Medium effect on the apparent second stage dissociation constants of some zwitterionic buffers for physiological research in various water plus organic solvent mixtures

The apparent second stage dissociation constants of piperazine-N,N-bis[2-hy droxypropanesulfonicacid](POPSO), N-[2-hydroxyethyl]piperazine-N'-3-propane sulfonic: acid (HEPPS), 2-[N-cyclohexylamino]-ethanesulfonic acid (CHES) a nd mono[tris(hydroxymethyl)aminomethane] phosphate (Trizma) were determined at (25.0 +/- 0.1) degrees C: and I = 0.1 mol dm(-3) (KNO3) by potentiometr ic pH titration in pure water and: different hydroorganic solvent media. Th e organic solvents used were methanol and ethanol as amphiprotic hydrogen b ond acceptor-donor (HBA-D) solvents, N,N-dimethylformamide (DMF), dimethyl sulfoxide, acetone, and dioxane as hydrogen bond acceptor solvents, and ace tonitrile as an HBA-HBD solvent. The ESAB2M computer program was used to re fine the initial estimates of the apparent second-stage dissociation consta nts of the four zwitterionic buffers studied, pk(a2)* values change with in creasing organic content of the solvent mixture. The results obtained are, discussed in terms of average macroscopic properties of the mixed solvents. The implications of the results with regard to specific solute;solvent int eracions, particularly stabilization of zwitterionic species, are discussed . The effects of coorganic solvents on the acid dissociation equilibria hav e been interpreted using solvatochromic quantitative values of Kamlet-Taft hydrogen bond acidity and basicity (alpha, beta) and dipolarity-polarizabil ity pi* of the solvent.