Potentiometric sensors fluoride electrode

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. 

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. 

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. 

Most potentiometric anion-selective electrodes are based on anion exchangers such as quaternary ammonium salts. The selectivity pattern of these sensors correlates with anion lipophilicity. Highly hydrated anions such as fluoride, bicarbonate and chloride are difficult to monitor due to significant interference from more lipophilic anions like perchlorate, salicylate and nitrate which may be present in the analyzed sample. The anions can be classified according to their lipophilicity resulting in the classical Hofmeister series (ClOi > SCN > salicylate > I > NOJ > Br >... 

Ion-selective electrodes (ISEs) are potentiometric sensors that include a selective membrane to minimize matrix interferences. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the H concentration in a solution. Other parameters that can be measured include fluoride, bromide, nitrate, and cadmium, and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen. ISEs do have their limitations including lack of selectivity and sensitivity and problems connected with conditioning of electrodes. Detection limits for nitrate-N, for example, are typically 0.098mgl for commercial field devices and have chloride as a major interferent. 

The accepted by EPA potentiometric methods involve selective electrodes for fluorides, cyanides, nitrates, ammonia, and sulfides detection [20]. The potentiometric characteristics of the anion-selective sensors are strongly dependent on the anion receptor design and properties [25]. At this time, molecular recognition of anions by synthetic receptors is an expanding research area. 

---