Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/535

Rh SILICON.] CHEMIST BY 523 in the preparation of paints, especially for mural painting, and in the manufacture of soap. Although silica is an extremely weak acid oxide, on account of its non-volatility, it is capable of decomposing the salts of all volatile acids at more or less elevated temperatures, and the salts formed from it are stable at the highest temperatures. A great variety of artificial and natural silicates are known, some of which may be regarded as derived from silicic acid, H 2 SiO 3, and others frum an acid which may be termed orthosilicic acid, H 4 Si0 4, but the majority are far more complex, those which occur as natural minerals being formed by the association of silica vith basic oxides in proportions included between the following limits : 4M./0 or 4M&quot;0 : Si0 2 and M O or M&quot;0 : 2Si0 2 2R 2 O 3 : Si0 2 and R 2 O 3 : 6Si0 2. On adding acids to a solution of an alkaline silicate, a gelatinous &quot; hydrate of silica &quot; is precipitated ; the whole of the silica may be retained in solution, however, if a dilute solution of an alkaline silicate be poured into a considerable excess of hydrochloric acid, but may be precipitated by gradually neutralizing the acid. The retention of the silica in solution is, perhaps, due to the formation of a soluble chlorhydrin (p. 565). If a stratum -$ of an inch in depth of the hydrochloric acid solution bs placed in a &quot; dialyzer &quot; formed by stretching wetted parchment paper across a light hoop of wood or gutta-percha, and the dialyzer be floated in a vessel of water, the water in the outer vessel being changed at intervals, after four or five days the hydrochloric acid and the soluble chlorides are found to have diffused completely into the water ; but the solution in the dialyzer still contains the silica, and a 5 per cent, solution prepared in this way may be concentrated by boiling down in an open flask until it contains nearly 14 per cent, of silica. The solution is tasteless, limpid, and colourless ; it has an acid reaction rather greater than that of carbonic acid; for every 100 parts of silica present, however, only 2 13 parts of potassium hydroxide are requisite to neutralize the acid reaction. The solution is not easily preserved for many days, as it becomes converted into a solid transparent jelly ; coagula tion is retarded by hydrochloric acid, and by small quantities of potassium or sodium hydroxide, but is effected in a few minutes by the addition of 10 1 00 part of any alkaline carbonate, although ammonia and its salts are without effect. Sulphuric, nitric, and acetic acids are also without action, but a few bubbles of carbon dioxide gas slowly cause coagulation. The condition in which the silica is present in the solution having these very remark able properties is not known, but on the assumption that a chlorhydrin is produced on decomposing the alkaline silicate by an excess of hydrochloric acid, it appears not improbable that, as the excess of the latter is removed by dialysis, the chlorhydrin is gradually decomposed by the water and, perhaps, converted into the hydroxide H 4 SiO 4 , or some other soluble hydroxide, and that the subsequent coagulation may be due to the conversion of this hydroxide into an insoluble hydroxide of different molecular composi tion. The solution evaporated at 15 C. in a vacuum leaves the silica in the form of a transparent, glassy, very lustrous hydrate, containing after two days exposure over sulphuric acid an amount of water which agrees very nearly with the formula H.,SiO 3. When silicon chloroform is added to water at the ordinary temperature, hydrogen is evolved, and hydrated silica is produced: SiHCl 3 + 2H 2 = H 2 + SiO, + 3HC1 ; but if water at zero is employed, a precipitate is obtained which, after drying over sulphuric acid, and then at 150 C., has the composition (HSiO). 7 O. This compound is analogous in composition to the unknown anhydride of formic acid : (HCO) 2 O = 2HCO(OH) - OH 2 ; its formation is probably preceded by that of the hydroxide SiH(OH) 3. It is a snow-white, voluminous, amorphous substance, slightly soluble in water ; it is decomposed by the least heat when in a moist state, with evolution of hydrogen, and is dis solved by ammonia, caustic alkalies, and alkaline carbonates, with evolution of hydrogen and production of an alkaline silicate. It is not acted upon by any acids except hydro fluoric acid, even concentrated nitric acid being without action. It decomposes when heated above 300 C. Silicon hexachloride and hexiodide are decomposed by water in a similar manner, being first converted into the hydroxide Si. 2 (OH) 6, which then furnishes siliconoxalic acid Si 2 O 2 (OH) 2. This is a white substance, and when heated is decomposed, like the preceding compound, with evolution of hydrogen, leaving a residue of silica ; it is also decom posed with evolution of hydrogen by even the weakest bases. It rapidly reduces potassium permanganate in the cold, but is only slowly oxidized by chromic acid, and is without action on solutions of gold chloride or selenious acid. On decomposing the chloride, Si 2 Cl 4, however, by water an hydrated oxide is obtained which not only reduces potassium permanganate and chromic acid, but also gold chloride and selenious acid. The behaviour of siliconoxalic acid on oxidation is precisely similar to that of its carbon analogue oxalic acid, C 2 O 2 (OH) 2, which, however, is a crystalli ne, strongly acid body, which dissolves readily in water and furnishes stable salts. By the action of anhydrous alcohol, silicon tetrachloride is converted into tetrethylorthosilicate, Si(OC 2 H 5 ) 4, which is a colourless liquid boiling at 166 C.; it is gradually decomposed by water, being converted into alcohol and gelatinous silica. By the joint action of zinc ethyl and sodium on this compound, it is possible to displace in succession each of the groups (OC 2 H 5 ) by ethyl, thus: 2Si(OC 2 H 5 ) 4 + Zn(C 2 H 5 ) 2 + 2Na = 2Si(C 2 H 5 )(OC 2 H 5 ) 3 + 2NaOC 2 H 5 + Zn. lu this manner the following compounds have been prepared : Boiling Point. Si(C 2 H 5 )(OC 2 H 5 ) 3 166 C. Si(C 2 H 5 ) 2 (OC 2 H 5 ) 2 159 Si(C 2 H 5 ) 3 (OC 2 H 5 ) ..... 155 Si(C,H 5 ) 4 153 Si(C 2 H 5 ) 3 H 107 These compounds are all colourless liquids, insoluble in water, and stable in the air ; the first only is decomposed when allowed to remain in contact with water. Silicon ethyl, Si(C 2 H 5 ) 4, may also be obtained by the action of zinc ethyl on silicon tetrachloride ; and from the latter compound and zinc methyl silicon methyl, Si(CH 3 ) 4, has been prepared ; similarly, the hexiodide is converted into the corresponding ethyl derivative Si 2 (C 2 Hr,) 6 by the. action of zinc ethyl. These silicon hydrocarbons exhibit the closest resemblance to their analogues the paraffins. Thus by the action of bromine on silicon triethylhydricle, Si(C 9 H-) 3 H, the bromide Si(C. 2 H 5 ) 3 Br is produced, which may be converted by the action of aqueous ammonia into the silicon alcohol triethylsilicol, Si(C.,H 5 ) 3 .OH ; this method of formation is precisely analogous to that by which the hydrocarbon C(CH 3 ) 3 H is converted into the alcohol C(CH 3 ) 3 .OH. Triethylsilicol also resembles the corre sponding alcohol triethylcarbinol, C(C 2 H 5 ) 3 .OH, in pro perties ; thus, both are colourless, viscid liquids, having a strong camphor-like odour, difficultly soluble in water ; the former boils at 154 C., the latter at 141 C. ; they dissolve sodium with evolution of hydrogen, being converted into corresponding sodium derivatives Si(C 2 H 5 ),,.OXa and