Electrode fluoride-sensitive

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

Perbromic acid and perbromates are most readily assayed by determination of their oxidizing power after reduction with hydrogen bromide, as described earlier in this article. Traces of fluoride in the acid or salts may be determined potentio-metrically, using a fluoride-sensitive electrode (Orion Research, Inc.) and an expanded-scale pH meter. Acid or alkaline solutions should be neutralized or buffered with acetic acid and sodium acetate before the determination. The electrode response should be calibrated against similar solutions of known fluoride content. 

The indicator electrode for a precipitation titration is often the metal from which the reacting cation is derived. Membrane electrodes that are sensitive to one of the ions involved in the titration process may be used. For example, fluoride-sensitive membrane electrode is used in the determination of the fluoride content of toothpastes. Lanthanum(III) solution is used as a precipitant. 

The fluoride formed was indicated at a fluoride sensitive electrode. The substrate, 4-fluorophenol, has a high reaction rate and a favorable diffusion behavior. The authors observed no interferences by ascorbic acid or uric acid glutathione interfered above 0.87 mg/ml. When immobilized by glutaraldehyde crosslinking with BSA, the enzymes were stable for 30 days. 

This paper describes a study of the aqueous actinide fluoride complexes. Only the Th(IV) data will be discussed. The experiments have been made at 25°C in a 1.00 M HCIO4 ionic medium. For the Th(IV)-fluoride system two experimental methods were used at different temperatures (1) potentiometiy using a fluoride sensitive electrode (at 3, 25 and 47°C) and (2) liquid-liquid extraction with dinonyl naphthalene sulphonic acid (at 10, 25 and 55°C). The experimental methods are briefly described, with no information on primaiy experimental data. However, the work was done in a specialist laboratory and this review therefore accepts the equilibrium constants proposed as listed in Table A-47. The authors have used the temperature variation of the equilibrium constants to deduce the enthalpy and entropy of reaction, but these quantities are very uncertain due to the experimental errors. These quantities for the reaction between Th" and F calculated using the dissociation constant of HF are given in Table A-46. 

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... 

Specific ion electrodes, similar in design to the glass electrode, have been developed to analyze for a variety of cations and anions. One of the first to be used extensively was a fluoride ion electrode that is sensitive to F- at concentrations as low as 0.1 part per million and hence is ideal for monitoring fluoridated water supplies. An electrode that is specific for Cl- ions is used to diagnose cystic fibrosis. Attached directly to the skin, it detects the abnormally high concentrations of sodium chloride in sweat that are a characteristic symptom of this disorder. Diagnoses that used to require an hour or more can now be carried out in a few minutes as a result, large numbers of children can be screened rapidly and routinely. 

The determination of fluoride ions has always been difficult, so the discovery of the lanthanum trifluoride electrode by Frant and Ross in 1966 was a great step forward. Until recently, this was the most important sensor in the ISE field, except for the glass electrode sensitive to hydrogen ions. The extraordinary specificity of this electrode made the greatest contribution to its usefulness. The only important interferent is the hydroxide ion. 

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. 

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. 

Electrodes that are sensitive to the concentration of a particular ion are called ion-selective electrodes, of which the glass electrode for pH measurement is just one example. Glass electrodes can be made sensitive to ions such as Na +, K+, or NH4 + by changing the composition of the glass. Other ions can be detected if an appropriate crystalline solid replaces the glass membrane. For example, a crystal of lanthanum(III) fluoride (LaF3) can be used in an electrode to measure [F-]. Solid silver sulfide (Ag2S) can be used to measure [Ag+] and [S2-]. Some of the ions that can be detected by ion-selective electrodes are listed in Table 11.2. 

Use of modified gold electrodes is not the only approach to achieve cytochrome c electrochemistry. Indeed, a number of studies have been reported on a variety of electrode surfaces. In 1977, Yeh and Kuwana illustrated (23) well-behaved voltammetric response of cytochrome c at a tin-doped indium oxide electrode the electrode reaction was found to be diffusion-controlled up to a scan rate of 500 mV sec Metal oxide electrodes were further studied (24, 25) independently in Hawkridge and Hill s groups. The electrochemical response of cytochrome c at tin-doped indium oxide and fluoride-doped tin oxide was very sensitive to the pretreatment procedures of the electrode surface. At thin-film ruthenium dioxide electrodes, variation of the faradaic current with pH correlating with the acid-base protonation of the electrode surface was observed. 

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/ ... 

Uchiyama et al. (1987,1988a) immobilized squid nerve tissue in front of a fluoride ion sensitive electrode. The tissue contains diisopropyl fluorophosphatase, the activity of which was used to measure diisopropyl fluorophosphate. The sensor was stable for 18 days. 

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