Fluoride-sensing systems

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. 

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]. 

Method A has been successfully applied in anion sensing. As mentioned previously, lanthanide ions can coordinate to anions such as acetate, fluoride, etc., which modulates the lanthanide luminescence. Examples of such systems will now be discussed. 

Yamaguchi S, Akiyama S, Tamao K. Colorimetric fluoride ion sensing by boron-containing -electron systems. J Am Chem Soc 2001 123 11372-5. 

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. 

Dealing with nomenclature issues, it is worthwhile to call attention to an incorrect practice in the composite community the use of the term phase instead of fcomponent . The term phase is very well defined in thermodynamics. Phase is frequently used in polymer physics to describe the various phases in one-component systems. Let us mention only poly(vinylidene fluoride) (PVDF) exhibiting five crystalhne polymorphic modifications (phases) and one amorphous phase, but PVDF is still a one-component system. The misuse of the term phase instead of component would require the definition of another term to describe the phase in the sense of thermodynamics. In this book, and particularly in the present chapter, care was taken to avoid such misuse. 

K. A. Sense et al., Vapor Pressure and Equilibrium Studies of the Sodium Fluoride-Beryllium Fluoride System, US.VEC Report BMI-1186, Battelle Memorial Institute, May 27, 1957. 

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