Determination of hydrogen ion activity in various binary dimethylformamide/water and ternary dimethylformamide/dioxane/water solvent systems

The practical pH values in dimethylformamide/water and dimethylformamide/dioxane/water solvent mixtures measured by a pH meter standardized against aqueous buffer solutions do not lie on the conventional scale of hydrogen ion activity referred to the standard state in the corresponding medium (pa^*_H). The values can be converted to pa^*_H by introducing a correction term δ. Values of δ were determined at 25°C. Simple interpolation equations are derived to express the variation of δ with solvent composition. In the ternary mixtures, the results show that the composite medium effect, described by a parameter b=dδ/du, (where u depends on the solvent composition), depends on the ratio of the organic solvent concentrations.