Determine the concentration of fluoride ion:

Problem:

What is the source of the potential of an ion selective electrode used to determine the concentration of fluoride ion?

Answer:

When ion selective electrode is dipped in the solution containing fluoride ions, the equilibrium will be established between both the fluoride ions in test solution and LaF₃ crystal and fluoride ions in inner solution and LaF₃. The activity of the F^- at the inner surface is likely to be different to that at the outer surface. This results in a development of the diffusion potential between two surfaces similar to liquid junction potential. Since the activity of internal solution is constant value then a diffusion potential is, therefore, dependent on the activity of F^- in the test solution.