Fluoride Sensing

More recently, it was demonstrated that the thermistor approach could be used to monitor specific interactions of fluoride ions with silica-packed columns in the flow injection mode. A thermometric method for detection of fluoride [56] was developed that relies on the specific interaction of fluoride with hydroxyapatite. The detection principle is based on the measurement of the enthalpy change upon adsorption of fluoride onto ceramic hydroxyapatite, by temperature monitoring with a thermistor-based flow injection calorimeter. The detection limit for fluoride was 0.1 ppm, which is in the same range as that of a commercial ion-selective electrode. The method could be applied to fluoride in aqueous solution as well as in cosmetic preparations. The system yielded highly reproducible results over at least 6 months, without the need of replacing or regenerating the ceramic hydroxyapatite column. The ease of operation of thermal sensing and the ability to couple the system to flow injection analysis provided a versatile, low-cost, and rapid detection method for fluoride. 

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Fig. 20 (a) Fluoride sensing with coumarin leuko dye 52 and (b) time-dependent spectroscopic response of the reaction TIPS = triisopropylsilyl group. (Reprinted in part with permission from [143], Copyright 2003 Wiley-VCH)... 

Figure 4 Fluoride sensing with luminescent triaiylboranes...

A receptor based on Co° (bpy)3 has been used by Sessler and coworkers for redox fluoride sensing [34]. In the cyclic voltammetry of 47 in DMSO solution addition of fluoride led to a complete disappearance of the Co(II)/(III) reduction wave. Addition of water to this solution restored this redox process, suggesting that the presence of a strongly bound fluoride anion renders the complex redox inactive. Chloride and dihydrogenphosphate produced cathodic shifts of 160 mV and 70 mV, respectively. It is proposed that... 

Nicolas. M. Fabre. B. Simonet, J. Boronate-function- 121. alized polypynole as a new fluoride sensing material. 

Fig. 12.10 Schematic representation of fluoride sensing by receptor 21 as achieved using an FDDA assay...

The most advantageous features of this type of sensor are that it operates at room temperature, and a fast response. Rare-earth fluorides, which are used for fluoride sensing... 

In the presence of fluorine anions, low-saturated (5 pM) solutions of one of the pyr-azoles, namely, 3-amino-5-(5-phenylpyrrol-2-yl)pyrazole-4-carboxamide, intensively fluoresce in blue region 424 nm, quantum yield 0.74), while other anions (Cl", Br, I, HSO4, H2PO4, AcO) do not affect the emission parameters [579]. The results indicate this compound is suitable for fluoride sensing by a naked eye detection. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

Selective removal of the less noble constituent has been demonstrated by chemical analysis in the case of nickel-rich alloys in fused caustic soda or fused fluorides ", and by etching effects and X-ray microanalysis for Fe-18Cr-8Ni steels in fused alkali chlorides. This type of excessive damage can occur with quite small total amounts of corrosion, and in this sense its effect on the mechanical properties of the alloy is comparable with the notorious effect of intercrystalline disintegration in the stainless steels. 

There are a few species in which the central atom violates the octet rule in the sense that it is surrounded by two or three electron pairs rather than four. Examples include the fluorides of beryllium and boron, BeF2 and BF3. Although one could write multiple bonded structures for these molecules in accordance with the octet rule (liable 7.2), experimental evidence suggests the structures... 

In the area of ion sensing, cation recognition by electrodes containing functionalized redox-active polymers has been an area of considerable interest. Fabre and co-workers have reported the development of a boronate-functionalized polypyrrole as a fluoride anion-responsive electroactive polymer film. The electropolymerizable polypyrrole precursor (11) (Fig. 11) was synthesized by the hydroboration reaction of l-(phenylsulfonyl)-3-vinylpyrrole with diisopinocampheylborane followed by treatment with pinacol and the deprotection of the pyrrole ring.33 The same methodology was utilized for the production of several electropolymerizable aromatic compounds (of pyrrole (12) (Fig. 11), thiophene (13 and 14) (Fig. 11), and aniline) bearing boronic acid and boronate substituents as precursors of fluoride- and/or chloride-responsive conjugated polymer.34... 

Tan W, Zhang D, Zhu D (2007) 4-N-Methyl-N-(2-dihydroxyboryl-benzyl) amino benzoni-trile and its boronate analogue sensing saccharides and fluoride ion. Bioorg Med Chem Lett 17(9) 2629-2633... 

B.D. MacCraith, V. Ruddy and S. McCabe, Remote flammable gas sensing using a fluoride fiber evanescent probe, Proc. SPIE, 1267 (1990) 43. 

It also appears, as we shall discuss presently, that some macromolecules, such as polyvinylidene fluoride noted above, have exceptional interaction properties, in which the segments may behave rather differently than the chemical monomer units would imply. For these and many related reasons, the studies of E. Helfand at Bell Laboratories on theoretical concepts of interfaces in polyphase systems of macromolecules give us a keen sense of the scope of future discoveries that are possible in this field. 

The sensing of fluoride with a reaction-based indicator can be based on its unique reactivity with silicon. As shown in Fig. 20, the O-Si bond cleavage with fluoride can be used to produce the coumarin dye 53. For this system, a 100-fold sensitivity increase was seen after incorporation of the reaction scheme into a conjugated polymer [143]. 

In Sect. 5.2, the strong 7r-donor ability of methoxide and fluoride has been elaborated. These two ligands effect a push-pull effect on the nitrosonium ion bound in Os(OEP)NO(OMe) [31c] and Os(OEP)NO(F) [3Id], as indicated by the low NO-stretching frequencies of the NO ion as compared with the dinitrosyl Os(OEP)(NO)2 [31e] and the perchlorato complex, 0s(0EP)N0(0C103) ([3If], Table 11). Thus, the a/7r-donor balance for the coordinated anions decreases in the series OMe > F > NO > OClOf. [31c] and [3Id] can be vaporized at 200°C/10-6 Torr in a mass spectrometer, while the dinitrosyl [31e] decomposes above 100 °C. This demonstrates the push-pull effect also in a chemical sense. 

The selectivity of the fluoride electrode is 1000 1 in the absence of OH , where the selectivity is a measure of how precise the measurement is. Consider an ISE that is monitoring the concentration of the anion X if the response of an ISE is such that nine out of every ten anions it detects is an X ion and the tenth ion is different, then the selectivity is 9 1. Clearly, such a selectivity is dreadful since any measurement can be up to 11% out common sense thus dictates that the selectivity should be maximized. 

The selectivities and activities of this reaction were greatly improved by running the catalysis solvent-free and by adding cocatalytic amounts of Schwesinger-type fluorides ( naked fluoride , e.g. phosphacenium-fluoride-P2 13) [14]. The fluoride effect is pronounced both in terms of activity and enantioselectivity (even inverting the sense of enantioselection in one example see Table 6.1). 

The solution pH affects the function of all ISEs, either through interference of hydroxonium or hydroxide ions in the membrane reaction (for example the interference of OH" with the function of fluoride ISEs at pH values greater than about 5.5), or through chemical interference in solution (for example, formation of poorly dissociated HF and HF in acid solutions, which are not sensed by a fluoride ISE or formation of HS and H2S with decreasing pH, which are not sensed by a sulphide ISE), or both. Moreover, the pH value can affect the equiUbria of interferents in the solution. The pH must thus be adjusted with all these effects in mind. Fortunately, it is usually sufficient to maintain the pH within a certain region rather than at a single precise value. 

Potentiometric titration is actually a form of the multiple known subtraction method. The main advantage of titration procedures, similar to multiple addition techniques in general, is the improved precision, especially at high determinand concentrations. ISEs are suitable for end-point indication in all combination titrations (acid-base, precipitation, complexometric), provided that either the titrand or the titrant is sensed by an ISE. If both the titrant and the titrand are electro-inactive, an electrometric indicator must be added (for example Fe ion can be titrated with EDTA using the fluoride ISE when a small amount of fluoride is added to the sample solution [126]). 

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