IONIZATION-CONSTANTS OF PH REFERENCE MATERIALS IN ACETONITRILE-WATER MIXTURES UP TO 70-PERCENT (W W)/

Ionization constants of pH reference materials in acetonitrile-water m ixtures containing 30, 40, 50 and 70% (w/w) acetonitrile were obtained . The ionization constant values determined were: pK1 and pK2 for tart aric and phthalic acid; pK1, pK2 and pK3 for citric acid and pK1 for b oric and acetic acid. These values are essential to determine the stan dard reference value, pHs, for various standard reference buffer solut ions. In the total composition range studied, pK values were linearly correlated with the mol fraction of acetonitrile and with the reciproc al of the dielectric constant of solvent mixtures. Considering the unl imited number of solvent mixtures, a multilinear regression procedure was applied to correlate pK values with % (w/w) and % (v/v) of acetoni trile, and the methodology of linear solvation energy relationships (L SERs) was used to relate pK data with solvatochromic parameters of ace tonitrile-water mixtures.