how can I measure the pH of a HF:H2O:CH3OH solution? more specifically, how can I precisely and accurately measure the H+ ion concentration?
The pH is a quantity that is measured in an aqueous solution by dipping a glass electrode in the solution. It is equal to -log[H+] in dilute solutions. For concentrations > 0.01 M, the pH deviates from log [H+], because the concentration [H+] should be calculated by dividing the number of moles H+ by the volume of the free water (and not the volume of the solution). Free water is the water not attached in the Debye layers around the ions. It is very difficult to calculate the volume of the free water in a given solution. For concentrations > 0.01 M, the volume of the free water is smaller than the volume of the solution.
In mixtures water + methanol, the problem becomes worse and cannot be solved by measuring the p. In this case, you have better determine the concentration of HF in a volumetric titration, by adding NaOH solution drop by drop, until the indicator changes its color. The indicator may be thymolphtalein. The volume of free water is smaller thant the volume of the solution.