POTENTIOMETRIC DETERMINATION OF THE DISSOCIATION-CONSTANTS OF L-HISTIDINE, PROLINE AND TRYPTOPHANE IN VARIOUS HYDROORGANIC MEDIA

The dissociation constant values of L-histidine, proline and tryptopha ne were determined at 25 +/- 0.1-degree-C by potentiometric pH titrati on in pure water and different hydroorganic solvent media. The organic solvents used were methanol, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, acetone and dioxane. Initial estimates of the dissociation constant values of the different amino acids studied have been refine d with ESAP2M computer program. It was observed that changing the medi um permittivity as the solvent is enriched in methanol or ethanol has little influence on the pK(a) values of the amino acids studied. The results obtained are discussed in terms of average macroscopic propert ies of the mixed solvents and the possible variation in microheterogen eity of the solvation shells around the solute.