Ionic interactions in aqueous solutions of organic electrolytes. Activity coefficients for (sodium chloride + sodium hydrogen malonate + sodium malonate) (aq) at 25°C. Application of Pitzer's equations

Abstract

Emf values for the cell glass |NaHMal(m), Na2Mal(m), NaCl(m), AgCl|Ag, where Mal stands for the malonate anion, were measured at 25°C in the ionic strength range from 0.05 to 4.0 mol kg−1. From these data and those obtained previously by Hamer and co-workers using the cell Pt, H2|NaHMal(m), Na2Mal(m), NaCl(m), AgCl|Ag in the ionic strength range from 0.005 to 0.2 mol kg−1, Pitzer's parameter values for Na2Mal were obtained together with the pKa2 value, where Ka2 is the second acidity constant of the malonic acid. Activity and osmotic coefficients for aqueous solutions of pure Na2Mal at several concentrations were determined from these parameter values. From the analysis of these activity coefficient values a strong ion—ion interaction of Na+-Mal2− nature can be deduced, which would justify the presence of a large formation of (Na-Mal)− ionic pairs.