NITRIC ACID (aqua fortis), an important mineral acid, It is mentioned in the De inventione veritatis ascribed to Geber, wherein it is obtained by calcining a mixture of nitre, alum and blue vitriol. It was again described by Albert le Grand ir. the 13th century and by Raimon Lull, who prepared it by heating nitre and clay and called it "eau forte." Glauber de vised the process in common use for many years, viz., by heating nitre with strong sulphuric acid. Its true nature was not de termined until the century, when A. L. Lavoisier (1776) showed that it contained oxygen, whilst in 1785 H. Cavendish determined its constitution and showed that it could be syn thesized by passing a stream of electric sparks through moist air. The acid is found to exist to a slight extent in the free condition in some waters, but chiefly occurs in combination with various metals, as nitrates, principally as nitre or saltpetre, and Chile saltpetre, It is formed when a stream of electric sparks is passed through moist air, and in the oxidation of nitro genous matter in the presence of water.
For experimental purposes it is usually obtained by distilling potassium or sodium nitrate with concentrated sulphuric acid. The acid so obtained usually contains more or less water and some dissolved nitrogen peroxide which gives it a yellowish red colour. It may be purified by redistillation over barium and silver nitrates and then under diminished pressure over urea. On the large scale it is obtained by three methods: (I) from saltpetre and concentrated sulphuric acid; (2) by the catalytic oxidation of ammonia; and (3) by the direct oxidation of nitrogen in the electric arc. In the first and oldest method, Chile saltpetre is distilled with concentrated sulphuric acid in horizontal cast iron stills, the vapours being condensed in a series of stone ware Woulfe's bottles. In practice the theoretical quantity of acid and Chile saltpetre is not used, but the charge is so regulated that the mixture of acid and neutral sodium sulphate formed in the retort remains liquid at the temperature employed, and con sequently can be readily removed. (2) The oxidation of ammonia is effected by passing this gas mixed with air or oxygen over a thin coating of spongy platinum deposited on a supporting material and kept at about 600° C. Platinum gauze may replace the sponge,
and the gases may be pre-heated at 300-400° C, the heat of reaction then being sufficient to maintain the whole at the re quired higher temperature. Other catalysts may be used and each has an appropriate suitable temperature; thus, with iron oxide the best temperature is 650-700°. Often a mixture of oxides is more efficient, one acting as a "promoter" for the other. The am monia must be free from impurities such as phosphine, and a fairly rapid stream of gas is used. The yields may exceed 95% of the theoretical. (3) For the production of nitric acid from air see NITROGEN, FIXATION OF. Fuming nitric acid consists of a solution of nitrogen peroxide in concentrated nitric acid and is prepared by distilling dry sodium nitrate with concentrated sulphuric acid.
Nitric acid is a colourless strongly fuming liquid, having a specific gravity of C) (V. Veley 1898). It is ex ceedingly hygroscopic and corrosive. On distillation, the pure acid begins to boil at 78.2° C (W. Ramsay), partial decomposition into water, oxygen and nitrogen peroxide taking place. The acid solidi fies when strongly cooled, the solid melting at —47° C. Concen trated nitric acid forms with water a constant-boiling mixture, the composition of which varies according to the pressure at which it is distilled : if distillation is at atmospheric pressure, the resulting acid boils at 120•5° C, contains 68% of and possesses a specific gravity of 1.414 C). If a more dilute acid than this be distilled, water passes over in excess and the residue in the retort reaches the above composition and boiling point; on dis tillation of a stronger acid, excess of passes into the distillate and the boiling point rises until the values of the constant-boiling mixture are reached. On the hydrates of nitric acid see V. Veley, Jour. Chem. Soc., 1903, 83, p. 1o15, and F. W. Kuster, Zeit. anorg. Chem., 1904, 41, p. 1. On mixing nitric acid with water there is a rise of temperature and a contraction in volume.