The acid is a powerful oxidizing agent. It attacks most metals readily, usually with production of a nitrate or hydrated oxide of the metal and one or other of the oxides of nitrogen, or oc casionally with the production of ammonium salts; magnesium, however, liberates hydrogen from the very dilute acid. Its action on metals depends in most cases on the temperature, strength of the acid, and the nature of the products of reaction. Thus in the case of copper, it is found that the diluted acid acts very slowly upon the metal at first, but as the reaction proceeds the copper dissolves more rapidly up to a certain point and then the rate of solution again diminishes. This is possibly due to the accelerating action of the nitrous acid which is produced in the direct action of the copper on the nitric acid and which, when a certain amount has been formed in the system, begins to decompose, thus = 2NO Iron when brought into contact with nitric acid under certain conditions, remains passive to the acid. Thus at C it is pas sive to an acid of specific gravity 1.42 and at 31° C to an acid of specific gravity 1.38. The cause of this passivity was for long a matter of dispute, but V. R. Evans in 1927 (J. Chem. Soc., p. 1020) succeeded in isolating an actual protective film from pas sivated iron. Nitric acid is without action on gold, platinum, iridium and rhodium.
A. Hantzsch (Berichte, 1925, 55, P. believes that con centrated nitric acid is chiefly composed of "nitronium" nitrate, [N(OH)3] (NO3)2. (See also ACIDS.) The salts of nitric acid, known as nitrates, are nearly all readily soluble in water and crystallize well. They are all decomposed when heated to a sufficiently high temperature, with evolution for the most part of oxygen and nitrogen peroxide, leaving a residue of oxide of the metal. They may be recognized by the fact that
on the addition of a solution of ferrous sulphate, followed by that of concentrated sulphuric acid (the mixture being kept quite cold), the ferrous sulphate solution becomes of a deep brown colour, owing to the reducing action of the ferrous sulphate on the nitric acid which is liberated by the action of the sulphuric acid on the nitrate. As an alternative method the nitrate may be warmed with some fragments of copper and sulphuric acid which has been diluted with its own volume of water, when characteristic brown vapours will be seen.
Nitric acid finds extensive application in the manufacture of sulphuric acid, certain coal-tar colouring matters, explosives, and in the production of various nitrates.
In medicine, nitric acid is used externally in a pure state as a caustic to destroy chancres, warts and phagadenic ulcers, and diluted preparations are employed in the treatment of dyspepsia, etc. Poisoning by strong nitric acid produces a widespread gastro enteritis, burning pain in the oesophagus and abdomen and bloody diarrhoea. There may also be blood in the urine. Death occurs from collapse or from secondary destructive changes in the in testinal canal. Characteristic yellow staining of the skin round the mouth from the formation of xanthoproteic acid serves to dis tinguish it from poisoning by other acids. The antidotes are mild alkalis, together with the use of opium to relieve pain.
(See A. Cottrell, The Manufacture of Nitric Acid and Nitrates, London, 1923.)