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NiTRATION AND NITRATING AGENTS 31

acid it was found that the compounds of HNO 3 and SO 3 are more stable. According

to Gelfman the compound HNO 3 .2SO 3 can be obtained in a crystalline form,

Moreover, Gelfman believed that the compounds HNO 3 .18SO 3 and 3HNO 3 .SO 3

do in fact exist.

Density measurements

Klimova, Zaslavskii and their co-workers [77,77a] have measured the den-

sities of anhydrous nitric and sulphuric acid mixtures. They obtained characteristic

slopes in the density curve of the mixtures at molar ratios HNO 3 :H 2 SO 4 = 1:2

and 1: 1. Hence they inferred that in the solution containing 24 by wt.% of HNO 3 ,

the cation H 3 NO 3 2+is present; with a decrease in H 2 SO 4 concentration this is con-

verted into the H 2 NO 3 + cation (39 by wt.% HNO 3 ).

Moreover, interpretation of the density curves of the acids and water mixtures

led them to the conclusion that on gradually adding water, hydrolysis of Hantzsch’s

ions occurs:

H 3 NO 3 2+ + H 2 O <-> H 2 NO 3 + + H 3 O+ (40)

H 2 NO 3 + + H 2 O <-> HNO 3 + H 3 O+ (41)

Since according to Hantzsch the H 2 NO 3 + ion is the principal nitrating agent,

these reactions could account for the fact that organic compounds are best ni-

trated with a nitric and sulphuric acid mixture with a water content of 10%.

Viscosity measurements

Measurements of the viscosities of nitric acid-sulphuric acid-water mixtures

have been the subject of research work by several authors. Zaslavskii, Klimova

and Guskova [78] have observed a maximum, corresponding to the combination

of ions:

(H 3 NO 3 )2+ (HSO 4 ) 2 -

With increase in water content the maximum in the curve shifts towards lower

concentrations of HNO 3 and disappears when the dilution has attained a degree

corresponding to 1 mole of water for 1 mole of the above compound.

Recently Swinarski and Dembinski [51] and Swinarski and Piotrowski [52],

have examined the viscosities of the three component solutions HNO 3 -H 2 SO 4 -H 2 O.

The diagram in Fig. 14 shows the results of these investigations, in terms of changes

in viscosity with increase in HNO 3 content. The authors pointed out a similarity

between the viscosity curve they obtained and the electric conductivity curve

(see Fig. 4, p. 18).

The curve shows a distinct maximum at 20 mole % HNO 3. The authors ex-
plained this by postulating the formation of a hydrated NO 2 + ion:

HNO 3 + 2H 2 SO 4 + H 3 NO 3 2+ + 2HSO 4 - (31)

An increase in concentration of HNO 3 causes a decrease in viscosity, presumably
because of dissociation: