Fluoride ions potential systemic effects

R. D. Shannon and C. T. Prewitt, Effective ionic radii in oxides and fluorides, Acta Crystallogr. Sec. B 25 925 (1969) Revised values of effective ionic radii, Acta Crystallogr. Sec. B 26 1046(1970). The fact that crystallographic ionic radii are the same as unsolvated ionic radii in aqueous solution is shown by G. Sposito, Distribution of potentially hazardous trace metals. Metal Ions Biol. Systems 20 1 (1986). 

A second class of chemical compounds whose reactions are catalyzed by the fluoride ion, and by CsF are organosilanes (42). The diversity of chemistry exhibited by these compounds is considerable, and with the potential for expanded valence in these compounds to five and six coordinate intermediate anions, the application of the salt/molecule technique was suggested. The gas phase ion/molecule reactions of several silane systems have been investigated through ion cyclotron resonance techniques (53,54), providing thermochemical information about the product ions in these systems. Perhaps the simplest reaction is that of F with SiFi, the intermediate anion SiFs has been stabilized under carefully controlled circumstances (55), while the 2 1 adduct SiFe is well known ( ). Less is known about the effects of alkylation on the stability of the product ions. 

The standard potential of the indicator system is not known exactly, but experiments have shown that in not too strongly acid solutions the sharp color change from colorless to violet, with green as a possible intermediate, occurs at a potential of about — 0.75 volt. The standard potential of the ferrous-ferric system is 0.78 whereas that of the di-chromate-chromic ion system in an acid medium is approximately — 1.2 volt hence a suitable oxidation-reduction indicator might be expected to have a standard potential of about — 0.95 volt. It would thus appear that diphenylamine would not be satisfactory for the titration of ferrous ions by acid dichromate, and this is actually true if a simple ferrous salt is employed. In actual practice, for titration purposes, phosphoric acid or a fluoride is added to the solution these substances form complex ions with the ferric ions with the result that the effective standard potential of the ferrous-ferric system is lowered (numerically) to about — 0.5 volt. The change of potential at the end-point of the titration is thus from about — 0.6 to — 1.1 volt, and hence diphenylamine, changing color in the vicinity of — 0.75 volt, is a satisfactory indicator. 

On the other hand, for measurements done at constant temperature (850 °C), Dp and Dpi continually decrease monotonously with the addition of CaF2. These results clearly show the effects of the composition and of the temperature on the self-diffusion. Such observations have been already described for fluorine self-diffusion coefficients in other fluoride systems [14,15]. Nevertheless, the fact that lithium is significantly affected by the composition was not seen in LiF-KF melts. Potassium and lithium cations have close ionic radii and have the same valence. Their ionic potentials corresponding to the polarizing strength of the ions... 

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