Thermodynamic dissociation constants of glycine and dl-alanine in water-ethanol mixtures

摘要

The thermodynamic dissociation constants of glycine and dl-alanine in 0,20,40,60 and 80% (w/w) water-ethanol mixtures at 30 deg C and at a constant ionic strength, I_s = 0.1 M KNO_3 have been measured pH-metrically using Irving and Rossotti technique. pK_1 and pK_2 values in aqueous medium have been found to agree well with previously reported ones. It is observed that the pK_1 values of these acids increase regularly with the increase of ethanol contents in the mixtures while pK_2 values show distinct minima ca. at 40% (w/w) ethanol content. The variation in pK values with the solvent composition is discussed in terms of the free energy transfer from water to ethanol-water mixtures. It is concluded that the specific solute-solvent interactions play a vital role in changing the pK values in ethanol-water mixtures.