Electrode, membrane: calcium fluoride

For more than the last half century experiments have been carried out to obtain electrodes with solid membranes containing inorganic insoluble compounds which the authors expect (consciously or not) to have ion-exchange properties (for a review, see [57]). These substances, for example insoluble cyanoferrates(II), phosphomolybdates, calcium fluoride, etc., are not, however, promising materials and probably only generate problems for the editors of specialized journals. 

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as 02, C02, CO, NO, N02 and S02, and enzyme electrodes. These electrodes fall beyond the scope of this book and are not discussed further. 

Early electrode systems. The first type of electrode used for calcium determination was an amalgam electrode which consisted of an amalgam of calcium oxalate and either lead or silver oxalate. Later, thin crystals of naturally occurring fluorite (calcium fluoride) were used as membranes for the potentiometric determination of calcium in pure aqueous solutions and in milk. Other early developments included a series of studies using membranes produced from pure paraffin containing calcium oxalate and a non-ionic detergent, and attempts to produce a glass electrode for calcium. ... 

Ke and Regier [71] have described a direct potentiometric determination of fluoride in seawater after extraction with 8-hydroxyquinoline. This procedure was applied to samples of seawater, fluoridated tap-water, well-water, and effluent from a phosphate reduction plant. Interfering metals, e.g., calcium, magnesium, iron, and aluminium were removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8). A buffer solution (sodium nitrate-l,2-diamino-cyclohexane-N,N,N. AT-tetra-acetic acid-acetic acid pH 5.5) was then added to adjust the total ionic strength and the fluoride ions were determined by means of a solid membrane fluoride-selective electrode (Orion, model 94-09). Results were in close agreement with and more reproducible than those obtained after distillation [72]. Omission of the extraction led to lower results. Four determinations can be made in one hour. 

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