Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/519

Rh SULPHUR.] CHEMISTRY 507 metallic salts to a solution of sodium thiosulphate, are very unstable, and rapidly decompose into a metallic sulphide and sulphuric acid, thus Ag 2 S 2 3 + H 2 = Ag 2 S Silver thiosulphate. Silver sulphide. Sulphuric acid. H 2 S0 4. Sodium thiosulphate is largely employed in photography for the purpose of dissolving the chloride, bromide, and iodide of silver. These salts are readily soluble in a solution of the thiosulphate until they are exposed to light, when they become insoluble. If, therefore, a sheet of paper coated with one of these silver salts be exposed to light under an object which is in part transparent and in part opaque, such as a piece of lace, for example, the silver salt is rendered insoluble in those parts upon which the light has fallen, and by then immersing the paper in a solution of sodium thiosulphate the unaltered silver salt is removed, and the picture is thus &quot; fixed.&quot; The solvent power of the thiosulphate is due to the formation of a soluble silver sodium thiosulphate Na 2 S 2 O 3 + AgCl - NaAgS 2 O 3 + NaCl ; many other metallic salts insoluble in water are dissolved by a solution of sodium thiosulphate, probably also in consequence of the formation of similar mixed salts. Anhydrosulphuric Acid, H 2 S 2 O~. This acid is obtained by dissolving sulphuric anhydride in sulphuric acid in the requisite proportions : H 2 S0 4 + S0 3 = H 2 S 2 O 7, It crystallizes in large colourless trans parent prisms. When gently heated it is decomposed into sulphuric anhydride, which distils over, and sulphuric acid, which remains. Water dissolves it, forming sulphuric acid. The so-called Nordhausen or fuming sulphuric acid consists chiefly of this acid. It is prepared at Nord hausen, in Saxony, by distilling an impure ferric sulphate, Fe 2 (S0 4 ) 3, obtained by exposing ferrous sulphate, FeSO 4 , to a moderate heat in contact with the air, the distillate, consisting chiefly of sulphuric anhydride, being received in sulphuric acid. This acid readily dissolves many hydrogenized carbon compounds which are only with difficulty acted upon by ordinary sulphuric acid, convert ing them into sulphonic acids ; it is employed on this account to dissolve indigo, and in the preparation of anthraquinonedisulphonic acid, from which alizarin is artificially produced. Salts of this acid, suck as Na 2 S 2 O 7, Ag 2 S 2 O T , BaS 2 O 7 , are obtained by combining the corresponding normal sulphates with sulphuric anhydride; water decomposes them, forming corresponding acid sulphates. The acid salt, KHS 9 O 7, or hydrogen potassium anhydrosulphate, is obtained by dissolving the normal salt in anhydro- sulphuric acid ; it crystallizes in prisms. The chloride of anhydrosulphuric acid, S.,O 5 C1, is pro duced by the action of sulphuric anhydride on many chlorinated compounds, thus 2S0 3 + CC1 4 = S,0 5 C1 2 + COC1 2 Sulphuric Carbon Anhydrosulphuric Carbonic anhydride. tetrachloride. chloride. chloride It is a colourless oily liquid, of specific gravity 1 829 at 18 C., and boils at 146 C. In contact with water it decomposes slowly and noiselessly, forming sulphuric and hydrochloric acids, and is thus distinguished from sulphuric chlorhydrate, S0 2 (OH)C1, which is rapidly decomposed with almost explosive violence when thrown in bo water. The lead chamber crystals (p. 504), when heated, furnish a magnificently crystalline body, which is also a derivative of anhydrosulphuric acid it distils at about 360 C. without decomposing. Dithionic Acid, H 2 S 2 6. The manganese salt of this acid is obtained by the action of a solution of sulphurous acid on manganese dioxide Mn0 2 + 2H 2 S0 3 = MnS 2 6 + 2H 2. This salt may be converted into the barium salt by treatment with barium hydroxide, and from a solution of the barium salt the acid is produced by adding exactly sufficient sulphuric acid to precipitate the barium. By concentrating the solution in a vacuum over sulphuric acid it may be obtained of specific gravity l - 347, but on further concentration the acid is resolved into sulphurous acid and sulphur dioxide A dilute solution decomposes in a similar manner when heated. In contact with the air dithionic acid is slowly oxidized to sulphuric acid. The dithionates produced by decomposing barium dithionate, BaS 2 O 6, with the corresponding sulphates, are all soluble in water and crystallize well. They exhibit considerable stability, but when heated are more or less readily converted into the corresponding normal sulphate and sulphur dioxide. Trithionic Acid, H^O^. Potassium trithionate is produced, together with potas sium thiosulphate, when a saturated solution of hydrogen potassium sulphite is gently heated with sulphur; it is not improbable that the thiosulphate is a secondary product, formed by the action of hydrogen sulphide resulting from the action of the sulphur on the hydrogen potassium sulphite 2HKSO + 2S = KSO + HS. Hydrogen potassium sulphite. 2S = K 2 S 3 O 6 Potassium trithionate. Trithionic acid may be obtained from a concentrated solution of the potassium salt by precipitating the potas sium by hydrofluosilicic acid. The solution has a sour and somewhat bitter taste, and is inodorous; it is per manent in a dilute state only, and on concentrating it in a vacuum over sulphuric acid it begins to decompose, even at C., sulphur dioxide being evolved, and sulphur de posited, while sulphuric acid remains in solution. The salts of trithionic acid are but little known ; they are extremely unstable, and even when boiled with water are decomposed and converted into sulphates, with evolution of sulphur dioxide and deposition of sulphur K 2 S 3 6 = K 2 S0 4 Potassium trithionate. Potassium sulphate. S0 S. Tetrathionic Acid, H 2 S 4 6. The salts of this acid are produced by the action of iodine on thiosulphates ; for example 2Na 2 S 2 3 + Sodium thiosulphate. I 2 = Na 2 S 4 Og + 2NaI. Sodium tetrathionate. The acid may be prepared from the barium salt by double decomposition with sulphuric acid. A dilute solu tion may be boiled without decomposing; but a con centrated solution is decomposed when heated, yielding sulphur, and sulphurous and sulphuric acids. Its salts are all soluble in water, and are much more stable than the trithionates, but for the most part their solutions cannot b&amp;lt;3 evaporated without decomposition. Pentathionic Acid, H 2 S 5 G. A solution of this acid is obtained by alternately passing sulphur dioxide and hydrogen sulphide gases through water 5H a SO 3 + 5H,S = H a Sj0 6 + 9H 2 O + 5S ;