POTENTIOMETRIC DETERMINATION OF THE 2ND-STAGE DISSOCIATION-CONSTANT OF N,N-BIS(2-HYDROXYETHYL)-2-AMINOETHANESULFONIC ACID IN VARIOUS WATER PLUS ORGANIC-SOLVENT MIXTURES

The second-stage dissociation constant values of N,N-bis(2-hydroxyethy l)-2-aminoethanesulfonic acid (BES) were determined at 25 +/- 0.1-degr ees-C by potentiometric pH titration in different solvent mixtures. Th e organic solvents used are methanol, ethanol, dimethylformamide (DMF) , dimethyl sulfoxide (DMSO), acetone, and dioxane. The ionization of B ES depends on both the proportion and the nature of the organic cosolv ent used. It is observed that the pK(a2) value is slightly influenced as the solvent is enriched in methanol and ethanol and remains practic ally 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 en riched in acetone or dioxane. These results are discussed in terms of various solvent characteristics. It is concluded that the electrostati c effect has only a relatively small influence on the dissociation equ ilibrium of the imino group. Other solvent effects such as solvent bas icity and different stabilization of the conjugate acid free base by h ydrogen-bonding interactions in aquoorganic solvent media relative to pure aqueous media as well as proton-solvent interaction play an impor tant role in the acid dissociation equilibrium.