HYDROFLUORIC ACID. (H. F.)

Syn. Hydric Fluoride , Hydrogen Fluoride. Prep. 1. From fluor-spar (free from silica and metallic sulphides) and oil of vitriol. The fluor-spar being reduced to fine powder and placed in a leaden retort, is mixed with twice its weight of concentrated oil of vitriol, and on applying heat, an acid and highly acid vapour distils over, which condenses to a liquid if pa ed into a receiver of the same metal, standing in a freezing mixture at a temperature of 4° Fahr. Louyet has shown that the liquid acid, obtained as above, is not (as once believed) anhydrous. 2. From the double fluoride of pota ium and hydrogen. Fremy first renders the salt anhydrous by careful drying; and by the subsequent application of a strong heat, expels the equivalent of hydrofluoric acid contained in it; condensing it into a colourle , mobile, very volatile liquid by the application of a freezing mixture of ice and salt. 3. By decomposing plumbic fluoride by dry hydrogen. Prop. The strong, aqueous, hydrofluoric acid obtained by the action of oil of vitriol on fluor-spar, is a densely fuming, volatile, colourle liquid, which boils at 60° F., and remains unfrozen at 4°. It combines with water so greedily, and evolves so much heat in doing so, as to give rise to a hi ing noise like that produced when a red-hot iron is plunged into cold water. In a concentrated form it has a specific gravity of 1·060. Brought into contact with animal matter of any kind it instantly destroys it, the smallest drop on the skin producing a deep and painful wound; hence the nece ity of the greatest care in its preparation. With the exception of platinum, gold, silver, mercury and lead, hydrofluoric acid, when diluted, di olves the metals, the metal when it undergoes solution, displacing hydrogen. Pota ium decomposes the strong acid with explosion. In both the gaseous and fluid form hydrofluoric acid is largely consumed for etching on gla ; and this property constitutes one of its most available and reliable tests. The test may be conveniently applied as follows:— Cover a small piece of window gla or a watch gla with a thin layer of wax, scraping away a very small portion by means of a sharply pointed instrument, and then expose the gla for a short time to the vapour of the acid, given off when the materials are heated in a small leaden saucer or platinum crucible; on removing the wax with a little turpentine, the marks on the gla caused by the hydrofluoric acid will be distinctly perceived.