MEDIUM EFFECT ON THE 2ND STAGE DISSOCIATION-CONSTANT OF N,N-BIS(2-HYDROXYETHYL)-GLYCINE (BICINE)

The second stage dissociation constant values of N,N-bis(2-hydroxyethy l)-glycine (bicine) were determined at 25.00 +/- 0.02 degrees C by pot entiometric pH titration in pure water and different mixed solvent mix tures (methanol, ethanol, N,N-dimethylformamide [DMF], dimethylsulfoxi de [DMSO], acetone and dioxane). It is observed that the pK(alpha 2) v alue is slightly influenced when the solvent is enriched in methanol o r ethanol, and remains practically constant in presence of different a mounts of DMF and DMSO. A pronounced change in the pK(alpha 2) value i s observed as the solvent is enriched in acetone or dioxane. These res ults were discussed in terms of various solvent characteristics. It is concluded that the electrostatic effect has only a relatively small i nfluence on the dissociation equilibrium of the imino group (-NH+). Ot her solvent effects such as: solvent basicity, different stabilization of the conjugate acid free base by hydrogen bonding interactions in t he aquo-organic solvents media, relative to pure aqueous media as well as proton-solvent interaction play an important role in the acid diss ociation equilibrium.