Applications of the fluoride-sensitive electrode to the study of metal-fluoride ion association constants

Abstract

An “Orion” fluoride-sensitive membrane electrode has been examined critically for the measurement of metal-fluoride ion association in aqueous solutions of constant ionic strength, with a calomel reference electrode and sodium chloride salt bridge. Measurements were made with magnesium, calcium, scandium, iron(III), europium(III) and gadolinium ions at 25°; yttrium and “protonation” of [F -] were studied at 15, 25 and 35° and enthalpy and entropy changes calculated. The electrode is limited to solutions 10 -6 M or greater in free [F -] aq, with a consequent limitation on the magnitude of the first association constant to ≦10 6. Further limitations are imposed by the insolubility of most metal fluoride salts. The electrode is less sensitive than the iron(III)-iron(II) electrode system, but is much more convenient. Good agreement with previous results was obtained from measurements at various temperatures.