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14 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

that a hypothetical “E-acid” is formed in the reaction of nitric acid with nitrogen

dioxide and is the entity responsible for nitration:

2HNO 3 + NO 2 <-> H 2 N 3 O 8 (10)

E-acid

According to those authors the nitration reaction occurs as follows:

2RH + H 2 N 3 O 8 -> 2RNO 2 + NO 2 + H 2 O (11)

There was no doubt at that stage of the investigation that nitric acid reacts

with sulphuric acid. Yet the research work described had put forward a problem

without giving a clear answer. It was only in recent investigations that, due to more

accurate physico-chemical methods, especially to the application of Raman spectrum

analysis, inferences about the interaction of the nitrating mixture components

have been confirmed.

MORE RECENT STUDIES

Cryometric investigations

Hantzsch’s work and his conclusions have lately been revised and criticized.

In his extensive work published in 1941, Titov [35] drew attention to the fact that

none of the existing view about the action of sulphuric acid on nitric acid explained

Hantzsch’s observation that the value of the van’t Hoff i-factor for nitric acid

dissolved in sulphuric acid may be close to 4.

Titov quotes the following equations, based on different existing views:

HNO 3 + 2H 2 SO 4 + (HO 3 S)ONO 2 + H 2 SO 4 .H 2 O (12)

(equation based on Markovnikov’s view)

2HNO 3 + H 2 SO 4 + N 2 O 5 + HSO 4 - + H 3 O+ (13)

(equation based on Sapozhnikov’s view) and Hantzsch’s eqns. (7) and (8) quoted

earlier.

According to Titov, the simplest way of explaining Hantzsch’s observations

is in the form of an equilibrium equation, in which the nitronium cation* occurs:

HNO 3 + 2H 2 SO 4 <-> NO 2 + + 2HSO 4 - + H 3 O+ (14)

Titov believed that Hantzsch’s nitracidium and hydronitracidium ions should

be considered as the hydrated nitronium ions:

H 2 NO 3 + + H+ -> NO 2 + + H 3 O+ (15)

H 3 NO 3 2+ -> NO 2 + + H 3 O+ (16)

This view was confirmed experimentally when in 1950 Ingold [36] and his co-workers

isolated crystalline nitronium salts (p. 19).

* Nitronium ion is sometimes called nitryl or (more correctly) nitroxyl ion (see p. 13).