PREPARATION AND PROPERTIES OF ACETIC ACID.

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or fermentable liquors contains a large quantity of foreign matters, from nearly all of which it may be freed by simple distillation: the acid liquid which comes over constitutes what is known in pharmacy as distilled vinegar. The strength of any vinegar is best ascertained by determining the quantity of anhydrous sodic carbonate which a given weight of it will neutralize : 100 parts of sodic, carbonate corresponding to 962 parts by weight of acetic anhydride. Care must be taken to ascertain the absence of sulphuric or of any other acid except the acetic, when operating in this way. Commercial vinegar is allowed by law to contain To of its weight of sulphuric acid; the presence of which prevents the vinegar from becoming mouldy.

Normal acetic acid is best procured in a pure form by distilling 82 parts, or I atom, of fused sodic acetate, with 98 parts or 1 atom, of oil of vitriol, at a gentle heat, and rectifying the product from a small quantity of potassic dichromate. Dried acetate of lead may be substituted for the sodium salt, but rather more than I equivalent of the salt must be taken for each equivalent of oil of vitriol, and the mixture must be digested for 24 hours before distilling. Either cupric acetate, or the acid potassic acetate, when submitted to distillation without any addition, also furnishes the concentrated acid with facility; but when prepared in this manner it is liable to be contaminated with

acetone.

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1278) Properties of Acetic Acid.-Normal acetic acid (H,H) is liquid at temperatures above 63° F., at which point it crystallizes in radiating tufts of plates: hence in this its state of greatest concentration, it is frequently known as glacial acetic acid. It may be mingled in all proportions with water, and its density increases on dilution, until nearly one-third of its weight of water has been added, when it contains a definite hydrate (HЄ2H,,,н ̧), with a density of 1079. If it be further diluted, the density diminishes according to the ordinary law. In consequence of this anomaly, normal acetic acid, and the same acid diluted with an equal weight of water, possess each the sp. gr. 1063. The concentrated acid has a sharp aromatic taste, and a peculiar pungent odour; it blisters the skin if applied to it for a sufficient length of time. It boils at 243° (117° C.) and may be distilled unchanged. The vapour of this acid is inflammable, producing by its combustion, water and carbonic anhydride. The vapour of acetic acid exhibits an anomalous dilatation when exposed to a gradually rising temperature:

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200

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230

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160

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At 320 and beyond this it undergoes no further alteration until the acid is itself decomposed. At 446° and upwards, I atom of the acid follows the usual law of yielding 2 volumes of vapour (Bineau).

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(1279) Acetic Anhydride (H ̧Ð ̧). Sp. gr. of liquid at 69°, 1073; of vapour 3'47; Rel. wt. 51; Boiling pt. 279°5 (137°5 C.).—This compound was obtained by Gerhardt by several methods; but the easiest consists in distilling 3 parts of phosphoryl chloride (POC) with 6 of anhydrous sodic acetate, returning the liquid which passes over, upon the residue, and redistilling, until the odour of the chloride has disappeared; finally the liquid is rectified. It is a colourless, very mobile liquid of high refracting power; with a very pungent smell resembling that of acetic acid, combined with that of the hawthorn blossom. It emits a vapour which is extremely irritating to the eyes. Acetic anhydride does not mix immediately with water, but sinks through it in oily drops, which become slowly dissolved, evolving heat, and forming the ordinary hydrated acid. This anhydride gradually absorbs moisture from the air, and becomes converted into normal acetic acid. Potassium acts rapidly upon it, an inflammable gas being given off, and the compound being by degrees converted into a crystalline mass of the anhydro-potassic acetate (2 KE,H ̧, ¤ ̧н), or binacetate. The same compound is also formed by dissolving dried potassic acetate in the anhydride.

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(1280) Acetates.—Acetic acid is monobasic. Many of its salts are largely used in the arts. Potassic Acetate (KЄ2H,2=98) which is employed medicinally as a diuretic, is an anhydrous, foliated, very deliquescent, fusible salt, abundantly soluble in alcohol. It combines with a second equivalent of acetic acid when dissolved in the concentrated acid, and forms a crystallizable hydro-potassic acetate (KH 2 CH12), which when subjected to distillation is decomposed into pure normal acetic acid and normal potassic acetate. Sodic Acetate (Na¤ ̧H ̧Ð1⁄2,3H ̧¤=82+54; sp.gr. 140) crystallizes in oblique rhombic prisms; it fuses readily. This salt is prepared in large quantities during the purification of wood. vinegar, the crude acid being neutralized sometimes directly by means of sodic carbonate, or soda ash; but in many manufactories, after the crude acid has been neutralized by chalk and thus converted into calcic acetate, the liquid is mixed with sodic sulphate in the pro

portion of 1 atom of the sulphate to I of the acetate. A soluble sodic acetate is thus obtained by double decomposition, whilst a heavy crystalline sodio-calcic sulphate is formed, which can be more easily freed from the mother liquor than the pasty mass which the simple calcic sulphate would produce. Ammonium Acetate is a very soluble salt, which is frequently used medicinally as a diaphoretic. Baric Acetate crystallizes below 60° in oblique rhombic prisms (Ba 2 €,H,,, 3 H2O); at higher temperatures its crystals assume the form of flattened prisms of sp. gr. 219, and retain only 1 H,O; it is more soluble in cold than in hot water, and is slightly soluble in alcohol; it is occasionally used as a precipitant for sulphuric acid; when distilled it furnishes acetone. Calcic Acetate (Ea 2 €2H ̧Ð2=158) crystallizes in silky, anhydrous needles, which are very soluble in water; if heated to 248° it becomes strongly phosphorescent by gentle friction.

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Acetates of Aluminum.-These compounds are extensively manufactured for the use of the dyer and calico-printer. The common red mordant is prepared by precipitating 100 parts of alum in solution by means of 120 of crystallized acetate of lead: the potassic sulphate of the alum remains undecomposed in the solution. Sometimes aluminic sulphate is used instead of alum. Crude calcic acetate is also extensively substituted for acetate of lead in this operation; calcic sulphate is precipitated, and acetate of aluminum remains in solution. When this liquor is used as a mordant it is thickened with gum, and applied by means of blocks to the cloth. The aluminic acetates have been carefully studied by Crum (Q. J. Chem. Soc., vi. 216), who has proved the existence of two insoluble aluminic diacetates (Al,„, 2 ЄH., 5 H2O), and (Al,,, 2 Є,H ̧3, 2H,O), besides a soluble diacetate (Al3, 2 Єн ̧Ð ̧, 4 H2O). The easiest method of paring the commercial aluminic acetate consists in decomposing a solution of aluminic trisulphate with one of acetate of lead; but although 3 equivalents of sulphuric acid are thus removed, and 3 equivalents of acetic acid are liberated, no solid aluminic triacetate can be obtained from this liquid. Indeed Crum is of opinion that, even when in solution, the salt which is thus formed is a diacetate, mixed with an equivalent of free acetic acid, since the liquid has a powerful odour of acetic acid; Al,Ꮎ,,3 ᎦᎾ, + 3 (Pb 2€ HyᎾ) + HᎾ =3PbᎦᎾ, + Al,0, 2 Ꮎ Ꮋ Ꮎ, + 2 HЄ,H ̧ ̧.

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If a concentrated solution, containing from 4 to 5 per cent. of alumina, prepared in the manner just directed, in such atomic proportions as to contain aluminic triacetate, be purified from

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ACETATES OF ALUMINUM.

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traces of lead by sulphuretted hydrogen, and from those of sulphuric acid by baric acetate, a liquid is obtained which, when kept at between 59° and 70° (15° and 21° C.) for a few days, gradually deposits a white coating upon the sides of the vessel; this deposit, when allowed to become dry spontaneously, forms white hard plates resembling porcelain in appearance. This is an aluminic diacetate (Al ̧Ð ̧, 2 ¤ ̧Н ̧Ð ̧, 5 н ̧‡), which is insoluble in water, but soluble in nitric acid. If the original solution of aluminic triacetate be boiled, a heavy white powder is rapidly deposited, and the solution retains scarcely a trace of alumina : Crum found this precipitate to be a different hydrate of aluminic diacetate (Al,,, 2 ¤ ̧н ̧Ð ̧, 2 н‚Ð). When dilute solutions of the aluminic acetate are employed, no precipitate is occasioned unless a considerable excess of free acetic acid be added. When the solution of the triacetate is evaporated rapidly by spreading it in very thin films over a sheet of glass, the heat not being allowed to rise above 100° F., and the oily drops as they run together being constantly spread out with a thin platinum knife, a dry substance may be obtained in transparent scales (Al‚Ð ̧, 2 ЄH ̧Ð ̧, 4 H2Ð), which may be redissolved easily and completely in water.*

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Several other acetates are also used as mordants in calicoprinting; for example, those of manganese, zinc (Zn, 2 ¤‚Í ̧Ð1⁄2, 3 H,→), and iron. Acetic acid dissolves iron readily with evolution of hydrogen; the crude acetate of iron, prepared by dissolving iron hoops in raw wood-vinegar, is employed for preserving wood by Boucherie's process (1110). Ferrous acetate crystallizes in silky needles, which absorb oxygen rapidly. Ferric acetate is of a deep red colour.

From this compound, the remarkable soluble aluminic hydrate, previously alluded to (662), may be prepared in the following manner :-If a dilute solution of this soluble aluminic diacetate, containing of its weight of alumina, be placed in a closed vessel which is immersed in water, maintained at the boiling point, for about ten days, the solution gradually loses its astringent taste, and acquires a strong odour of acetic acid. If this liquid be now diluted with rather more than an equal bulk of water, and be boiled briskly in a shallow dish, the layer of liquid not being more than a quarter of an inch in depth, and water being supplied in proportion as it evaporates, the acetic acid may in about an hour and a half be driven off so as to be no longer sensible to litmus paper. The liquid remains limpid and transparent, but is quite tasteless. When I grain of sulphuric acid in 1000 grains of water is mixed with 8000 grains of such a solution (a quantity which contains 20 grains of alumina), the whole is converted into a solid, transparent jelly by pressure in a bag, the volume of this jelly may be reduced to one-sixtieth of its original bulk, the solid coagulum retaining nearly the whole of the sulphuric acid, which amounts to about 1 equivalent of the acid to 15 of alumina. Two atoms of the tribasic citric acid produce as powerful an effect in coagulating the solution as 3 atoms of sulphuric acid, and I atom of tartaric acid as much as one of sulphuric acid, but of hydrochloric and nitric acids

Acetates of Lead.—Acetic acid forms several compounds with

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The normal acetate and the tribasic acetate are important salts. The normal acetate of lead (Pb 2 Є,H,,, 3 H2O=325+54) may be procured by dissolving litharge in excess of acetic acid: by evaporating the solution, the salt may be obtained crystallized in right rhombic prisms, which are sometimes opaque, but generally transparent: its more usual form, however, is that of a mass of confused minute white crystals, much resembling loaf-sugar; it also has a sweetish metallic taste, and from this circumstance it derives its common name of sugar of lead: when exposed to the air it is efflorescent. It is soluble in less than twice its weight of water, and is also freely soluble in alcohol. When heated it first becomes anhydrous, and then fuses to a clear liquid. If the heat be raised further, acetone, ¤ ̧H., is formed, carbonic anhydride escapes, and the mass becomes solid. In this state it consists of sesquibasic acetate of lead (2 [Pb 2 ¤‚Í‚Ð2], PьЂ͂¤); this salt is readily soluble both in alcohol and in water; it has a distinctly alkaline reaction, and crystallizes in pearly scales: the same salt may also be formed by adding the normal acetate in the proportion of 57 parts, or 3 atoms, to 38, or 1 atom, of a concentrated boiling solution of the tribasic acetate of lead.

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Tribasic acetate of lead (Pb 2 €2H ̧Є, 2 Pb‡‚Í ̧‡), formerly known as Goulard's Extract, is prepared by digesting 7 parts of finely powdered litharge with 6 parts of normal acetate of lead, dissolved in 30 parts of water: it has a strongly alkaline reaction. The solution of oxide of lead is effected much more rapidly if a silver basin be used than if one of glass or porcelain be employed.

not less than 600 atoms are required to produce an effect of coagulation equal to that occasioned by 1 atom of sulphuric acid. One grain of potash dissolved in 1000 of water coagulates 9000 grains of the solution; and soda, ammonia, and lime have an equally powerful effect. An excess of potash, as well as of sulphuric or hydrochloric acid, dissolves the coagulum, converting the alumina into its ordinary modification. Solutions of the sulphates coagulate the solution as readily as free sulphuric acid.

Alumina, in this soluble condition, has lost its power of acting as a mordant. When the precipitated aluminic diacetate is kept for an hour or two in 200 parts of boiling water with constant agitation, it becomes gradually dissolved, and may then, by digestion, be converted into free acetic acid and the soluble hydrate of alumina. The soluble hydrate of alumina, when evaporated at 2120, retains 2 atoms of water (Al,,, 2 H2O).