Lanthanum fluoride

Sodium fluoroborate [13755-29-8] M 109.8, m 384 , d 2.47, pK -4.9 (for fluoroboric acid H30 BF4 ). Crystd from hot water (50mL/g) by cooling to 0 . Alternatively, purified from insoluble material by dissolving in a minimum amount of water, then fluoride ion was removed by adding cone lanthanum nitrate in excess. After removing lanthanum fluoride by centrifugation, the supernatant was passed... 

A single crystal electrode is exemplified by the lanthanum fluoride electrode in which a crystal of lanthanum fluoride is sealed into the bottom of a plastic container to produce a fluoride ion electrode. The container is charged with a... 

The lanthanum fluoride crystal is a conductor for fluoride ions which being small can move through the crystal from one lattice defect to another, and equilibrium is established between the crystal face inside the electrode and the internal solution. Likewise, when the electrode is placed in a solution containing fluoride ions, equilibrium is established at the external surface of the crystal. In general, the fluoride ion activities at the two faces of the crystal are different and so a potential is established, and since the conditions at the internal face are constant, the resultant potential is proportional to the fluoride ion activity of the test solution. 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

Lanthanum fluoride (and fluorides of some other lanthanides) has an unusual type of defect (see Section 6.3.2), namely Schottky defects of the molecular hole type (whole LaF3 molecules are missing at certain sites). Charge carriers (F ) are formed as the result of interaction of LaF3 with this hole, leading to dissociation with formation of LaF2+ and F . 

All ion-exchanger membranes with fixed ion-exchanger sites are porous to a certain degree (in contrast to liquid membranes and to membranes of ion-selective electrodes based on solid or glassy electrolytes, such as a single crystal of lanthanum fluoride). 

The lanthanum fluoride electrode (discussed in Section 2.6) is used to determine F ions in neutral and acid media. After the pH-glass electrode, this is the most important of this group of electrodes. 

ScF3 and LaF3. Both separate and multilayer scandium and lanthanum fluoride films have been grown from nitrates and lanthanum chloride and sodium and ammonium fluorides. XPS analysis revealed that the films were stoichiometric and that the films contained both hydroxide groups and water.129... 

These incorporate membranes fabricated from insoluble crystalline materials. They can be in the form of a single crystal, a compressed disc of micro-crystalline material or an agglomerate of micro-crystals embedded in a silicone rubber or paraffin matrix which is moulded in the form of a thin disc. The materials used are highly insoluble salts such as lanthanum fluoride, barium sulphate, silver halides and metal sulphides. These types of membrane show a selective and Nemstian response to solutions containing either the cation or the anion of the salt used. Factors to be considered in the fabrication of a suitable membrane include solubility, mechanical strength, conductivity and resistance to abrasion or corrosion. 

Variation op the Solubility Product for Lanthanum Fluoride with Sodium Perchlorate Concentration0... 

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 ... 

We have briefly encountered the solid-state fluoride electrode, which has a fully nemstian response down to c. 10 mol dm . The fluoride electrode is iiiunersed in a test solution of fluoride ion (usually aqueous), and the emf is then determined. At its heart is a single crystal of lanthanum fluoride doped with erbium fluoride, (see Figure 3.10). Like the pH electrode, a full fluoride electrode also contains a small reference electrode, meaning that a fluoride electrode is in reality a cell. The fluoride electrode does not suffer from interference from CP, so an AgCl Ag reference is the normal choice owing to its convenience and compact size. 

Solid-state crystal of doped lanthanum fluoride... 

Fig. 6.3. The effect of pH on the potential of the Orion Research lanthanum fluoride electrode in NaF solutions of various concentrations. The potential change with pH in the acidic region is caused by the formation of HFJ. (After Butler [53].)...

Lanthanum fluoride (La + 3 F " —> LaF ) is a white powder used to coat the inside of phosphorus lamps and lasers. 

The addition of hydrofluoric acid to an aqueous solution of lanthanum chloride precipitates out lanthanum fluoride, LaFs ... 

Lanthanum fluoride is used in phosphor lamp coating. Mixed with other rare earths, it is used in carbon arc electrodes and lasers. Also, the fluoride is used in the production of lanthanum metal, an intermediate step in the manufacture of high purity metal. 

Anhydrous lanthanum fluoride also may be made by passing dry hydrogen fluoride over lanthanum oxide. This process, however, produces trace amounts of lanthanum oxyfluoride, LaOF. Highly purified material may be obtained by passing dry purified HF over molten fluoride in a platinum crucible. 

Methanol and Water. Methanol and water mixtures have been a popular choice for workers interested in free energies of transfer of ions from water into a mixed solvent. Such mixtures exhibit a drop in dielectric constant with increasing methanol content. Hence the electrical term must be estimated in order to compare spectroscopic and thermodynamic quantities. Feakins and Voice (28) have presented new data and revised earlier data for the alkali metal chlorides. In advance of carefully determined and extrapolated emf data for fluorides, using the solid state fluoride selective electrode based on lanthanum fluoride, some data of moderate accuracy have been presented (27). On the... 

Here, the potentiometric selectivity coefficient is given with respect to the hydroxyl ion. Single-crystal lanthanum fluoride is a wide bandgap semiconductor in which the electrical conductivity is due only to the hopping mobility of fluoride ions through the defects in the crystal. It does not respond to the La3+ ion because of the slow ion exchange of that ion. Hydroxyl ion is the only other ion that has appreciable mobility, and is the only known interference. For this reason, the measurements with a fluoride electrode are always done below pH 7, which circumvents this interference. As shown later, the consideration of ionic and/or electronic conductivity of the membrane plays a critical role also in the design of the internal contact in nonsymmetric potentiometric sensors. 

Fig. 5.1. Enthalpies of formation AH 1, for barium and lanthanum fluorides and iodides experimental values for BaX2 and LaX3, otherwise calculated values as explained in text.

---