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Pure Appl. Chem., Vol. 70, No. 7, pp.1419-1422, 1998




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

(Technical Report)

S. Rondinini, P. R. Mussini, T. Mussini, and A. Vertova

Abstract: 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, e , 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-ol, ethane-1,2-diol, 2-methoxyethanol ("methylcellosol ve"). 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.


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