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8 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

They believe that this formula is better than the others in explaining the ability

of nitric acid to dissociate. Moreover it is consistent with the low basicity of

the nitro groups (which is discussed later, see p. 218).

A more detailed review of research work on the absorption spectra of nitric

acid is given later (see p. 19-27).

Nitric acid forms hydrates with water: HNO 3. H 2 O (m. p. -38°C) and

HNO 3 .3H 2 O (m. p.-18.5°C). The chief evidence for these hydrates is obtained

from the thermal analysis of the system nitric acid-water (Fig. 2). Other experimental

Mole % HNO3

FIG. 2. Thermal analysis of nitric acid-water system.

facts also confirm the existence of addition compounds of nitric acid and water.

Thus the refractive index shows, according to Veley and Manley [12], a linear

relation over the range from 78 to 91% concentration. At 91% a sharp inflection

occurs, and above 98.5% the slope of the curve is reversed. The electrical conduc-

tivity also shows anomalies over this concentration range, passing through a min-

imum.

The considerable heat evolved on dilution of nitric acid with water proves that

the nitric acid hydrates are stable. With an excess of nitric acid the heat amounts

to 3.5 kcal per mole of water. It has been suggested that the nitric acid hydrate,

HNO 3 .H 2 O, present in concentrated nitric acid is undissociated and that the water

molecules are attached to the nitric acid molecules by hydrogen bonds. Thus there

would be the equilibrium :

(2)

Nitric acid acts on organic compounds both as a nitrating and as an oxidizing

agent. The reaction depends on the compound being nitrated and on the concen-

tration of the nitric acid. In accordance with a general rule, a concentrated nitric

acid acts mostly as a nitrating agent. When diluted its nitrating action diminishes