182 CONCENTRATION OF SULPHURIC ACID. Beyond this point it is useless to carry the operation, as the concentrated acid distils over. Indeed during the whole operation some acid passes over with the water, which is therefore preserved, and returned to the leaden chamber. The acid that remains in the retort after it has thus been boiled down, is the concentrated oil of vitriol of commerce; it is a definite compound,* consisting of H2SO The following table gives the proportion of sulphuric acid contained in solutions of the densities therein mentioned :+ Strength of Sulphuric Acid of Different Densities at 60° F. (Ure.) * De Marignac (Ann. de Chimie, III. xxxix. 189) finds that it always contains a slight excess of water beyond the atomic proportion calculated from the formula H,e,se; instead of 18.36 per cent. of water, he always obtained 1962; and a similar observation was made by Gay-Lussac. Play fair states that if the concentration of the acid be effected by a temperature not exceeding 500°, the true compound H2SO, of sp. gr. 1848 is obtained; but if heated to ebullition, it is partially decomposed in the manner stated by De Marignac. + Bineau has more recently made a careful determination of the strength of sulphuric acid of different densities (Ann. de Chimie, III. xxiv. 341), but his results differ but slightly from those of Ure, as may be seen from the annexed table (temp. 59° F.): HYDRATES OF SULPHURIC ACID. 183 (414) H2SO4 or HO,SO, often called Protohydrate of Sulphuric Acid.-The oil of vitriol of commerce forms a dense, oily-looking, colourless liquid, without smell, and of specific gravity 1-842. It is intensely caustic, and chars almost all organic substances, from its powerful attraction for moisture. With water it mixes completely in all proportions, and the mixture, when cold, occupies less bulk than the two liquids did when separate. Great heat is given out at the moment the mixture is made; the dilution should therefore be performed gradually, always pouring the acid into the water, not the water into the acid. So powerful is the attraction of the acid for moisture, that if it be exposed in a shallow dish to the air for a few days, it frequently doubles its weight by absorbing aqueous vapour from the air. In the laboratory, advantage is very often taken of this property, which enables it to be employed in a variety of cases as a desiccating agent (66 and 185). The acid of commerce is often of a dark brown colour, occasioned by its charring action on fragments of organic matter, such as straw or wood, which have accidentally fallen into it. Sulphuric acid does not evaporate at the ordinary temperature of the air. If a drop of the diluted acid fall upon a cloth, the water gradually evaporates until the acid which is left behind acquires a certain degree of concentration. On approaching a fire or other source of heat, a further portion of the water is expelled, and the acid becomes more concentrated, until it chars or destroys the cohesion of the fibres; this is one cause of the destructive action of sulphuric acid upon linen, even when very much diluted. De Marignac finds that the true sulphuric acid (H2SO) when heated emits a small quantity of the vapour of the anhydride, and the remaining liquid boils at 640° F. Bineau states that just above the boiling-point of the acid the vapour has a sp. gr. of 2.15, which would represent 2 volumes of the anhydride and 2 volumes of steam (1 atom of each) condensed into the space. of 3 volumes, but it continues to expand by heat until at 880° an atom of the compound occupies the space of 4 volumes, which would reduce the density of the vapour to 1692. This by some chemists is supposed to be produced by the separation of the compound into aqueous vapour and anhydride by the process of dissociation (see note; Part I. p. 100). After the acid has been frozen, it melts at 51°, but it may be cooled much below this point without solidifying. On dropping into the cooled acid a crystal of the acid previously frozen, congelation immediately occurs, and the temperature rises to 51°. The concentrated acid of commerce does not usually freeze till it has been cooled to about -30°; but 184 SULPHURIC ANHYDRIDE, OR when frozen it does not become liquid till the temperature reaches 32°. 2 (H2SO H2O) or Second Hydrate of Sulphuric Acid.-If water be added to sulphuric acid, until the density is reduced to 178, a definite hydrate is formed. It freezes at 47°, and crystallizes in splendid rhombic prisms, the sp. gr. of which is 1951; from this property it is often termed glacial sulphuric acid. According to Dalton, it boils at 435°. Graham found that this hydrate may be obtained by heating a more diluted acid to 400° till it ceases to give off water. Another hydrate (H2SO4, 2H,O) may, according to Graham, be procured by evaporating a dilute acid in vacuo at 212°, till it ceases to lose weight. The density of this hydrate is 1632, and its boiling-point is 348°. (415) Nordhausen Sulphuric Acid.-For the purpose of dissolving indigo in the process of dyeing Saxony blue, an acid of still higher concentration than oil of vitriol is required. Such an acid is principally prepared at the town of Nordhausen, in Saxony, and is hence known as Nordhausen oil of vitriol. The old name for sulphate of iron was green vitriol, and this circumstance, taken in conjunction with the oily consistence of the concentrated acid, gave rise to the name of oil of vitriol, by which the concentrated acid of commerce is still frequently known, and which is convenient as distinguishing it from more diluted acids. In preparing the Nordhausen acid, sulphate of iron is dried at a moderate heat to expel its water of crystallization, and is then distilled in earthen retorts; a dense, brown, fuming liquid passes over, of sp. gr. about 1.9. (416) Sulphuric Anhydride (SO).—If this fuming Nordhausen acid be placed in a glass retort, furnished with a receiver which is kept cool by ice, and a gentle heat be applied to the retort, white fumes pass over, which solidify into a white, silky-looking fibrous mass. This is the compound frequently called anhydrous sulphuric acid. The remainder in the retort, after all the anhydride is expelled, consists of ordinary oil of vitriol. Sulphuric anhydride may also be obtained from the acid sulphate of sodium (NaHSO), which melts at a dull red heat, and is deprived of its hydrogen in the form of water; after which, if distilled in an earthen retort, it yields white fumes of the anhydride, whilst neutral sulphate of sodium remains in the retort. ANHYDROUS SULPHURIC ACID. 185 The anhydride forms with oil of vitriol a compound (H2SOS) that crystallizes in plates which fuse at 95°. Sulphuric anhydride, however, possesses no acid properties. It is tough, ductile, and can be moulded in the fingers, like wax, without charring the skin. It fumes in the air, and is very deliquescent: when thrown into water the heat emitted is so intense that it hisses as a hot iron would do. The solution has all the properties of ordinary sulphuric acid. The anhydride melts at 65°, and boils at about 110°, forming a colourless vapour, which, if passed through ignited porcelain tubes is decomposed into 2 volumes of sulphurous anhydride and I of oxygen; I volume of sulphur vapour and 3 of oxygen being condensed in the anhydride into the space of 2 volumes of vapour. The specific gravity of this vapour was found by Mitscherlich to be 3'01, or somewhat higher than its calculated amount, which is 2.764: for— According to De Marignac, sulphuric anhydride exists under two modifications; one of which melts at about 65°, and is produced by distillation, or by fusion at a high temperature; but when once it has been solidified, it passes rapidly into the other form, which melts near 212°, at which temperature it is slowly volatilized, and becomes reconverted into the first variety. Sulphuric anhydride in some cases combines with the anhydrous bases; if its vapour be passed over baryta heated to the point of redness, the two combine with incandescence, and sulphate of barium is formed. Mercury when heated in the vapour, is converted into mercuric sulphate with liberation of sulphurous anhydride. Phosphorus takes fires in its vapour, setting sulphur free. Sulphuric anhydride combines with sulphur, forming solutions which have a brown, green, or blue colour, according to the proportion of sulphur; the blue compound containing the smallest proportion. It likewise dissolves iodine, and with one-tenth of its weight of iodine forms a green crystalline compound. It also combines with hydrochloric acid, and forms a liquid termed chlorhydrosulphuric acid (HCl,SO), which boils at 293°, and is decomposed by water. Williamson obtained it by the action of pentachloride of phosphorus upon sulphuric acid; H2SO+PCI: HCI,SO2+ HCl + POCI,; hydrochloric acid and oxychloride of phosphorus being formed at the same time. 3 = 186 COMMON IMPURITIES OF SULPHURIC ACID. We are therefore acquainted with the following definite compounds of sulphuric anhydride with water; starting with the Uses.—The applications of sulphuric acid in the arts are very numerous. Immense quantities of it are consumed in the manufacture of sulphate of sodium as a preliminary process in making carbonate of sodium; and it is in constant requisition for the preparation of nitric, hydrochloric, and other volatile acids. Its applications in the laboratory are too numerous to be specified. (417) Impurities common in the Commercial Acid.-The oil of vitriol of commerce is never pure: it always contains lead, derived from the vessels in which it is made. The greater part of the sulphate of lead is precipitated as a white powder when the acid is diluted. It is also frequently contaminated with arsenic, derived from the pyrites: the diluted acid in this case gives a yellow precipitate when exposed to a current of sulphuretted hydrogen gas. The arsenic is still more easily recognised by what is termed Marsh's test, which will be described under the head of arsenic (846). On the large scale this impurity is effectually removed by adding a small quantity of sulphide of barium to the acid sulphide of arsenic and sulphate of barium are formed; they are both insoluble in the acid, and may be separated by subsidence and decantation. The greater part of it may also be got rid of by adding hydrochloric acid and boiling the liquid, when the arsenic is expelled in the form of chloride of arsenic with the excess of hydrochloric acid. Nitric acid and some of the lower oxides of nitrogen are also often present: a strong solution of green vitriol in water, when added to the undiluted acid, shows the presence of these impurities by striking a characteristic purplish-red colour at the point of contact of the two liquids. Sulphurous acid may likewise sometimes be detected in the acid, as may also hydrochloric acid and sulphate of potassium. When required in a pure form, the acid must be re-distilled with a little sulphate of ammonium; this salt decomposes any |