NONSTOICHIOMETRIC DISSOLUTION OF LANTHANUM FLUORIDE (LAF3) AND ITS RELEVANCE TO A PROCESS OF ION-SELECTIVE CHARGE SEPARATION AT THE SOLID-SOLUTION INTERFACE

The solubilities of lanthanum fluoride (LaF3) powder and single crysta ls were measured in various aqueous electrolyte solutions. In contrast with the classical assumption of the stoichiometric dissolution of La F3, it was found that the dissolution of the component cation La3+ was undetectable and the fluoride ion dissolved preferentially: c(F) - > 3c(La3+) where c is the ion concentration. X-ray photoelectron spectro scopy (XPS) of the LaF3 surface treated with electrolyte solutions cor roborated this observation. As a result of the preferential fluoride d issolution, charge separation was expected to occur in such a way that the LaF3 surface would have positively charged ''fluoride-vacancies'' . This was confirmed by changes in membrane potential as a function of the fluoride dissolution and the adsorption of an anionic adsorption marker (eosin Y) on the solid surface. It was also found that the pref erential dissolution of fluoride ions was accompanied by simultaneous adsorption of almost the same amount of electrolyte anions (e.g. NO3-) onto the LaF3 surface. This was confirmed by both solution measuremen ts and surface analysis by diffuse reflectance IR Fourier transform (D RIFT) spectrometry. The result of this anion adsorption was explained in terms of ion exchange to ensure electroneutrality in the bulk of th e adjacent electrolyte solution, which would otherwise be violated by the preferential fluoride dissolution. The anionic adsorption marker a nd DRIFT measurements were used to characterize further the above-ment ioned ion exchange and selective uptake of fluoride ions into the ''fl uoride vacancies''. Release of the adsorbed eosin Y or electrolyte ani ons, such as NO3-, from the LaF3 surface was found to be induced by F- or OH- ions only. On the basis of this observation, the response mech anism of the fluoride ion-selective electrodes was discussed.