Non-stoichiometric dissolution of lanthanum fluoride (LaF 3) and its relevance to a process of ion-selective charge separation at the solid ⋯ solution interface

Abstract

The solubilities of lanthanum fluoride (LaF 3) powder and single crystals were measured in various aqueous electrolyte solutions. In contrast with the classical assumption of the stoichiometric dissolution of LaF 3, it was found that the dissolution of the component cation La 3+ was undetectable and the fluoride ion dissolved preferentially: c F − > 3 c La 3+ where c is the ion concentration. X-ray photoelectron spectroscopy (XPS) of the LaF 3 surface treated with electrolyte solutions corroborated this observation. As a result of the preferential fluoride dissolution, charge separation was expected to occur in such a way that the LaF 3 surface would have positively charged “fluoride-vacancies”. This was confirmed by changes in membrane potential as a function of the fluoride dissolution the adsorption of an anionic adsorption marker (eosin Y) on the solid surface. It was also found that the preferential dissolution of fluoride ions was accompanied by simultaneous adsorption of almost the same amount of electrolyte anions (e.g. NO 3 −) onto the LaF 3 surface. This was confirmed by both solution measurements and surface analysis by diffuse reflectance IR Fourier transform (DRIFT) spectrometry. The result of this anion adsorption was explained in terms of ion exchange to ensure electroneutrality in the bulk of the adjacent electrolyte solution, which would otherwise be violated by the preferential fluoride dissolution. The anionic adsorption marker and DRIFT measurements were used to characterize further the above-mentioned ion exchange and selective uptake of fluoride ions into the “fluoride vacancies”. Release of the adsorbed eosin Y or electrolyte anions, such as NO 3 −, from the LaF 3 surface was found to be induced by F − or OH − ions only. On the basis of this observation, the response mechanism of the fluoride ion-selective electrodes was discussed.