acid is commonly applied. The hydrate of acetic acid is also soluble in alcohol. 6 6 2. The acetates undergo decomposition at a red heat; among the products of this decomposition we generally find hydrate of acetic acid, and almost invariably acetone (C, H, O). The acetates of the alkalies and alkaline earths are converted into carbonates in this process; of the acetates with metallic bases many leave the metal behind in the pure state, others in the form of oxide. Most of the residues which the acetates leave upon ignition are carbonaceous. Nearly all acetates dissolve in water and in alcohol; most of them are readily soluble in water, a few only are difficult of solution in that menstruum. If acetates are distilled with dilute sulphuric acid, the free acetic acid is obtained in the distillate. 3. If sesquichloride of iron is added to acetic acid, and the acid is then nearly saturated with ammonia, or if a neutral acetate is mixed with sesquichloride of iron, the fluid acquires a deep dark red color, owing to the formation of ACETATE OF SESQUIOXIDE OF IRON. Upon boiling, the fluid becomes colorless if it contains an excess of acetate, the whole of the sesquioxide of iron precipitating as a basic acetate, in the form of brown-yellow flakes. Ammonia precipitates from it the whole of the sesquioxide of iron as hydrate. Upon addition of hydrochloric acid, a fluid which appears red from the presence of acetate of sesquioxide of iron turns yellow (difference from sulphocyanide of iron). 4. Neutral acetates (but not free acetic acid) give with nitrate of silver white, crystalline precipitates of ACETATE OF SILVER (Ag O, C, H, 0), which are very sparingly soluble in cold water. They dissolve more easily in hot water, but separate again upon cooling, in the form of very fine crystals. Ammonia dissolves them readily; free acetic acid does not increase their solubility in water. 5. Nitrate of suboxide of mercury produces in solutions of acetic acid, and more readily still in solutions of acetates, white, scaly crystalline precipitates of ACETATE OF SUBOXIDE OF MERCURY (Hg, Ó, C, H, 03), which are sparingly soluble in water and acetic acid in the cold, but dissolve without difficulty in an excess of the precipitant. The precipitates dissolve in water upon heating, but separate again upon cooling, in the form of small crystals; in this process the salt undergoes partial decomposition: a portion of the mercury separates in the metallic state, and imparts a gray color to the precipitate. If the acetate of suboxide of mercury is boiled with dilute acetic acid instead of water, the quantity of the metallic mercury which separates is exceedingly minute. 6. When acetates are heated with concentrated sulphuric acid, HYDRATE OF ACETIC ACID is evolved, which may be known by its pungent odor. But if the acetates are heated with a mixture of about equal volumes of concentrated sulphuric acid and alcohol, ACETIC ETHER (C, H, O, C, H, O2) is formed. The odor of this ether is highly characteristic and agreeable; it is most distinct upon shaking the mixture when somewhat cooled, and is much less liable to lead to mistakes than the pungent odor of the free acetic acid. 7. If acetates are distilled with dilute sulphuric acid, and the distillate is digested with an excess of oxide of lead, part of the latter dissolves as basic acetate of lead, which may be readily recognised by its alkaline reaction. $171. b. FORMIC ACID (H O, C, H 0,). 1. The hydrate of formic acid is a transparent, colorless, slightly fuming liquid, of a characteristic and exceedingly penetrating odor. When cooled to below 32° F., it crystallizes in colorless plates. It is miscible in all proportions with water and with alcohol. When exposed to the action of heat, it volatilizes completely; the vapors are inflammable and burn with a blue flame. 2. The formates, like the corresponding acetates, leave upon ignition either carbonates, oxides, or metals behind, the process being attended with separation of charcoal, and escape of carbide of hydrogen, carbonic acid, and water. All the compounds of formic acid with bases are soluble in water; alcohol likewise dissolves some of them. 3. Formic acid presents the same deportment with sesquichloride of iron as acetic acid. 4. Nitrate of silver fails to precipitate free formic acid, and decomposes the alkaline formates only in concentrated solutions. The white, sparingly soluble, crystalline precipitate of FORMATE OF SILVER (Ag O, C, HO,) acquires very rapidly a darker tint, owing to the separation of metallic silver. Complete reduction of the oxide of silver to the metallic state takes place, even in the cold, after the lapse of some time; but immediately, upon applying heat to the fluid containing the precipitated formate of silver. The same reduction of the oxide of silver to the metallic state takes place in a solution of free formic acid, and also in solutions of formates so dilute that the addition of the nitrate of silver failed to produce a precipitate in them. But it does not take place in presence of an excess of ammonia. The rationale of this reduction is as follows: the formic acid, which may be looked upon as a compound of carbonic oxide with water, deprives the oxide of silver of its oxygen, thus causing the formation of carbonic acid, which escapes, and of water, whilst the reduced silver separates in the metallic state. 5. Nitrate of suboxide of mercury gives no precipitate with free formic acid; but in concentrated solutions of alkaline formates this reagent produces a white, sparingly soluble precipitate of FORMATE OF SUBOXIDE. OF MERCURY (Hg, O, C, H O2), which rapidly becomes gray, owing to the separation of metallic mercury. Complete reduction ensues, even in the cold, after the lapse of some time, but is immediate upon application of heat. This reduction is also attended with the formation of carbonic acid and water, and takes place, the same as with the oxide of silver, both in solutions of free formic acid and in fluids so highly dilute that the formate of suboxide of mercury is retained in solution. 6. If formic acid or an alkaline formate is heated with chloride of mercury to from 140° to 158° F., SUBCHLORIDE OF MERCURY precipitates. If the mixture is heated to 212° F., metallic mercury separates along with the subchloride. 7. If formic acid or a formate is heated with concentrated sulphuric acid, the formic acid is resolved into water and carbonic oxide gas, which latter escapes with effervescence, and if kindled, burns with a blue flame. The fluid does not turn black in this process. The rationale of the decomposition of the formic acid is this: the sulphuric acid withdraws from the formic acid the water or the oxide necessary for the existence of the latter acid, and thus occasions a transposition of its elements (C, HO, 2 CO+HO). Upon heating formates with dilute sulphuric acid in a distilling apparatus, free formic acid is obtained in the distillate, and may mostly be readily detected by its odor. Upon heating a formate with a mixture of sulphuric acid and alcohol, formic ether is evolved, which is characterized by its peculiar arrack-like smell. 8. If dilute formic acid is heated with oxide of lead, the latter dissolves. On cooling the solution, which, if necessary, is concentrated by, evaporation, the FORMATE OF LEAD (Pb O, C, H O2) separates in brilliant prisms or needles. $172. Recapitulation and remarks.-Acetic acid and formic acid may be distilled over with water, and form with sesquioxide of iron soluble neutral salts which dissolve in water, imparting to the fluid a blood-red color, and are decomposed upon boiling. These reactions distinguish the two acids of the third group from the other organic acids. From each other the two acids are distinguished by the odor of their hydrates and ethyle compounds, and by their different deportment with salts of silver and salts of mercury, oxide of lead, and concentrated sulphuric acid. The separation of acetic acid from formic acid is effected by heating the mixture of the two acids with an excess of oxide of mercury or oxide of silver. Formic acid reduces the oxides, and suffers decomposition, being resolved into carbonic oxide and water; whilst the acetic acid combines with the oxides, forming acetates, which remain in solution. |