pH Measurements In Non/Aqueous And Mixed Solvents: Predicting pH(PS) of Potassium Hydrogen Phthalate for Alcohol-Water Mixtures

Predictive equations for estimating pH(PS) (PS = Primary Standard) of the potassium hydrogen phthalate buffer in any aqueous-alcoholic mixture are given as functions of the alcoholic moiety mole fraction, x, in the mixture and of the relative permittivity, ε, and the autoprotolysis constant, pKap, of the pure alcohol at 298.15 K. The two equations, one valid for 298.15 K and the other covering the 263 - 318 K temperature range, have been optimized by using the experimental data of pH(PS) in aqueous mixtures with the following alcohols: methanol, ethanol, propan-2-o1, ethane-1,2-diol, 2-methoxyethanol ("methylcellosolve"). The equations, tested on subsets generated from the full-set by removing an entire co-solvent at a time, provide predicted pH(PS) values which differ from the "true" ones by less than 0.1 pH over the entire range of co-solvent composition: 0 < x < 0.75. The predictive equations are recommended for calculating pH(PS) only in those aqueous-alcoholic media for which the corresponding experimental values are not available, and not as smoothing equations for water and its mixtures with the above-mentioned solvents.