Fluoride Electrode Methods

These are indirect methods based around computational procedures, using estimates of the monomeric inorganic aluminium fraction and free and total fluoride concentrations. Normally, free fluoride is determined by direct measurement with an ion-selective electrode. The total fluoride concentration is determined using the same ion-selective electrode technique, but after the addition of a total ionic strength buffer (TISAB). LaZerte [177] 

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Separation methods The methodology for their separation often consists of fractionation methods (see Section 3.3.4) and chromatographic methods of the soluble species. Common detection methods are AAS, ICP-OES, and fluoride electrode methods. 

Figure 2. Fluorohydrolase activity as a function of pH for dimethyl pimelimidate crosslinked CCMP. Activity was measured using 5 mM PMSF as the substrate at 30°C. Each activity is the average of two separate measurements obtained by HPLC and fluoride electrode methods described in the text. 

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. 

The fluorine titration of chromium oxide was carried out at the Elf-Atochem Research Center, Pieire-Benite.The catalyst mineralization was carried out in a Parr bomb by reaction with sodium peroxyde. Fluorine ions were then titrated by a potentiometric method with a specific fluoride electrode. 

The final laboratory report favoured the use of the fluoride electrode for routine determinations and the alizarin-complexone procedure as an alternative or referee method. These two showed the most consistent agreement during the analysis of a variety of water samples from rivers, wells and reservoirs, the former giving 100% recoveries with spiked samples. 

The selectivity of the fluoride electrode is usually superb - perhaps the best for any of the solid-state ISEs. The reason why hydroxide ought to be avoided in tandem with a fluoride ISE is because the OH and F ions have a very similar ionic radius, and so move through the doped LaF3 lattice with a similar velocity and activation energy, i.e. the OH and F" ions are virtually indistinguishable by this method. 

Elemental composition Zn 63.24%, F 36.76%. ZnF2 may be characterized from its x-ray and other physical properties. The water of crystaUization in the tetrahydrate may be determined by thermogravimetric method. A small amount of compound is dissolved in water (anhydrous salt is very slightly soluble in water) and analyzed for fluoride ion by the electrode method or by ion chromatography. A diluted acid solution of the compound is analyzed for zinc by various instrumental methods (See Zinc). 

S. Esala, E. Vuori, L. Niinisto, Determination of nanogram amounts of fluorine in breast milk by ashing-diffusion method and the fluoride electrode, Mikrochim. Acta I (1983) 155-165. 

L. forma, Collaborative study of ion selective electrode method for the direct determination of fluoride in feeds, J. AOAC 58 (1975) 477-481. 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

Vickery and Vickery [9] have investigated the interference by aluminium and iron in the ion-selective electrode method for the determination of fluoride in plant extracts. They demonstrated that plant ashes may contain sufficient of these two elements to seriously interfere in the determination of fluoride when using the fluoride-selective electrode. In the presence of these metals, the known additions method gives erroneous results, as did that involving the attempted formation of complexes with ethylene diamine tetraacetic acid (disodium salt) or 1,2-cyclohexylenedinitrilotetraacetic acid. 

A method for measuring samples as small as 10 juL by pressing a combination fluoride electrode on confined spot test filter paper (S S Yagoda 211-Y) has been described (24). An adaptation of this technique may permit measurements with as little as 10 juL of remoistening solution in our application. 

Fluoride ion electrode method, internal standard, dilutions at 0.1, 0.02, 0.01. Colorimetric method with internal standard, dilution at 0.004. 

Laboratory analysis of drinking water may be required to assess possible fluoride excess in natural well waters and may also be necessary during incidents of failure of the equipment used to treat drinking water. The determination of fluoride in urine can be used to assess exposure to different sources of fluoride. For drinking water and urine, direct determination using a fluoride-specific electrode is employed. For food, feces, and tissue, prior separation of fluoride from the sample matrix is required using a Conway diffusion procedure. The combination of the fluoride-electrode with flow injection has allowed a rapid and sensitive method to be used for serum and urine fluoride analysis/ ... 

Determination of total fluoride emissions from stationary sources—SPADNS zirconium lake method Determination of total fluoride emissions from stationary sources—specific ion electrode method Determination of fluoride emissions from potroom roof monitors for primary aluminum plants Determination of total fluoride emissions from selected sources at primary aluminum production facflities Determination of hydrogen sulfide, carbonyl sulfide, and carbon disulfide emissions from stationary sources Determination of total reduced sulfur emissions from sulfur recovery plants in petroleum refineries Semicontinuous determination of sulfur emissions from stationary sources Determination of total reduced sulfur emissions from stationary sources (impinger technique)... 

With the advent of good specific ion electrodes, methods were developed to liberate the bound fluorine and directly measure the fluoride concentration. The reagent sodium-biphenyl followed by oxidation with hydrogen peroxide is used to liberate the organically bound fluorine in fluorouracil. A fluoride specific ion electrode is used, in conjunction with a high-ionic-strength-buffer solution, for direct measurement of the liberated fluoride (22). 

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. 

Ion-selective electrodes have a role for monitoring milk both for its calcium content and as a possible vehicle for health fluoridation programmes. The ion-selective electrode method appears to give higher ionic calcium levels (< 2.71 mmol dm" ) than the method of EDTA titration of ion-exchange eluates ( 2.52 mmol dm ) possibly because of the smaller pH changes in the former case. However, in passing from raw milk to sterilised milk ( 2.27 mmol dm ) and pasteurised milk ( 2.04 mmol dm" ) there is a fall in the ionic calcium level [275]. 

Fluoride in dentrifices is usually present as sodium fluoride or monofluoro-phosphate or tin(II) fluoride [192,313,341,358,359]. Analysis of aqueous extracts or slurries of such dentrifices take less than 10 min with the fluoride electrode and closely match those obtained by a diffusion—thorium(IV) titration method but which requires about 45 min per sample [358]. 

Determination of Extent of Reaction of 3 - and 5 -FSBA with Proteins. There are three relatively simple methods to detect the extent of reaction of these adenosine analogs with proteins. First, the release of fluoride ion from the compound can be followed with a specific fluoride electrode during incubation with a known amount of protein. Such data can be used to estimate the number of sites on the protein that have been modified, providing appropriate corrections are made for any reaction between the compounds and the buffer under the same conditions. Second, the extent of incorporation of sulfonylbenzoyladenosine groups can be approximated from the difference in absorbance at 259 nm between the isolated modified enzyme and native enzyme, using the extinction coefficient of free 3 - or 5 -FSBA at that wavelength. Third, and most directly, the number of moles of reagent incorporated per mole of protein can be determined with radioactive 3 - or 5 -FSBA. 

Before the introduction of ion-selective fluoride electrode, measuring fluoride concentration was quite a difficult task. Fewer methods are available for this than for the analysis of other halides. 

The limited number of well functioning, classical or spectrophotometric methods is available for measuring fluoride ion concentration in different samples. Therefore, after the invention of lanthanum fluoride crystal-based ISE [15], its use as a detector in standardized methods becomes almost general. For example, the Environmental Protection Agency (ERA) METHOD 9214 [44] is for measuring the concentration of fluoride ions in water samples as well as in soil extracts. It is a direct potentiometric method using the ion-selective fluoride electrode and the conventional or double junction reference electrode. The lower limit of detection is 0.025 mg dm. Fluoride concentrations from 0.025 to 500 mg dm can be measured. 

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. 

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