Determination of hydrogen ion activities in various ternary water/methanol/dioxane solvent systems

The practical pH values for solutions in ternary water/methanol/dioxane solvents measured by a pH meter standardized with 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). These values can be converted to pa^*_H by the introduction of a correction term δ = $\text{E}\text{?}$_j ? log(_mγ_H) (where $\text{E}\text{?}$_j is a term incorporating the liquid junction potential, which depends on the solvent composition and _mγ_H is the medium effect on hydrogen ion). Values of δ were determined for various ternary solvents at 25°C and were found to be constant in each medium independent of the solute composition.