MEDIUM EFFECT ON THE 2ND STAGE DISSOCIATION-CONSTANT OF [TRIS(HYDROXYMETHYL)METHYLAMINO-PROPANE SULFONIC-ACID] (TAPS)

The second stage dissociation constant values of [tris(hydroxymethyl)m ethylaminopropane sulphonic acid] (TAPS) were determined at 25.00+/-0. 02-degrees-C by potentiometric pH titration in pure, water and differe nt mixed solvent mixtures (methanol, ethanol, N,N-dimethyl-formamide [ DMF], dimethylsulfoxide [DMSO], acetone and dioxane). It is observed t hat the pKa2 Value is slightly influenced as the solvent is enriched i n methanol, ethanol and remains practically constant in presence of di fferent amounts of DMF and DMSO. A pronounced change in the PKa2 value is observed as the solvent is enriched in acetone or dioxane. These r esults were 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 group (-NH2+). Other solvent effects such as solvent basicity, different stabilization of the conjugate acid free base by hydrogen bonding interactions in aquo- organic solvents media relative to pure aqueous media as well as proto n-solvent interaction play an important role in the acid dissociation equilibrium.