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NITRATION AND NITRATING AGENTS 19

Two phenomena seem to be superposed. One is the normal increase of conduc-

tivity with temperature, the other is the equilibrium (32) which is shifted to the

right at higher temperatures:

NO 2 +.H 2 O + NO 3 - <-> 2HNO 3 <-> N 2 O 5 + H 2 O (32)

On the basis of their experiments on electrolysis of 80-100% nitric acid Swi-

narski and Bialozynski [53] came to the conclusion that the nitronium ion exists

mainly in hydrated form NO 2 +.nH 2 O.

Isolation of salts in which nitric acid is a cation. Hantzsch’s experiments on

the preparation of nitracid and hydronitracid perchlorates have also been verified.

Ingold and his co-workers [36,34,39] have indeed prepared salts of empirical for-

mula corresponding to (H 3 NO 3 )2+(C1O 4 ) 2 -. They also stated that the substance

was a mixture of nitronium perchlorate (NO 2 )+(ClO 4 )- and the well known

perchloric acid hydrate (H 3 O)+ (ClO 4 )-. They separated the two components by

fractional crystallization from nitromethane. But they did not succeed in the

preparation of the compound (H 2 NO 3 )+(ClO 4 )-.

To Weber’s compound (p. 12) the formula of nitronium pyrosulphate is at

present assigned (NO 2 )+(HS 2 O 7 )-.

Examination of Raman and infra-red absorption spectra (which will be dis-

cussed later) confirmed the nitronium salt structure of all these compounds.

Ingold and his co-workers regarded nitric anhydride as nitronium nitrate

(NO 2 )+(NO 3 )- (see also eqn. 24). They obtained nitronium di- and trisulphates

by treating nitric anhydride with SO 3 :

(NO 2 ) 2 + (S 2 O 7 )2-and (NO 2 ) 2 + (S 3 O 10 ) 2 2-

Woolf and Emeléus [54] have prepared a number of other crystalline nitronium

salts, for example:

NO 2 + HS 2 O 7 -

(NO 2 +) 2 S 2 O 7 2-

(NO 2 +) 2 S 3 O 10 2-

NO 2 +BF 4 -

NO 2 +SbF 6 - etc.

Examination of the nitric acid spectrum (alone and in mixture with sulphuric acid)

Ultra-violet absorption spectrum. The first investigations into the ultra-violet
absorption spectra of nitric acid, containing certain quantities of water, were

carried out by Hartley [55]. He observed that the band at about 302 mμ charac-

teristic for the nitrate ion disappears gradually with increase in concentration of

nitric acid.

Among other investigations special attention should be paid to Schafer’s in-