NITRIC ACID.—(FR., Acids nitrique or azotique ; GER., Salpetersaiire), sometimes, in a diluted form, called aquafortis (eau forte, schcidewasser). Constituents, hydrogen, nitrogen, and oxygen. Formula, Nitric acid may be viewed as a compound of one equivalent of the anhydrous acid with one equivalent of water. When pure it is a transparent colourless liquid of 1.52 sp. gr., giving off, in moist air, white fumes with a well-known sour taste and pungent odour. The commercial acid is usually more or less coloured, owing to the presence of certain lower oxides of nitrogen. Its mixture with water develops heat, a definite acid liquid of 1.42 sp. gr. being formed, containing 60 per cent. of the anhydrous acid. At 86° nitric acid boils and undergoes partial decomposition into water, oxygen and peroxide of nitrogen. After about three-fourths of the liquid has distilled over, the residue loses all colour, and contains 95.8 per cent. of real acid. Beyond this the boiling point rises continually, a strong acid coming over, and a weaker being left behind. When this residue attains a percentage of only 68 it is found to boil unaltered at a temperature of 120°, under the ordinary pressure of the atmosphere. Whatever may be the strength of the acid experimented upon this final constant liquid is always obtained. This is shown in the following table which gives the results of a series of experiments by Roscoe. Column I. gives the volume of acid employed ; column II. the percentage quantity of real acid in the liquid before distillation ; column III. the volume of liquid remaining in the retort ; column IV. the percentage of real nitric acid contained in the residual liquid after distillation If the pressure under which the operation is conducted be varied, a new, but always finally fixed, acid is obtained, with a constant boiling point. Thus under a pressure of 1.22 m. of mercury a 68.6 per cent. acid distils over unchanged, whilst under a pressure of 0.070 m. an acid with constant composition of 66.7 per cent. comes over at a temperature of from 65° to 70°. When aqueous
nitric acid is subjected to a current of dry air, a stronger or weaker acid is volatilized, according to the strength or temperature, until a residual liquid is obtained which volatilizes unchanged. Thus at a temperature of 100° the residual acid shows a composition of 66.2 per cent. ; at a tempera ture of 60° per cent., while at 15° the residue has a fixed composition of 64 per cent. real acid. In all these respects aqueous nitric acid resembles hydrochloric acid. For further details relating to these matters the reader is referred to the paper published by Dr. Roscoe in the Chem. Soc. Qu. J.,' vol. xiii. 150.
I. Kolbe (' An. Ch. Phys.' [4] X. 136) has published a new table of the exact specific gravity of nitric acid of various strengths:— Nitric acid is a strong oxidizing agent, attacking nearly all the metals, the non-metallic sub stances and organic bodies. The final products of the combustion of the last-named substances are, usually, acetic, formic, and oxalic acids, with various intermediate and resulting compounds. Most of the metals are converted into nitrates, while the non-metallic bodies—e. g. phosphorus, arsenic, sulphur, carbon, silicon—are dissolved into their respective acids. Gold, platinum, and titanium resist the solvent. The potency, and resulting action of nitric acid depend to a great extent upon its strength and the temperature at which the operation is conducted. When a strong acid is used, as a rule, nitric oxide is evolved ; when a weaker solvent, a lower oxide of nitrogen. Charcoal digested with a strong acid, at a temperature below 0°, liberates oxygen, with the evolution of nitric peroxide. Occasionally nitrogen is set free. Many of the proto-salts—e. g. arsenites—are converted by the action of nitric acid into per-salts. It stains many animal substances a deep yellowish brown colour. The pure liquid is much less potent than when it contains nitrous acid or nitric oxide, forming the well known, red, fuming liquid.