Such a crystal is an ionic conductor by virtue of the mobility of fluoride ions which jump between lattice vacancies.
An electrochemical cell may be constructed using such a crystal as a membrane separating two fluoride solutions.
Generally such "alkaline error" can be avoided by buffering the sample to a pH below 7.
The cell diagram of a typical experimental arrangement is: where: Some commercially available reference electrodes have an internal junction which minimizes the liquid junction potential between the sample solution and the electrolyte in the reference electrode (KCl).
The internal electrolyte is at fixed composition and the electrode response is given by the Nernst equation: where: Martin S. Frant and James W. Ross Jr. Electrode for Sensing Fluoride Ion Activity in Solution.