FLUORINE, a gaseous chemical element of the halogen group having a greenish-yellow colour and a choking smell (symbol F, atomic number 9, atomic weight 19.0o) . It is never found in the uncombined condition, but in combination with calcium as fluorspar it is widely distributed ; it is also found in cryolite in fluor-apatite, and in minute traces in sea-water, in some mineral springs, and as a constituent of the enamel of the teeth. It was first isolated by H. Moissan in 1886 by the electrolysis of pure anhydrous hydrofluoric acid con taining dissolved potassium fluoride. The U-shaped electrolytic vessel and the electrodes are made of an alloy of platinum-iridium, the limbs of the tube being closed by stoppers made of fluorspar, and fitted with two lateral exit tubes for carrying off the gases evolved. Whilst the electrolysis is proceeding, the apparatus is kept at a constant temperature of —23°C. by means of liquid methyl chloride. The fluorine, which is liberated as a gas at the anode, is passed through a well-cooled platinum vessel in order to free it from any acid fumes that may be carried over, and fi nally through two platinum tubes containing sodium fluoride to remove the last traces of hydrofluoric acid; it is then collected in a platinum tube closed with fluorspar plates. Fluorine possesses a specific gravity of 1.265 (H. Moissan) ; it has been liquefied, the liquid also being of a yellow colour and boiling at —187°C. It is the most active of all the chemical elements ; in contact with hydrogen, combination takes place between the two gases with explosive violence, even in the dark, and at as low a tem perature as —253°C.; finely divided carbon burns in the gas, forming carbon tetrafluoride ; water is decomposed even at ordi nary temperatures, with the formation of hydrofluoric acid and "ozonized" oxygen; iodine, sulphur and phosphorus melt and then inflame in the gas; from chlorides it liberates chlorine, with which it does not combine, but it combines with most metals in stantaneously to form fluorides. Organic compounds are rapidly attacked by the gas.
Only one compound of hydrogen and fluorine is known, namely hydrogen fluoride or hydrofluoric acid, HF or which was first obtained by C. Scheele in 1771 by decomposing fluorspar with concentrated sulphuric acid, a method still used for the com mercial preparation of the aqueous solution of the acid, the mix ture being distilled from leaden retorts and the acid stored in leaden or gutta-percha bottles. The perfectly anhydrous acid is a very volatile colourless liquid and is best obtained, according to G. Gore, by decomposing the double fluoride of hydrogen and potassium at a red heat in a platinum retort fitted with a platinum condenser surrounded by a freezing mixture, and having a plat inum receiver luted on. It can also be prepared in the anhydrous condition by passing a current of hydrogen over dry silver fluoride. The pure acid thus obtained is a most dangerous substance to handle, its vapour, even when highly diluted with air, having an exceedingly injurious action on the respiratory organs, whilst in halation of the pure vapour is followed by death. The anhydrous acid boils at 19.5°C. (H. Moissan) and melts at —92.3°C. (K. Olszewski). It is a non-conductor of electricity (see ACIDS). Potassium and sodium readily dissolve in the anhydrous acid with evolution of hydrogen and formation of fluorides. The aqueous solution is strongly acid to litmus and dissolves most metals directly; it is a conductor of electricity, but inferior to hydrochloric acid in this respect. Its most important property is that it rapidly attacks glass, reacting with the silica of the glass to form gaseous silicon fluoride, and consequently it is used for etching. T. E. Thorpe determined the vapour density of hydrofluoric acid at different temperatures, and showed that there is no approach to a definite value below about 88°C., where it reaches the value I o• 29 corresponding to the molecular formula HF; at temperatures below 88°C. the value increases rapidly, showing that the molecule is more complex in its structure. The probable existence of is cited by T. M. Lowry as an ex ample of the co-ordination of hydrogen, thus: (HF,)H. The aqueous solution behaves on concentration similarly to the other halogen acids; E. Deussen found the solution of constant boiling point to contain 43.2% HF and to boil at 1 io° (75o mm.).
Fluorine oxide (oxygen fluoride), F,O, according to P. Lebeau and A. Damiens (1927), results as a by-product when fluorine is prepared by the electrolysis of acid potassium fluoride, as in Moissan's method (above). It is less reactive than fluorine and is stable in the presence of water or glass even at high temperatures; it liberates iodine from potassium iodide, and is only sparingly soluble in water, but fairly soluble in alkalis.
The salts of hydrofluoric acid are known as fluorides and are easily obtained by the action of the acid on metals or their oxides, hydroxides or carbonates. The fluorides of the alkali metals, of silver, and of most of the heavy metals are soluble in water ; those of the alkaline earths are insoluble. In many of these solubility relationships fluorides differ from chlorides, bromides and iodides. A characteristic property of the alkaline fluorides is their power of combining with a molecule of hydrofluoric acid and with the fluorides of the more electronegative elements to form double fluorides, a behaviour not shown by other metallic halides. Fluor ides can be readily detected by their power of etching glass when warmed with sulphuric acid ; or by warming them in a glass tube with concentrated sulphuric acid and holding a moistened glass rod in the mouth of the tube, the water apparently gelatinizes owing to the decomposition of the silicon fluoride formed. The atomic weight of fluorine according to Moissan is 19.05, and E. Moles and T. Batuecas, from the vapour density of methyl fluor ide, give 19.0o. For the introduction of fluorine into the benzene nucleus, see DIAzo-COMPOUNDS.
See H. Moissan, Le Fluor et ses composes (i 900) .