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16 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

3HNO 3 -> NO 3 - + NO 2 + + HNO 3 .H 2 O (25)

Calculations have shown, that the NO 2 + and NO 3 - ions content in anhydrous

nitric acid at -40°C is 1.2 and 1.7% by weight respectively. The content of water

is 0.5% (in form of the nitric acid hydrate).

Bennett and his co-workers [43] confirmed this interpretation of the cryometric

investigations. To prove definitely the existence of the NO 2 + ion, they attempted

to show that when electrolysed, the ion is transported towards the cathode. They

did not succeed in obtaining full evidence for this, although they found that nitric

acid moves away from the anode. It was only when the electrolysis was carried out

in the presence of oleum and barium salts, that the transport of nitric acid towards

the cathode was confirmed. Studying the cathodic polarization of nitric acid Mint

[44] observed the evolution of nitrogen dioxide at the cathode. This may be an

additional piece of evidence for the transport of an ion containing nitrogen (prob-

ably NO 2 +) towards the cathode.

Brand [45] gave a very simple method for the analysis of nitrating mixtures

confirming the theory of the formation of the nitronium ion in solutions of

nitric acid in oleum. It is well known that free SO 3 present in a solution may

be titrated with water until fuming disappears. This method gives fairly accurate

results. The volume of water added for the titration of a mixture HNO 3 + oleum

is smaller than the quantity used for the titration of pure oleum without HNO 3.

This would be evidence of the reactions:

HONO 2 + NO 2 + + OH- (26)

OH- + H+ -> H 2 O (27)

The volume of water added in the titration of SO 3 is smaller by this amount.

Brand has shown that in a solution of nitric acid in sulphuric acid, the formation

of water and the HSO 4

• ion occurs, which combine with free SO 3. The amount of
  the SO 3 bound corresponds to the equation:

HNO 3 + 2H 2 SO 4 -> NO 2 + + 2HSO 4 - + H 3 O+ (18)

However, it is not only sulphuric acid added to nitric acid that increases the
latter’s nitrating properties, Such substances as boron fluoride (R. J. Thomas et al.
[46]) or hydrogen fluoride (Simons et al. [47]) when added to nitric acid have a
similar effect.
These phenomena can be accounted for by assuming formation of the nitronium
ion according to the equations:

BF 3 + HNO 3 -> NO 2 + + BF 3 OH- (28)

2HF + HNO 3 -> NO 2 + + H 3 O+ + 2F- (29)

Conductometric studies

Further conductometric studies of nitric acid by Médard [48] and later Dalmon

[49] have shown that on adding water to the anhydrous acid, conductivity falls