pH measurements in non-aqueous and mixed solvents: Predicting pH(PS) of potassium hydrogen phthalate for alcoholwater mixtures (Technical Report)
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, pK,,, 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-2-01, ethane-I ,2-diol, 2-methoxyethanol (“methylcellosolve”). The equations, tested on subsets generated from the full-set by removing an entire cosolvent 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 I 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. INTRODUCTION Recently, recommended pH(PS) (PS = Primary Standard) values of potassium hydrogen phthalate 0.05 m buffer solution in aqueous-organic solvent mixtures have been reported (ref. 1) for the following organic co-solvent in the mixtures: methanol (ref. 2), ethanol (ref. 1,3), propan-2-01 (ref. 2,4), ethane-1,2-diol (ref. 1 3 , 2-methoxyethanol (ref. 1,6), acetonitrile (ref. 7), 1,4-dioxane (ref. 8) and dimethyl sulfoxide (ref. 9). Although the above-mentioned set of recommended values covers some of the most often used aqueous-organic solvent mixtures, it is evident that the set is far from being complete. Hence, urgent need emerges of providing reliable estimates of the pH(PS) values, especially for those solvent mixtures for which direct experimental determinations have not yet been performed. Therefore, it is proposed here that, in absence of directly determined values in mixed solvents, the pH(PS) be calculated by using a predictive equation that relates the sought values to selected, easily accessible properties of the relevant pure organic co-solvents at 298.15 K. RESULTS Among the 8 co-solvents studied so far (ref. l), alcohols constitute a wide, homogeneous and highly significant set, since they include differently functionalised molecules with quite different physicochemical properties. For the sake of clarity, the pH(PS) domains for the five families of alcohol-water mixtures are represented in Fig. 1 as a function of the alcohol mole fraction, x. For each family, the shaded area marks the variations of pH(PS) within the appropriate temperature range. The predictive equation is restricted to alcohol-water mixtures. Out of many physico-chemical properties, relative permittivity (&) (ref. 10-13), autoprotolysis constant (pKap) (ref. 14 Note A ) of the pure alcohol component at 298.15 K, temperature and alcohol mole fraction (x) of the mixture, have been chosen as the most significant variables by a multiregressional analysis procedure. Note A : for the autoprotolysis constant of 2-methoxyethanol, in the absence of specific literature value, that of 2ethoxyethanol (pKap = 19.3) has been taken as the best approximation