fluorine F 2 , which is recognised by the fact that it liberates iodine from KI and easily combines with silicon, forming SiF 4 . This method gives chemically pure fluorine, and is based upon the breaking up of the higher compound—tetrafluoride of lead, PbF 4 , corresponding to PbO 2 , into free fluorine, F 2 , and the lower more stable form—bifluoride of lead, PbF 2 , which corresponds to PbO; that is, this method resembles the ordinary method of obtaining chlorine by means of MnO 2 , as MnCl 4 here breaks up into MnCl 2 and chlorine, just as PbF 4 splits up into PbF 2 and fluorine.
Among the compounds of fluorine, calcium fluoride, CaF2, is somewhat widely distributed in nature as fluor spar,56 whilst cryolite, or aluminium sodium fluoride, Na3AlF6, is found more rarely (in large masses in Greenland). Cryolite, like fluor spar, is also insoluble in water, and gives hydrofluoric acid with sulphuric acid. Small quantities of fluorine have also in a number of cases been found in the bodies of animals, in the blood, urine, and bones. If fluorides occur in the bodies of animals, they must have been introduced in food, and must occur in plants and in water. And as a matter of fact river, and especially sea, water always contains a certain, although small, quantity of fluorine compounds.
Hydrofluoric acid, HF, cannot be obtained from fluor spar in glass retorts, because glass is acted on by and destroys the acid. It is prepared in lead vessels, and when it is required pure, in platinum vessels, because lead also acts on hydrofluoric acid, although only very feebly on the surface, and when once a coating of fluoride and sulphate of lead is formed no further action takes place. Powdered fluor spar and sulphuric acid evolve hydrofluoric acid (which fumes in the air) even at the ordinary temperature, CaF2 + H2SO4 = CaSO4 + 2HF. At 130° fluor spar is completely decomposed by sulphuric acid. The acid is then evolved as vapour, which may be condensed by a freezing mixture into an anhydrous acid. The condensation is aided by pouring water into the receiver of the condenser, as the acid is easily soluble in cold water.
In the liquid anhydrous form hydrofluoric acid boils at +19°,