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10 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

nitric acid gradually becomes dehydrated with increase in concentration of sul-

phuric acid. Finally, at x =n, nitric acid becomes anhydrous.

HNO 3. nH 2 O + nH 2 SO 4 + HNO 3 + nH 2 SO 4. H 2 O (4)

At high concentrations (mixtures with a low water content) Sapozhnikov found

certain anomalies in the vapour pressure of HNO 3 and in its electric conductivity.

He ascribed them to the formation of nitric anhydride N 2 O 5. Thus the equation

will be:

2HNO 3 + H 2 SO 4 -> N 2 O 5 + H 2 SO 4. H 2 O (5)

Later, to confirm his theory, Sapozhnikov gave some data on the O-nitration

of cellulose with nitric acid (see Vol. II) and on the nitration of naphthalene (p. 427).

FIG.. 3. Isobars of partial vapour pressure of nitric acid in nitric acid-

sulphuric acid-water mixtures (Sapozhnikov [16]).

The nitration curves for these compounds when nitrated with nitrating mixtures

are similar to the vapour pressure curves of nitric acid in mixtures, presented after

Sapozhnikov in Fig. 3.

Sapozhnikov’s view was partly confirmed by Walden [17]. Halban and Eisen-

brand found [18] that the absorption spectrum of pure nitric acid in the ultra-

violet differed completely from its spectrum when in solution in anhydrous sul-

phuric acid. The authors explained this observation by assuming the existence

of the nitric anhydride or the mixed nitric-sulphuric anhydride. This assumption

seemed to be all the more probable as in some earlier papers the idea had already

been expressed that N 2 O 5 might exist in concentrated nitric acid. Thus in 1898