Solvent effect on stability Constants of (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid with zinc in different aqueous solution of methanol

The metal-ligand stability constants of HL ligand, HL= (+)-(S)-2-(6-methoxynaphthalen-2-yl) propanoic acid, with zinc in 50, 60, 70, 80 and 90% (v/v) methanol+water mixtures have been determined at an ionic strength of 0.10 M Me4NCl and at 25.0 °C under an argon atmosphere. A potentiometric method was used and the calculation of the stability constants has been carried out using the BEST computer program. Species distribution curves of complexes has been plotted as function of pH using the SPE and SPEPLOT program to visualize the presence of various species in equilibrium in the pH range 2–12. The results are discussed in terms of the effect of solvent on complexation and show that the stability constants of the metal-ligand complex increases with increasing amount of methanol. It is known that the ligand must compete with solvent molecules for the cation in the complexation process. Thus, variation of the solvent is expected to change the apparent binding properties of the ligand. This may be due to the decrease in dielectric constant of the solutions and hydrogen bonding and increase in solvation of protonated species by the organic solvents [1-3]. The different species ML2, ML2H, ML2H2, ML2H-1 and ML2H-2 formed depending on pH of solution. The main species formed are ML2 and ML2H-1 in the pH range of 5-7 and in pH higher than 10, respectively.