3. Nitrate of soda (cubic nitre, Chili saltpetre) is introduced, in quantities varying between 4 and 10 lbs., into a cylindrical iron retort, which it will only half fill, and after the lid is luted on and the connection made with the condensers, an equivalent of oil of vitriol is poured in through an aperture provided for the purpose, and the charge is worked off with a gradually increased heat. The condensing apparatus consists of a series of 5 or 6 salt-glazed stoneware receivers, about 1⁄6th part filled with cold water. The product of this process is the strongest brown and fuming ‘NITROUS ACID’ of commerce (AQUAFORTIS, FUMING NITRIC ACID; ACIDUM NITROSUM;
ACIDUM NITRICUM FUMANS), and has usually the sp. gr. 1·45. It is rendered colourless by gently heating it in a glass retort, when it forms COMMERCIAL NITRIC ACID (sp. gr. 1·37 to 1·4.)
4. (Pure monohydrated nitric acid.) By mixing the strongest commercial acid with about an equal quantity of oil of vitriol; redistilling; collecting apart the first portion which comes over, and exposing it, in a vessel slightly warmed and sheltered from the light, to a current of dry air made to bubble through it until the nitrous acid is completely removed.
Prop. Pure liquid nitric acid is colourless, highly corrosive, and possesses powerful acid and oxygenising properties. The sp. gr. of the strongest liquid acid (monohydrated nitric acid) has the sp. gr. 1·517 at 60° Fahr. “On boiling nitric acid of different degrees of concentration at the ordinary atmospheric pressure, a residue is left boiling at 249° Fahr., and 29 in. barometer, having a sp. gr. 1·414 at 60° Fahr.” (Fownes.) Acid of less density than 1·414 parts with water, and gradually becomes stronger by boiling; but acid of less sp. gr. than 1·414 is weakened by exposure to heat. When exposed to intense cold, liquid nitric acid freezes. It is rapidly decomposed, with loss of oxygen, by contact with most organic and many metallic and non-metallic bodies. In many cases these reactions occur with considerable violence, and the production of light and heat.
Pur. The nitric acid of commerce is generally contaminated by hydrochloric acid, nitrous acid, sulphuric acid, or chlorine, or by their soda or potassa salts, and, occasionally, iodine, together with an excess of water. The last is readily detected by the sp. gr., and the others by the appropriate tests. “Colourless. Contains 70% of HNO3. Sp. gr. 1·42. 90 gr. by weight, mixed with 1⁄2 oz. of distilled water, require for neutralisation 1000 grain measures of the volumetric solution of soda. Evaporated, it leaves no residue. Diluted with six volumes of distilled water, it gives no precipitate with chloride of barium or nitrate of silver—indicating absence of sulphuric and hydrochloric acids.” (B. P.) 5 measures of acid, sp. gr. 1·5, mixed with 2 of water, condensed into 61⁄2 measures, and makes the sp. gr. 1·42. “Free from colour. Sp. gr. 1·42. Exposed to the air, it emits very acrid vapours. Totally volatilised by heat. Diluted with 3 times its volume of water, it gives no precipitate with either nitrate of silver or chloride of barium. 100 gr. of this acid (sp. gr. 1·42) are saturated by 161 gr. of crystallised carbonate of soda.” (Ph. L.) The Ph. E. states the density of commercial nitric acid is 1·380 to 1·390. “If diluted with distilled water it precipitates but slightly, or not at all, with solution of nitrate of baryta or nitrate of silver.” The best ‘double aquafortis’ of the shops (aquafortis duplex) has usually the sp. gr. 1·36; and the single aquafortis
(aquafortis simplex), the sp. gr. 1·22; but both are commonly sold at much lower strengths.
Tests.—1. It stains the skin yellow.—2. When mixed with a little hydrochloric acid or chloride of ammonium, it acquires the power of dissolving gold leaf.—3. Morphia, brucia, and strychnia, give it a red colour, which is heightened by ammonia in excess.—4. When placed in a tube, and a solution of protosulphate of iron is cautiously added, a dark colour is developed at the line of junction, which is distinctly visible when only 1⁄24,000th part of nitric acid is present. This test may be often conveniently modified by dropping into the liquid a crystal of protosulphate of iron; the fluid immediately surrounding this crystal then acquires a dark brown colour, which disappears upon simple agitation of the fluid, or by heating it.—5. When mixed with a weak solution of sulphate of indigo, and heated, the colour of the latter is destroyed.—6. When saturated with carbonate of potassium or sodium, and evaporated to dryness, the residuum deflagrates when thrown on burning coals.—7. When the mixture of a nitrate with cyanide of potassium, in powder, is heated on a piece of platinum, a vivid deflagration follows, attended with distinct ignition and detonation. (Fresenius.) It is stated that sulphate of aniline is an extremely delicate test for nitric acid. The following is the method of its application:—About a cubic centimètre of pure concentrated sulphuric acid (sp. gr. 1·84) is placed in a watch-glass; half a cubic centimètre of a solution of sulphate of aniline (formed by adding ten drops of commercial aniline to 50 c.c. of diluted sulphuric acid in the proportion of 1 to 6) is poured on, drop by drop; a glass tube is moistened with the liquid to be tested, and moved circularly in the watch-glass. By blowing on the mixture during the circular agitation, when a trace of nitric acid is present, circular striæ are developed of a very intense red colour, tinting the liquid rose. With more than a trace of nitric acid the colour becomes carmine, passing to a brownish red. This process serves to detect the presence of nitric acid in the sulphuric acid of commerce. It will also reveal the presence of nitrates in water.[46]—8. Take a quantity of diphenylamine, about the size of a mustard seed, put it into a test-tube, and pour a little sulphuric acid over it, then add a drop or two of water, so as to increase the temperature sufficiently to effect the solution of the diphenylamine, and the test is ready for use. Now add very gently the solution to be tested, and if only a trace of nitric or nitrous acid be present, a beautiful and very permanent blue colouration is produced at the junction of the two liquids, but if there be any quantity of the nitrogen compound, the colour becomes almost black. This reaction is so delicate and certain that, in
the case of a solution of nitric acid containing about 1 part B. P. acid in 10,000 of water, the reaction is most distinct; one part of nitrite of potassium in 30,000 of water gives also almost unmistakable evidence of the presence of the nitrogen acid.
[46] ‘Pharmaceutical Year Book.’
The following process for the quantitative estimation of nitric acid is by Fischer:[47]—Indigotin prepared by reduction of indigo by means of grape sugar, alcohol, and caustic soda, oxidation in the air, and solution in sulphuric acid, may be kept unchanged for years. Five c.c. of such a solution, diluted with water and mixed with 30 c.c. of pure sulphuric acid, is titrated by adding a standard nitric acid solution until the blue colour gives place to a light green; the indigo solution is then diluted, so that 1 c.c. shall be equal to 0·0025 milligramme equivalents of nitric acid, or 0·2525 milligramme of potassium nitrate. If a water is being examined it is run into 4 c.c. of the titrated indigo solution, mixed with 20 c.c. of sulphuric acid, until the blue colour changes to light green. Ten, divided by the number of c.c. of water used, expresses the milligramme equivalents of nitric acid per litre; thus, if 4 c.c. of water are used, there are 2·5 milligramme equivalents of nitric acid, equal to 252·5 milligrammes of potassium nitrate per litre. If a preliminary test with brucine has shown that the water contains very little nitric acid, 2 c.c. only of the indigo solution must be used, or sometimes as little as 1 c.c. If more than 8 c.c. of water is required to destroy the blue colour, 100 c.c. must be evaporated down to the volume of 8 c.c. and then titrated. The volume of sulphuric acid must be at least double the sum of the volumes of indigo and water; the temperature must not sink under 110°.