206 ACTION OF SULPHURIC ACID ON ORGANIC MATTERS. When the perfectly pure acid is required, it is actually distilled over so as to leave the solid impurities (sulphate of lead, &c.,) behind in the retort. Some fragments of rock crystal should be introduced into the retort to moderate the bursts of vapour, and heat applied by a ring gas-burner with somewhat divergent jets. Divested of working details, this most important chemical manufacture may be thus described:: A mixture of sulphurous acid, air, steam, and a little vapour of nitric acid, is introduced into a leaden chamber containing a layer of water. The nitric acid is reduced by the sulphurous acid to the state of nitric oxide (NO), which takes up oxygen from the air (forming NO,), and gives it to the sulphurous acid, which it converts into sulphuric acid. This is absorbed by the water, forming diluted sulphuric acid, which is concentrated by evaporation, first in leaden pans, and afterwards in glass retorts or platinum stills. Properties of oil of vitriol.-The properties of concentrated sulphuric acid are very characteristic. Its great weight (sp. gr. 1·842), freedom from odour, and oily appearance, distinguish it from any other liquid commonly met with, which is fortunate, because it is difficult to preserve a label upon the bottles of this powerfully corrosive acid. Although, if absolutely pure, it is perfectly colourless, the ordinary acid used in the laboratory has a peculiar grey colour, due to traces of organic matter. Its high boilingpoint (640° F.) has been already noticed; and although its vapour is perfectly transparent in the vessel in which the acid is boiled, as soon as it issues into the air it condenses into voluminous dense clouds of a most irritating description. Even a drop of the acid evaporated in an open dish will fill a large space with these clouds. Oil of vitriol solidifies when cooled to about -30° F., but the acid once solidified requires a much higher temperature to liquefy it again. Oil of vitriol rapidly corrodes the skin and other organic textures upon which it falls, usually charring or blackening them at the same time. Poured upon a piece of wood, the latter speedily assumes a dark brown colour; and if a few lumps of sugar be dissolved in a very little water, and stirred with oil of vitriol, a violent action takes place, and a semi-solid black mass is produced. This property of sulphuric acid is turned to account in the manufacture of blacking, in which treacle and oil of vitriol are employed. These effects are to be ascribed to the powerful attraction of oil of vitriol for water. Woody fibre (CHO) (which composes the bulk of wood, paper, and linen), and sugar (CHO,,), may be regarded, for the purpose of this explanation, as composed of carbon associated with 10 and 11 equivalents of water, and any cause tending to remove the water would tend to eliminate the carbon. The great attraction of this acid for water is shown by the high temperature (often exceeding the boiling point of water) produced on mixing oil of vitriol with water, which renders it necessary to be careful in diluting the acid. The water should be placed in a jug, and the oil of vitriol poured into it in a thin stream, a glass rod being used to mix the acid with the water as it flows in. Ordinary oil of vitriol becomes turbid when mixed with water, from the separation of sulphate of lead (formed from the evaporating pans), which is soluble in the concentrated but not in the diluted acid, so that if the latter be allowed to stand for a few hours, the sulphate of lead settles to the bottom, and the clear acid may be poured off free from lead. Diluted sulphuric acid has a smaller bulk than is occupied by the acid and water before mixing. ACTION OF SULPHURIC ACID ON METALS. 207 Even when largely diluted, sulphuric acid corrodes textile fabrics very rapidly, and though the acid be too dilute to appear to injure them at first, it will be found that the water evaporates by degrees, leaving the acid in a more concentrated state, and the fibre is then perfectly rotten. The same result ensues at once on the application of heat; thus, if characters be written on paper with the diluted acid, they will remain invisible until the paper is held to the fire, when the acid will char the paper, and the writing will appear intensely black. If oil of vitriol be left exposed to the air in an open vessel, it very soon increases largely in bulk from the absorption of water, and a flat dish of oil of vitriol under a glass shade (fig. 194) is frequently employed in the laboratory for drying substances with out the assistance of heat. The drying is of course much accelerated by placing the dish on the plate of an airpump, and exhausting the air from the shade, so as to effect the drying in vacuo. It will be remembered also that oil of vitriol is in constant use for drying gases. At a red heat, the vapour of oil of vitriol is decomposed into water, sulphurous acid, and oxygen HO. SO3 = HO + Fig. 194.-Drying over oil of vitriol. SO, + 0. This decomposition takes place most easily when the vapour is passed through a strongly-heated tube of platinum, and it has been taken advantage of for the preparation of oxygen, the sulphurous acid being absorbed by passing the mixed gases through lime. Reflecting upon the manufacture of oil of vitriol, it will be perceived that the oxygen thus obtained was originally derived from the air. When sulphur is boiled with oil of vitriol, the latter gradually dissolves the melted sulphur, converting it into sulphurous acid = 3SO, + 2HO. S + 2(HO.SO3) All ordinary metals are acted upon by concentrated sulphuric acid when heated, except gold and platinum (this last even does not quite escape when long boiled with the acid), the metal being oxidised by one portion of the acid, which is thus converted into sulphurous acid, the oxide combining with another part of the sulphuric acid to form a sulphate. Thus, when silver is boiled with strong sulphuric acid, it is converted into sulphate of silver, which is soluble in hot water = Ag + 2(HO. SO3) AgO. SO + 2HO + SO2. Should the silver contain any gold, it is left behind in the form of a dark powder. Sulphuric acid is extensively employed for the separation or parting of silver and gold. This acid is also employed for extracting gold from copper; and when sulphate of copper is manufactured by dissolving that metal in sulphuric acid (see p. 196), large quantities of gold are sometimes extracted from the accumulated residue left undissolved by the acid. If the sulphuric acid contains nitric acid, it dissolves a considerable quantity of gold, which separates again in the form of a purple powder when the acid is diluted with water. 208 COMPOSITION OF OIL OF VITRIOL. Some of the uses of sulphuric acid depend upon its specific action on certain organic substances, the nature of which has not yet been clearly explained. Of this kind is the conversion of paper into vegetable parchment by immersion in a cool mixture of two measures of oil of vitriol and one measure of water, and subsequent washing. The conversion is not attended by any change in the weight of the paper. Beside oil of vitriol, sulphuric acid forms other definite combinations with water. By evaporating diluted sulphuric acid in vacuo at 212° F., an acid is left which has the composition HO. SO, + 2HO (sp. gr. 1.63). If this acid be evaporated in air at 400° F., as long as steam escapes, the remaining acid has the composition HO. SO, + HỎ (sp. gr. 1·78). This acid is called glacial sulphuric acid, because it solidifies to a mass of icelike crystals at 47° F. Composition of oil of vitriol.*-It is found by experiment that in order to neutralise 1 equivalent (47 parts by weight) of potash (KO) there are required 49 parts by weight of oil of vitriol. If 49 parts of oil of vitriol be heated with a weighed quantity of pure oxide of lead, more than sufficient to combine with the acid, 9 parts of water are expelled, and the weight of the oxide of lead is increased by 40. One equivalent (49 parts by weight) of oil of vitriol, therefore, contains 40 parts (1 equivalent = 16 sulphur + 24 oxygen) of anhydrous sulphuric acid, and 9 parts (1 equivalent) of water. The specific gravity (or weight of 1 volume) of vapour of oil of vitriol (at 880°) is stated to be 1.692, so that 1 equivalent (49 parts) would occupy 4 volumes (8 parts 0 = 1 volume)— Weight of 4 volumes HO. SO, (1·692 × 4) •622 × 2) = 6.768 5.524 The difference (5-524) represents the weight of the vapour of anhydrous sulphuric acid contained in 4 volumes of vapour of oil of vitriol. Experiment has proved that when the vapour of anhydrous sulphuric acid is passed through red-hot porcelain tubes, it yields a mixture of 2 volumes of sulphurous acid and 1 volume of oxygen, showing it to contain 1 equivalent of SO, (= 1 equivalent or 1 volume of sulphur and 2 equivalents or 2 volumes of oxygen) and 1 equivalent of oxygen; hence the anhydrous sulphuric acid contains 1 volume sulphur vapour, weighing 2.2300 3.3168 5.5468 Now the weight of 1 volume (sp. gr.) of vapour of anhydrous sulphuric acid has been found by experiment to be 3:01, so that it would appear that the above number (554) really represents 2 volumes of the vapour, though the difference is somewhat greater than usual, between the results of experiment and calculation. The number 5.546, however, closely approximates to that above given (5.524), as representing the weight of the vapour of the anhydrous acid contained in 4 volumes of vapour of oil of vitriol. * Pure oil of vitriol (HO. SO,) can only be obtained by crystallisation, for when concentrated by boiling, a portion of the hydrate is decomposed, anhydrous sulphuric acid passing off, until the residual acid in the retort contains 98.7 per cent. of HO. SO,. SULPHURIC ANHYDRIDE-SULPHATES. 209 It would appear, therefore, that 4 volumes (1 equivalent) of vapour of oil of vitriol contain 2 volumes (1 equivalent) of vapour of water, and 2 volumes (1 equivalent) of vapour of anhydrous sulphuric acid, or 2 volumes of hydrogen (contained in 2 volumes HO), 1 volume of sulphur, and 4 volumes of oxygen (3 volumes belonging to the SO, and 1 volume to the HO). = The molecular formula of oil of vitriol would therefore be written ᎻᎾ. ᎦᎾ, or H,ᎦᎾ (Ꭶ = 32, Ꮎ 16). This formula would represent a molecule of the acid to occupy 4 volumes (H = 1 volume) instead of 2, the ordinary molecular volume of compound vapours. Some chemists explain this by assuming that the oil of vitriol vapour is decomposed into water and anhydrous sulphuric acid at the temperature (880° F.) at which its specific gravity is determined. 147. Anhydrous sulphuric acid or sulphuric anhydride.-The mode of preparing this substance from the fuming sulphuric acid has already been noticed. It is more commonly obtained by expelling the water from bisulphate of soda (NaO. HO.2SO) by fusing it at a dull red heat, and afterwards distilling the anhydrous bisulphate (NaO. 2SO3) in an earthen retort, when neutral sulphate of soda (NaO. SO,) is left, and the anhydrous sulphuric acid passing off as vapour may be condensed in a receiver cooled by ice. Anhydrous sulphuric acid forms a white mass of crystals resembling asbestos; it fumes when exposed to air, since it emits vapour which condenses the moisture of the air, and it soon deliquesces from absorption of water, becoming hydrated sulphuric acid. When thrown into water it hisses like red-hot iron from the sudden formation of steam. It fuses at 65° F., and boils at 110° F. The vapour is decomposed, as mentioned above, into sulphurous acid and oxygen when passed through a red-hot tube. Phosphorus burns in its vapour, combining with the oxygen and liberating sulphur. Baryta glows when heated in the vapour of anhydrous sulphuric acid, and combines with it to form sulphate of baryta. Anhydrous sulphuric acid is capable of combining with olefiant gas (CH) and oil-gas (CH), and absorbs these from mixtures of gases. In the analysis of coal-gas, a fragment of coke wetted with Nordhausen sulphuric acid is passed up into a measured volume of the gas standing over mercury, to absorb these illuminating hydrocarbons. An interesting method of obtaining the anhydrous sulphuric acid consists in pouring 2 parts by weight of oil of vitriol over 3 parts of anhydrous phosphoric acid, contained in a retort cooled in ice and salt, and afterwards distilling at a gentle heat, when the phosphoric acid retains the water, and the anhydrous sulphuric acid may be condensed in a cooled receiver. 148. Sulphates. Action of sulphuric acid upon metallic oxides.-At common temperatures sulphuric acid has a more powerful attraction for bases than any other acid, and is therefore capable of displacing all other acids from their salts; many cases will be remembered in which this power of sulphuric acid is turned to account. So great is the acid energy of sulphuric acid, that when it is allowed to act upon an indifferent or acid metallic oxide, it causes the separation of a part of the oxygen, and combines with the basic oxide so produced. Advantage is sometimes taken of this circumstance for the preparation of 210 ACID AND DOUBLE SULPHATES. oxygen; for instance, when binoxide of manganese is heated with sulphuric acid, sulphate of manganese is produced, and oxygen disengaged— MnO. SO, O + HO. MnO2 + HO. SO3 = Again, if chromic acid be treated in the same way, sulphate of sesquioxide of chromium will be produced, with liberation of oxygen 2CrO3 + 3(HO. SO3) = Cr2O. 3SO3 + Оg + 3HO. 3 A mixture of bichromate of potash (KO. 2CrO,) and sulphuric acid is sometimes used as a source of oxygen. Many bases are capable of forming two salts with sulphuric acid, a neutral sulphate and an acid sulphate. The acid sulphates may be represented as compounds of the neutral sulphates with hydrated sulphuric acid; thus, the neutral sulphate of potash is KO. SO, and the bisulphate is KO. SO, HO. SO3. The latter, being a solid salt which possesses, at high temperatures, almost all the acid energy of sulphuric acid, is most useful in blowpipe and metallurgic experiments. When strongly heated, this salt parts with hydrated sulphuric acid, and neutral sulphate of potash is left. It has been seen that bisulphate of soda (NaO. SO, HO. SO2) parts with its water when heated, and becomes NaO. 280. Crystals of anhydrous bisulphate of potash KO. 2SO, have also been obtained. Sulphuric acid forms a large number of double salts in which two sulphates are combined together. The large class of alums yields examples of these, in which one of the sulphates contains an alkaline base, and the other a basic sesquioxide. Potash-alum, for example, is represented by the formula KO. SO, ALO ̧ . зSO3 + 24Aq, being a double sulphate of alumina and potash. In consequence of the tendency of sulphuric acid to break up into sulphurous acid and oxygen at a high temperature, most of the sulphates are decomposed by heat; sulphate of copper, for example, when very strongly heated, leaves oxide of copper, whilst sulphurous acid and oxygen escape; CuO. SO3 CuO + SO2 + 0. Sulphate of iron is more easily decomposed, because of the attraction of the protoxide of iron for the oxygen, with which it combines to form sesquioxide— = part of the acid escaping in the anhydrous state. Sulphate of zinc (ZnO. SO) has been proposed as a source of oxygen upon the large scale, since it is a very cheap salt, and when strongly heated, yields a residue of oxide of zinc which is useful as a white paint, whilst sulphurous acid and oxygen gases escape, the former of which may be absorbed by lime or soda, yielding sulphites which are useful in the arts. The neutral sulphates of potash, soda, baryta, strontia, lime, and oxide of lead are not decomposed by heat, and sulphate of magnesia is only partly decomposed at a very high temperature. When a sulphate is heated with charcoal, the carbon removes the whole of the oxygen, and a sulphide of the metal remains, thus Hydrogen, at a high temperature, effects a similar decomposition. |