Formation and stability of sodium - carboxylate ligand complexes in aqueous solution

The determination of weak complex formation constants needs the use of techniques with high accuracy, in order to record low perturbation of the system properties, as a consequence interaction ligand - metal ion and it requires the capacity to distinguish between a variation of activity and the complex formation. The last condition is achieved if the system under study is kept at constant ionic strength, or if the variation of activity coefficients with the ionic strength can be evaluated with a good accuracy for all the terms of equilibrium equation. In order to obtain an experimental evidence of the formation of sodium complexes, we have planned an experimental procedure which needs the use a sodium-selective electrode. The experiments were carried out with a negligible change of ionic strength of the solution during the measurements. The formation constants of sodium complexes with phthalate, citrate and nitrilotriacetate (NTA) were determined at 25°C. For Na+ - citrate3- and Na+ - phthalate2- systems, I = 0.3 mol L-1 while for Na+ - NTA3-, I = 0.1 mol L-1. Solutions containing the ligand were titrated with a NaCl solution. The ionic strength of working solutions is due to the concentration of the deprotonated ligand and to the amount of tetrabutylammonium cation (tetrabutylammonium hydroxide was added to reach pH ~ 10, necessary to avoid the H+ interference on Na-selective electrode and to assure the complete deprotonation of the ligand molecules). The per cent variation of ionic strength over the titration exceeds 1% only for very few experimental points, for which the same variation can reach 3%. The titrations and the external electrode calibrations were regularly alternated in order to strictly control the electrode performance and the changes of standard potential during the analyses. The calibration solutions were prepared at the same ionic strength of working solutions by using tetrabutylammonium bromide, as background salt, and tetrabutylammonium hydroxide to reach pH ~ 10. The Na+ concentration ranged between 2.5 10-4 mol L-1 and 5.0 10-3 mol L-1. Both the calculations relative to the standard potential of electrode and the estimation of formation constants were performed with a least-squares computer program. The elaboration of data was carried out using the theoretical value of Nernst equation slope (59.16 mV), since we found experimental values very near the theoretical one. In our calculations we always took into account the eventual presence of sodium as impurity in the reagents employed, so as sodium exchanged between the solution under examination and the surface of the titration vessel. The estimated formation constants values are: logK = 0.60 ± 0.03, logK = 1.12 ± 0.02, logK = 1.46 ± 0.02, for Na(phthalate)-, Na(citrate)2- and Na(NTA)2- complexes, respectively. The experimental procedure proposed in this work allows us to have a direct evidence of Na+ complex formation and to avoid that changes in activity coefficients consequent to significant changes in ionic strength conditions can be erroneously interpreted in terms of weak complex formation. Previously some data have been reported in the literature concerning the formation of sodium complexes with these ligands, mainly studied by means of pH-metric measurements. The fairly good agreement we found between the results from pH-metric measurements and the actual ones confirm that pH-metric technique can be usefully employed in investigating weak complex formation.