Fluoride electrode

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

Ion hopping is a familiar concept in the chemistry of solid-state conductors, e.g. in the semiconductor industry. In the fluoride electrode, fluoride vacancies in.side the solid LaF lattice allow for conduction of charge (see Figure 3.11), in turn registered by the electrode as a potential. The emf is zero if the internal and external solutions are the same because the same numbers of fluoride ion enter the crystal from either face. 

WARNER AND HOOVER Measurement of Sea-Salt Aerosols 71 Table III. Response of Chloride Electrode with Ion-Selective Reference Electrode (Fluoride) ... 

Metal wire electrodes Fluoride, chloride and oxide wire electrodes... 

In the cases where hydrogen ions are released or consumed upon introduction of the gas into the aqueous phase, pH glass electrodes can be used to indicate the change in pH of the test solution. In the last three cases, ion-selective electrodes (fluoride and silver-selective electrodes) can be employed. 

Graphite reacts with alkali metals, for example potassium, to form compounds which are non-stoichiometric but which all have limiting compositions (for example K C) in these, the alkaU metal atoms are intercalated between the layers of carbon atoms. In the preparation of fluorine by electrolysis of a molten fluoride with graphite electrodes the solid compound (CF) polycarbon fluoride is formed, with fluorine on each carbon atom, causing puckering of the rings. 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

Representative Method Ion-selective electrodes find application in numerous quantitative analyses, each of which has its own unique considerations. The following procedure for the analysis of fluoride in toothpaste provides an instructive example. 

Fluoride-free toothpaste is added as a precaution against any matrix effects that might influence the ion-selective electrode s response. This assumes, of course, that the matrices of the two toothpastes are otherwise similar. 

The titration of AF+ with E in a water-acetonitrile mixture is followed potentiometrically using a fluoride ion-selective electrode. The cumulative formation constant for AlEs is determined from the titration curve. 

The following data were collected for the analysis of fluoride in tap water and in toothpaste, (a) For the analysis of tap water, three 25.0-mL samples were each mixed with 25.0 mL of TISAB, and the potential was measured with an F ISE relative to a saturated calomel electrode. Five 1.00-mL additions of a standard solution of 100.0-ppm F were added to each, measuring the potential following each addition. 

Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Barium fluoride is used commercially in combination with other fluorides for arc welding (qv) electrode fluxes. However, this usage is limited because of the availabiUty of the much less expensive naturally occurring calcium fluoride. 

The direct deterrnination of fluoride using ion-selective electrodes has allowed analysis of fluorspar without the tedious distillation step (see... 

Eluorspar assay may be completed by fluoride determination alone, because the mineralogical grouping rarely iacludes fluorine minerals other than fluorite. Calcium can be determined as oxalate or by ion-selective electrodes (67). SiUca can be determined ia the residue from solution ia perchloric acid—boric acid mixture by measuriag the loss ia weight on Aiming off with hydrofluoric acid. Another method for determining siUca ia fluorspar is the ASTM Standard Test Method E463-72. 

The JnitedSfates Pharmacopeia (76) specifications for sodium monofluorophosphate require a minimum of 12.1% fluoride as PO F (theoretical 13.2%) and a maximum of 1.2% fluoride ion reflecting unreacted sodium fluoride. Analysis for PO F is by difference between total fluoride ia the product less fluoride ion as determined by a specific ion electrode. The oral LD q of sodium monofluorophosphate ia rats is 888 mg/kg. 

Other limitations of electrochemical fluorination ate that compounds such as ethers and esters ate decomposed by hydrogen fluoride and cannot be effectively processed. Branching and cross-linking often take place as a side reaction in the electrochemical fluorination process. The reaction is also somewhat slow because the organic reactant materials have to diffuse within 0.3 nm of the surface of the electrode and remain there long enough to have all hydrogen replaced with fluorine. The activated fluoride is only active within 0.3 nm of the surface of the electrode. 

Operation and Control. Control of a chromium phosphate conversion coating bath requires monitoring chromium and aluminum concentrations, active fluoride level, and temperature. Coating weight is very sensitive to active, ie, uncomplexed, fluoride. An innovative electrochemical method using a siHcon electrode (25) is employed for measuring active fluoride. A special precaution in chromium phosphate bath operation is the... 

Fluoride. A fluoride concentration of ca 1 mg/L is helpful in preventing dental caries. Eluoride is deterrnined potentiometrically with an ion-selective electrode. A buffer solution of high total ionic strength is added to the solution to eliminate variations in sample ionic strength and to maintain the sample at pH 5—8, the optimum range for measurement. (Cyclohexylenedinitrilo)tetraacetic acid (CDTA) is usually added to the buffer solution to complex aluminum and thereby prevent its interference. If fluoroborate ion is present, the sample should be distilled from a concentrated sulfuric acid solution to hydrolyze the fluoroborate to free fluoride prior to the electrode measurement (26,27). 

Chlorine and fluorine in beryUium metal are isolated by pyrohydrolysis or by distUlation (21). Fluoride and chloride in the condensate are determined by ion-selective electrode or colorimetricaUy. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

The availability of lon-selecti ve electrodes has revolutionized the field of analytical chemistry The fluoride lon-selective electrode has greatly assisted in the task of... 

There are two types of fluoride lon-selective electrodes available [27] Onon model 96-09-00, a combination fluoride electrode, and model 94-09-00, which requires a reference electrode The author prefers to use Onon model 94-09-00 because it has a longer operational life and is less expensive When an electrode fails, the reference electrode is usually less expensive to replace The Fisher Accumet pH meter, model 825 MP, automatically computes and corrects the electrode slope It gives a direct reading for pH, electrode potential, and concentra tion in parts per million The fluoride lon-specific electrode can be used for direct measurement [2S, 29] or for potenPometric titration with Th" or nitrate solutions, with the electrode as an end point indicator... 

Direct analysis with the fluoride lon-selective electrode requires addition of total ionic strength adjustor buffer solution (TISAB) to the standard and to unknown samples Some advantages of this addition are that it provides a constant background ion strength, ties up interfenng cations such as aluminum or iron, which form a complex with fluoride ions, and maintains the pH between 5 0 and 5 5 According to the manufacturer s claim, reproducibility of direct electrode measurement IS 2 0%, and the accuracy for fluonde ion measurement is 0 2% [27]... 

Orion Research Inc Instruction Manual, Fluoride Electrodes, Orion Cambridge, 1990... 

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