Single crystal LaF3 membrane electrode

Example Fluoride-ion Electrode In this particular instance the membrane essentially comprises of a single crystal of lanthanum fluoride (LaF3), usually doped with a slight trace of europium (II), Eu2+, so as to initiate the crystal defects required for establishing its electrical conductivity. Therefore, the potential developed at each surface of the membrane is finally determined by the exact status of the equilibrium ... 

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

An example is described here for the measurement of fluoride ions in solution. The fluoride electrode uses a LaF3 single crystal membrane and an internal reference, bonded into an epoxy body. The crystal is an ionic conductor in which only fluoride ions are mobile. When the membrane is in contact with a fluoride solution, an electrode potential develops across the membrane. This potential, which depends on the level of free fluoride ions in solution, is measured against an external constant reference potential with a digital pH/mv meter or specific ion meter. The measured potential corresponding to the level of fluoride ions in solution is described by the Nernst equation ... 

Crystal-membrane electrodes, such as the fluoride electrode. Here a crystal is responsible for the exchange of ions in the case of the fluoride electrode, it is a LaF3 crystal. The advantage of this type of electrode is their high selectivity towards one single species because of the unique affinity of the ion in solution (e.g. fluoride) for the crystal (e.g. LaF3)2. 

The fluoride ion selective electrode is the most commonly used single crystal ISE. It is shown schematically in Fig. 15.9. The membrane is a single crystal of LaF3 doped with EuF2. The term doped means that a small amount of another substance (in this case, EUF2) has been added intentionally into the LaF3 crystal. (If the addition were not intentional, we would call the europium an impurity or contaminant ) Note that the two salts do not have the same stoichiometry. Addition of the europium fluoride creates fluoride ion vacancies in the lanthanum fluoride lattice. When exposed to a variable concentration of F ion outside the membrane, the fluoride ions in the crystal can migrate. Unlike the pH electrode, it is the F ions that actually move across the membrane and result in the electrode response. The F ISE is extremely selective for fluoride ion. The only ion... 

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