Instruction in chemical analysis : (Qualitative) / With a preface by Prof. Liebig. Edited by J.L. Bullock.
- Carl Remigius Fresenius
- Date:
- 1846
Licence: Public Domain Mark
Credit: Instruction in chemical analysis : (Qualitative) / With a preface by Prof. Liebig. Edited by J.L. Bullock. Source: Wellcome Collection.
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![HYD110FLU0SILIC1C ACID. r>() [§ 40. § 46. 9. HYDHOFLUOSILICIC ACID. (Si Fl5, FI H.) Preparation.—A mixture of one part of sand and one part of fluor-spar powder is introduced into a glass flask, and six parts of sulphuric acid poured over it; the aperture of the flask is closed with a perforated cork, into which the one end of a bent tube is fitted air-tight. The other limb of the tube is made to pass to the bottom of a tall flat-bottomed glass jar, into which a small amount of mercury is poured sufficient to cover the ex- tremity of the tube to the extent of several lines; the mercury is then covered wdtli four parts of water. The disengagement of fluosikclc gas, which commences even in the cold, is promoted by exposing the flask to a gentle heat, in the sand-bath. Every gas bubble ascending through the mercury produces in the water a precipitate of hydrated silicic acid. The rationale of this pro- cess is, that of every three equivalents of fluoride of silicon, (Si Fl2), one equivalent transposes with two equivalents of water into silicic acid (Si 02), which separates, and hydrofluoric acid which combines with the two undecomposed equivalents of fluo- ride of silicon to hydrofluosilicic acid. [3 Si FI2 +2 H 0=2 (Si Fl2, FI H) -f Si 02.] The precipitated hydrate of silicic acid renders the liquid gela- tinous, and it is for this reason that the aperture of the exit tube must be placed under mercury, since it would speedily be choked if this precaution were neglected. It sometimes happens in the course, and especially towards the end, of the operation, that the gas forms complete tubes or channels of silica in the gelatinous liquid, through which it subsequently gains the surface without undergoing decomposition, if they are not occasionally broken by stirring the liquid. When the evolution of gas has completely ceased, the gelatinous paste is thrown upon a piece of linen, and the fluid squeezed through and subsequently passed through a filter. The filtrate is kept for use.](https://iiif.wellcomecollection.org/image/b29300940_0078.jp2/full/800%2C/0/default.jpg)
