Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/518

Rh 506 CHEMISTKY [ACIDS OP Sulphuric acid enters into reaction with a large number of hydrogenized carbon compounds in such a manner that water is formed and hydrogen displaced by the monad group SO 2. OH ; for example HO.SOj.OH + C H 6 = C 6 H 5 .S0 2 .OH + OH 2. Sulphuric, acid. Benzene. Benzenesulplionic acid. The sulphonic acids, as the compounds thus produced are termed, are powerful monobasic acids. But the action may proceed further, both OH groups in sulphuric acid being displaced by monad compound radicles, in which case neutral bodies called sulphones are formed ; thus HO.S0 2 .OH Sulphuric acid. 2C 6 H 6 = C H 5 .S0 2 .C 6 H 5 + 20H 2. Benzene. Benzenesulphone. Selenic Acid, H 2 Se0 4. This acid may be obtained by oxidizing selenious acid by the action of chlorine, or bromine, and water HSe0 + Br + H0 = 4 + 2HBr. It is a transparent colourless liquid, which in the most concentrated state boils at 280 C., and has a specific gravity of 2 -6. In this state it is not quite pure selenic acid, but contains a little water, which cannot be driven off without decomposing the acid into selenium dioxide, oxygen, and water. Selenic acid resembles sulphuric acid in many of its properties, being very hygroscopic, and when it is added to water a considerable amount of heat is deve loped. It is reduced to selenioua acid when boiled with hydrochloric acid H 2 Se0 4 + 2HC1 - H 2 Se0 3 + C1 2 + H 2 ; but it is not decomposed by the hydrogen produced when zinc or iron is dissolved in its solution. Like sulphuric acid it precipitates barium salts, even in presence of other mineral acids. The selenates or salts produced by displacing the hydro gen in selenic acid by metals correspond in composition, and also very closely in their properties, to the sulphates, with which they are isomorphous. Telluric Acid, H 2 Te0 4. The potassium salt of this acid is obtained by fusing tellurium or tellurous oxide with potassium nitrate. From this salt barium tellurate may be prepared by precipitating its solution with a barium salt, and the acid is produced by decomposing barium tellurate with sulphuric acid. Telluric acid crystallizes from water in large prisms of the composition H 2 TeO 4 + 2H 2 O. This hydrate has a metallic taste, and reddens litmus slightly. It dissolves slowly in cold water, but freely in boiling water. It loses its water of crystallization at a little above 100 C. The compound H 2 TeO 4 is nearly insoluble in cold water, but dissolves on boiling ; when heated to a temperature below redness it furnishes the oxide Te0 3. It is reduced to tellurous acid by boiling with hydrochloric acid, and it is also decomposed by sulphurous acid, which is without action on selenic acid, with precipitation of tellurium; In addition to acid and normal tellurates, such as KHTeO 4 and K 2 TeO 4, and hyperacid tellurates, of which the salt KHTe0 4 + H 2 Te0 4 is an example, telluric acid also furnishes so-called di- and tetra-tellurates, such as (NH 4 ) 2 Te 2 7 and (NH 4 ) 2 Te 4 13 or (NH 4 ) 2 TeO 4 + 3TeO 3 , and basic tellurates, such as Ag 6 Te0 6 and Ag 6 Te 2 O 9. The ditellurates may be regarded as salts of the acid H 2 Te 2 O 7, analogous to anhydrosulphuric acid, and the basic salt Ag 6 TeO 7 as formed from the hydrate H 6 Te0 6 or H 2 Te0 4 + 2H 2 0; whilst salts such as Ag 6 Te 2 9 are ap parently derivatives of an acid formed by the withdrawal of the elements of three molecules of water from two molecules of the acid H G Te0 6. Other Acids of /Sulphur. In addition to sulphurous and sulphuric acids, a number of acids, more or less closely related to them, may be obtained. The following is a complete list of the known acids of sulphur : Hyposulphurous acid H 2 S0 2 Sulphurous acid H 2 S0 3 Sulphuric acid H 2 S0 4 Thiosulphuric acid H 2 S 2 3 Anhydrosulphuric acid H 2 S 2 7 Dithionic acid H 2 S 2 O a Trithionic acid H 2 S 3 6 Tetrathionic acid H 2 S 4 6 Pentathionic acid H 2 S 6 6 Hyposulphurous Add. This acid is obtained by the action of zinc on a solution of sulphurous acid, which dissolves the metal, forming zinc sulphite ; the hydrogen, which is the accessory product of this reaction, is not evolved, however, but reduces a portion of the acid, forming hyposulphurous acid ; thus H 2 S0 3 + Zn = 2H -r- ZnS0 3 Sulphurous acid. H 2 S0 3 + Sulphurous acid. Zinc sulphite. 2H = H 2 S0 2 Hyposulphurous H 2 O. The solution thus obtained is very unstable, and rapidly decomposes with separation of sulphur. Sodium hypo sulphite, NaHS0 2, is a more stable substance, and is produced in a similar manner by the action of zinc on a solution of sodium hydrogen sulphite. This salt crystallizes in slender colourless needles ; it is soluble in water, but insoluble in alcohol. When exposed to the air in a moist state it becomes very hot, and is converted by oxidation into hydrogen sodium sulphite, bat in the dry state it is not affected by oxygen. The remarkable observation has been made, however, that when a solution of sodium hyposulphite is oxidized by free oxygen, that is to say, when water saturated with oxygen is added to a solution of the hyposulphite, only one-half the oxygen is employed in causing the conversion of the hyposulphite into the sulphite, the remaining half becoming affixed to water, forming hydrogen dioxide ; the reaction may be expressed by the following equation NaHSOo + + H = NaHSO, 2 W 2 Sodium hyposulphite. H 2 = NaHS0 3 + H 2 2. Sodium hydrogen Hydrogen sulphite. dioxide. Hyposulphurous acid has a much greater decolorizing and reducing power than sulphurous acid ; it immediately reduces the metals from mercury and silver salts, and it precipitates copper hydride, Cu 2 H 2, from a solution of copper sulphate. Thiosulphuric Acid, H 2 S 2 3. This acid is formed from sulphurous acid by combining it with sulphur; thus, when a solution of sodium sulphite is digested with sulphur, sodium thiosulphate is produced Na 2 S0 3 + S = Na 2 S 2 O 3. Sodium sulphite. Sodium thiosulphate. It will be obvious that this reaction is precisely analogous to that which occurs when sodium sulphite is converted into sodium sulphate by the action of oxygen. Thiosulphuric acid cannot be isolated, on account of its instability, and when an acid hydrochloric acid, for ex ample is added to a solution of a thiosulphate, the thio- sulphuric acid which is produced rapidly decomposes into sulphur and sulphurous acid : H 2 S 2 O 8 = H 2 S0 3 + S. The thiosulphates of alkali and alkaline earth metals are crystalline and soluble in water, and are fairly stable salts ; the thiosulphates of the heavy metals, however, which are precipitated on the addition of solutions of