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

The second-stage dissociation constants of NN-bis(2-hydroxyethyl)glyci ne (bicine) were determined at 25.00 +/- 0.02-degrees-C by potentiomet ric pH titration in pure water and different mixed solvent mixtures (m ethanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, acetone, and dioxane). It is observed that the pK(a2) value is slightly influen ced as the solvent is enriched in methanol or ethanol and remains prac tically constant in the presence of different amounts of DMF and DMSO. A pronounced change in the pK(a2) value is observed as the solvent is enriched in acetone or dioxane. These results are discussed in terms of various solvent characteristics. It is concluded that the electrost atic effect haa only a relatively small influence on the dissociation equilibrium of the imino group (-NH+). Other solvent effects such as s olvent basicity and different stabilization of the conjugate acid free base by hydrogen-bonding interactions in aquo-organic solvent media r elative to pure aqueous media as well as proton-solvent interaction pl ay an important role in the acid dissociation equilibrium.