264 PRINCIPLES OF CHEMISTRY 



Nitric acid so obtained always contains water. It is extremely 

 difficult to deprive it of all the admixed water without destroying a 

 portion of the acid itself and partially converting it into lower oxides, 

 because without the presence of an excess of water it is very unstable. 

 When rapidly distilled a portion is decomposed, and there are obtained 

 free oxygen and lower oxides of nitrogen, which, together with the 

 water, remain in solution with the nitric acid. Therefore it is neces- 

 sary to work with great care in order to obtain a pure hydrate of nitric 

 acid, HNO 3 , and especially to mix the nitric acid obtained from nitre, 

 as above described, with sulphuric acid, which takes up the water, and 

 to distil it at the lowest possible temperature that is, by placing the 

 retort holding the mixture in a water or oil bath and carefully heating 



Nevertheless, both sulphuric and nitric acids evince a certain action on cast iron, and 

 therefore the acid obtained will contain traces of iron. In practice sodium nitrate (Chili 

 saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to 

 take potassium nitrate, because it is purer and does not froth up so much as sodium 

 nitrate when heated with sulphuric acid. In the action of an excess of sulphuric acid on 

 nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of 

 nitrogen, which are dissolved in the nitric acid. A portion of the sulphuric acid itself is 

 also carried over as spray by the vapours of the nitric acid. Hence sulphuric acid occurs 

 as an impurity in commercial nitric acid. A certain amount of hydrochloric acid will 

 also be found to be present in it, because sodium chloride is generally found as an im- 

 purity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. Com- 

 mercial acid further contains a considerable excess of water above that necessary for the 

 formation of the hydrate, because water is first poured into the earthenware vessels 

 employed for condensing the nitric acid in order to facilitate its cooling and condensation. 

 Further, the acid of composition HNOj decomposes with great ease, with the evolution 

 of oxides of nitrogen. Thus the commercial acid contains a great number of impurities. 

 Generally its specific gravity is 1'33 (36 Baume), and it contains 53 p.c. of nitric acid. 

 The acid employed in medicine and in the laboratory contains one-third of nitric acid and 

 two-thirds of water, and its specific gravity is T2. The commercial acid is often purified 

 in the following manner : Lead nitrate is first added to the acid because it forms non- 

 volatile and almost insoluble (precipitated) substances with the free sulphuric and 

 hydrochloric acids, and liberates nitric acid in so doing, according to the equations 

 Pb(NO 3 )o + 2HCl = PbCl 2 + 2NHO 5 and Pb(NOs). 2 + H.,,SO 4 = PbSO4 + 2NHO 3 . . Potas- 

 sium chromate is then added to the impure nitric acid, by which means oxygen is 

 liberated from the chromic acid, and this oxygen, at the moment of its evolution, 

 oxidises the lower oxides of nitrogen and converts them into nitric acid. A pure nitric 

 acid, containing no impurities other than water, may be then obtained by distilling 

 the acid, manipulated as above described, with care, and particularly if only the middle 

 portions of the distillate are collected. Such acid should give no precipitate, either 

 with a solution of barium chloride (a precipitate shows the presence of sulphuric acid) 

 or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric 

 acid), nor should it, after being diluted with water, give a coloration with starch con- 

 taining potassium iodide (a coloration shows the admixture of other oxides of nitrogen). 

 The oxides of nitrogen may be most easily removed from impure nitric acid by heat- 

 ing for a certain time with a small quantity of pure charcoal. By the action of nitric 

 acid on the charcoal carbonic anhydride is evolved, which carries off the NO, NOa, 

 and other volatile substances. On redistilling, pure acid is obtained. The oxides of 

 nitrogen occurring iu solution may also be removed by passing air through the nitric 

 acid. 



