[152] “A manufactory of sal-ammoniac, near Magdeburgh, which uses urine, is able to supply flowers of benjamin by the cwt.” (Gray.)
Prop. When obtained by sublimation benzoic acid forms soft, light, feathery, white, flexible crystals, which are transparent or semi-transparent, with more or less of a mother-of-pearl lustre; when by slowly cooling its aqueous solution, or by precipitation from a solution of a benzoate, it forms either thin plates or scales, or a dazzling white crystalline powder. It is inodorous when cold,[153] but acquires a faint balsamic odour when gently warmed; fuses at about 212° Fahr., and begins to sublime freely at a temperature a little above it, but does not boil until heated to about 460°; burns with a bright yellow flame; is very soluble in alcohol, dissolves in about 200 parts of cold water, and about 25 parts of boiling water; resists the action of ordinary nitric acid even when boiling; and forms salts (BEN′ZOATES) with the bases. Sp. gr. 0·667. Its vapour, which is very suffocating and irritating, has a density of 4·27. Added to fat and fatty substances it either prevents, or greatly retards, the accession of rancidity.
[153] That of the shops usually smells slightly of benzoin, owing to the presence of a trace of volatile oil.
Test, &c. It may be recognised—1. By its physical properties (appearance, fusibility, volatility, odour, &c.) already enumerated:—2. By its ready solubility in solutions of the alkalies; and by being precipitated from these solutions, on the addition of one of the stronger acids, under the form of a dazzling white powder, which is only sparingly soluble in cold water:—3. By its neutral salts with the alkalies, or its neutral solution in an alkali, giving a bulky, flesh-coloured precipitate with perchloride of iron, which is insoluble in water:—4. By its solution not being precipitated by acetate of lead until after neutralisation with a fixed alkali, when the acetate produces a white, flocculent precipitate:—5. By a mixture of alcohol, ammonia, and solution of chloride of barium, neither disturbing a solution of the free acid, nor that of one of its salts with the alkalies.
It is chemically distinguished from cinnamic acid by not yielding essential oil of almonds when it is distilled with oxidising agents, as chromic acid or a mixture of bichromate of potassium and sulphuric acid; and from succinic acid, by its different deportment with sesquichloride of iron (Test 3, antè), and with a mixture of alcohol, ammonia, and solution of chloride of barium (T. 5, antè).
Estim.—1. By weighing it as benzoic acid, obtained either by precipitation, or by very careful sublimation in a glass apparatus:—2. By neutralising its alcoholic or aqueous solution, by the usual method of acidimetry:—3. By precipitating its neutral solution with acetate of lead, or with sesquichloride of iron, and weighing the carefully washed and dried
precipitate either as benzoate of lead, or as ferric benzoate.
Pur., &c. White crystalline silky plates and needles, have an aromatic odour. Solubility in cold water, 1 in 300; in boiling water, 1 in 12; in spirit, 1 in 4. Also soluble in caustic alkalies and lime. Borax considerably increases its solubility in water; 1 of benzoic acid and 1 of borax are soluble in 100 of water. It sublimes without residue when heated. It is sometimes met with adulterated with hippuric acid, which may be easily detected by its altered form, by its diminishing solubility in cold water, and by its exhaling an odour of tonquin-beans, and afterwards of hydrocyanic acid, when sublimed. The presence of succinic acid may be readily detected by its greatly increased solubility in cold water; that of sugar, not only by its increased solubility, and partial volatility, but also by the odour of caramel being evolved on the application of sufficient heat, and the residuum being black and carbonaceous; that of camphor, by its peculiar odour when gently heated. Spermaceti, specially prepared for the purpose, is also an occasional adulterant, easily detected by its insolubility and other well-known properties. All these substances either destroy or lack the proper crystalline form of benzoic acid, which is one of the best proofs of its purity. They also greatly increase its sp. gr.
Uses, &c. Its chief use in medicine is as a stimulant and expectorant. It is an ingredient in the compound tincture of camphor (paregoric elixir) of the pharmacopœia.—Dose, 10 to 30 gr., dissolved in water by the aid of a little ammonia or potassa; in old coughs, &c.
BENZOIC AL′COHOL. A peculiar oily fluid, discovered by M. Cannizzaro, and obtained by the action of an alcoholic solution of potassa on pure oil of bitter almonds.