Lanthanum fluoride electrode

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). 

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. 

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. 

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 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. 

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. 

A fluoride electrode, in which the membrane is a single crystal of lanthanum fluoride doped with europium to increase the conductivity, is one of the best ion-selective electrodes available. Conduction through the membrane is facilitated by the movement of F" ions between anionic lattice sites which in turn is influenced by the F ion activities on each side of the membrane. If the electrode is filled with a standard solution of sodium fluoride, the membrane potential is a function of the fluoride activity in the sample solution only. Thus,... 

Ellis, A. J. The effect of temperature on the ionization of hydrofluoric acid. J. Chem. Soc., 4300-4304 (1963). Vanderborgh, N, E. Evaluation of the lanthanum fluoride membrane electrode response in acidic solutions. Talanta 15, 1009-1013 (1968). 

An important practical example of the second case is the lanthanum fluoride single-crystal electrode, permeable to fluoride ion but not to cations. Construct a cell... 

One of the most important uses of lanthanum compounds is in carbon arc lamps. In a carbon arc lamp, an electrical current is passed through the lamp electrode. The electrode is made of carbon and traces of other materials that have been added. The current causes the carbon to heat up and give off a brilliant white light. The exact color of the light depends on the other materials that have been added to the carbon. Lanthanum fluoride (Laf3) and lanthanum oxide (La203) are usually used for this purpose. 

A crystalline electrode for fluoride ion is available from commercial sources. The membrane consists of a slice of a single crystal of lanthanum fluoride that has been doped with europium(II) fluoride to improve its conductivity. The membrane, supported between a reference solution and the solution to be measured, shows a theoretical response to changes in fluoride ion activity from 10 to 10 M. The electrode is selective for fluoride ion over other common anions by several orders of magnitude only hydroxide ion appears to offer serious interference. 

Potentiometric sensors for analysis of solutions were enabled by the development of the glass electrode, and its coupling to a volt meter for measuring pH. This work was later extended to ion-selective electrodes using membrane materials such as Lanthanum Fluoride and valinomycin/Polyvinyl Chloride. 

The construction of these electrodes is shown in Figure 13.12. The most successful example is the fluoride electrode. The membrane consists of a single crystal of lanthanum fluoride doped with some europium(II) to increase the conductivity of the crystal. Lanthanum fluoride is very insoluble, and this electrode exhibits Nerstian response to fluoride down to 10 M and non-Nerstian response down to 10 M (19 ppb ). This electrode has at least a 1000-fold selectivity for fluoride ion over chloride, bromide, iodide, nitrate, sulfate, monohydrogen phosphate, and bicarbonate anions and a 10-fold selectivity over hydroxide ion. Hydroxide ion appears to be the only serious interference. The pH range is limited by the formation of hydrofluoric acid at the acid end and by hydroxide ion response at the alkaline end a pH range of 4 to 9 is claimed... 

The ion selective membrane electrode (the membrane is of lanthanum fluoride) is becoming more widely used for the determination of fluorides at concentrations > 0.02 mg 1 [13, 14, 43-46]. 

Further procedures are based on a discoloration of organic dyes with zir-conium(IV) salts (zirconium-eriochromcyanine R), leading to the formation of a blue fluoride complex with trivalent lanthane and alizarine complexone, or they depend on the measurement with a fluoride ion selective electrode based on lanthanum fluoride [18]. 

A lanthanum fluoride membrane electrode was used to measme free fluoride concentrations in test solutions that contain mixtmes of NaF, HF or HNO3 and Th(N03)4 in... 

Lanthanum fluoride, Lal- u is a nearly ideal substance for the preparation of a crystalline membrane electrode for the determination of fluoride ion. Although this compound is a natural conductor, its conductivity can be enhanced by doping with europium fluoride, LuF,. Membranes are prepared by cutting disks from a single crystal of the doped compound. 

The magnitude of the charge depends on the fluoride ion concentration of the solution. Thus, the side of the membrane in contact vith the lower fluoride ion concentration becomes positive with respect lo the other surface. This charge produces a potential difference that is a measure of the difference in fluoride concentration of the two solutions. The potential ofa cell containing a lanthanum fluoride electrode is given by an equation analogous to Hquaiion 2.1-l.T That is,... 

Commercial lanthanum fluoride electrodes come in various shapes and si/.es and are available from several sources. Most are rugged and can he used at teniper-aiure.s between O and 80 C The response of Ihe fluoride eleclrode is linear down lo 10 M (0.02 ppm), where the solubility of lanthanum fluoride begins to contribute to the concentration of fluoride ion in the analyte solution. The only ion that interferes directly with fluoride measurements is hydroxide ion this interference becomes serious at pH > 8. At pH 5, hydrogen ions also interfere in total fluoride determinations. Under these conditions, undissociaied hydrogen fluoride forms, and the eleclrode does not re.spond to... 

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... 

Glass electrodes have been standard items of laboratory equipment for about forty years, but the present widespread interest in ion-selective electrodes stems from the commercial availability in 1966 of the fluoride solid-state lanthanum fluoride electrode. Ion-selective electrodes are now commercially available for about twenty common anions and cations [1]. Their development has coincided with the demand for convenient methods of ion determinations in the biomedical field, stimulated by the successes of the fluoride electrode in determining trace fluoride in water fluoridation and dental health programmes. 

Liquid junction problems can be avoided by using as reference a solid-state lanthanum fluoride membrane electrode in conjunction with spiked fluoride samples [9j. On details, Moore [46,53] has highlighted the need for careful sampling and subsequent processing by his studies on technique, selectivity, calibration, sample freezing and pH. 

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