Solubility and activity coefficients of 2,2′-bipyridyl, 1,10-phenanthroline and 2,2′,6′,2″-terpyridine in NaCl(aq) at different ionic strengths and T = 298.15 K

The solubility of 2,2′-bipyridyl (Bipy), 1,10-phenanthroline (Phen) and 2,2′,6′,2″-terpyridine (Terpy) was determined in NaCl(aq) at different salt concentrations (cNaCl = 0–5 mol L−1) at 298.15 K. Some solubility data were also obtained for 1,10-phenanthroline in (CH3)4NCl(aq) ( c ( C H 3 ) 4 NCl = 0 – 3   mol   L − 1 ). To better characterize the acid–base properties of these azines, the protonation constants were determined potentiometrically in the same supporting electrolyte concentrations. The solubility of neutral species is 43.1, 18.8 and 5.4 mmol L−1 for Bipy, Phen and Terpy, respectively, in pure water. In NaCl(aq) solubility decreases by ∼1.2 orders of magnitude. The opposite trend is shown by the solubility of Phen in (CH3)4NCl(aq), with a small increasing effect. By the dependence on salt concentration Setschenow coefficients were also determined. All the considered azines (Bipy, Phen and Terpy) form a monoprotonated species HA+ and, in addition, Phen forms the diligand species HA2+, and Terpy the diprotonated species H2A2+. The dependence on ionic strength of protonation constants was modelled by the SIT (Specific ion Interaction Theory) and Pitzer approaches. The activity coefficients of different species were obtained by using solubility and protonation data, in the molal concentration scale. Activity coefficients, for the neutral species, in (CH3)4NCl(aq) (only for Phen) show an opposite trend with respect to that in NaCl(aq), and this was interpreted in terms of weak complex formation.