Study of monoprotic acid–base equilibria in methanol-water association using spectrophotometry and equilibrium modeling

Methanol–water mixtures are widely used as solvents in chemistry such as HPLC separations. Previous studies showed a methanol-water mixture cannot be considered as a simple binary system. So the system is assumed to contain clusters of methanol (M), water (W), and methanol–water (M-W), contains water and methanol in the molecular ratio of unity [1]. So the solute molecules in such mixture are known to be distributed between different solvent clusters. To clarify the issue, we considered acid - base equilibria of dye as indicator in methanol-water mixture. In this model, the protonated and deprotonated form of indicator can be distributed between M, W and M-W clusters, with different distribution constants. So, three components exist for each of the protonated and deprotonated forms. Also it should be noted that there is an acid-base equilibrium with a different protonation constant in each clusters (Schematic 1). (A)M + (H+)M (AH+)M (A)W + (H+)W (AH+)MW (A)MW + (H+)MW (AH+)MW Schematic 1: A model for acid–base equilibrium of the indicator in methanol-water mixture. where, (A)W, (A)M and (A)MW are deprotonated indicator in water, methanol and methanol-water clusters with the protonation constants of , and respectively. The protonated indicator in water, methanol and methanol-water clusters are shown as (AH+)W, (AH+)M and (AH+)MW, respectively. k1, k2, k3, k4, k5 and k6 were distribution constants of desired analytes between water, methanol and methanol-water clusters. At first, the solvation of protonated and deprotonated form of the indicator in different methanol/water compositions was studied spectrophotometrically to calculate of distribution coefficients of solute between clusters and association constant methanol-water cluster. Then, the spectrophotometric data obtained from acid-base titration of the indicator in methanol and water was used for calculating of protonation constant of dye in these solvents. Finally, global analysis of spectrophotometric data obtained from acid-base titration of the indicator in mixture of methanol/ water was used for calculating of protonation constant of dye in M-W cluster