+–K+–Lz−. Moreover, by using suitable calculation methods, it is possible to calculate the formation constants of simple and mixed ion pairs. As an example, for 1,2,3,4-butanetracarboxylic anion (L4−), we calculated K(Na+ + HiL(4−i)− = NaHiL(3−i)−) = 0.67, 0.33 and 0.13; K(K+ + NaHiL(3−i)− = KNaHiL(2−i)−) = 1.41, 1.29 and 0.9 for i = 0, 1 and 2, respectively, indicating a significant tendency to form mixed alkali metal ion pairs.