Behavior the electrode of fluoride  (monocrystal of LaF3)  is  well investigated at present. Procedure of method became classical. Method is founded on use the masking reagent for cations Fe or Al. The citrate or ethylene diamine tetraacetate is this masking reagent.
Except masking reagent, solution must contains sodium chloride or potassium chloride for stabilization of ionic force. Concentration these salts must be less 0,1M. pH value must be between 5 and 5,5 units, because alkaline reaction will obstruct determination of the fluoride. Ionometric method allows to determine fluoride from 10-5M to 10-2M.

Distinctive  feature of procedure is long response time for small concentration of fluoride (pF=4-5). Some investigators told us about reduction the response time in acid solutions. But you must pay for this. Masking reagents become less effective. Silver chloride reference electrode become very uncertain. And at last, the solution contains less the free anions of fluoride. In spite of this experts consider new procedure reliable.