Medium Effect on the Second-Stage Dissociation Constant of N,N-Bis(2-hydroxyethyl)glycine

The second-stage dissociation constants of N,N-bis(2-hydroxyethyl)glycine (bicine) rvere determined at 25.00±0.02 o C by potentiometric pH titration in pure water and different mixed solvent mixtures (methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, acetone, and dioxane). It is observed that the pK a2 , value is slightly influenced as the solvent is enriched in methanol or ethanol and remains practically 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 electrostatic effect has only a relatively small influence on the dissociation equilibrium of the imino group (-NH + )