urbaf2

NITRATION AND NITRATING AGENTS 27

anhydride. It is produced by addition of sodium nitrate, potassium dihydrogen

phosphate, water or larger amounts of acetic anhydride.

Recently Hofman, Stefaniak and T. Urbanski [70] examined the spectra of

nitric acid alone and in solutions in sulphuric acid. The following are the fre-

quencies they obtained for nitric acid of 98.2-99.5% concentration:

1665 cm-1 a medium band, assymmetric stretching vibrations of the NO 2 group

1368 cm-l a very strong band of the nitrate ion (NO 3 - )

1297 cm-1 a strong band, symmetric stretching vibrations of the NO 2 group

948 cm-l a weak band, bending vibrations of NO 2 group

774 cm-l a weak band, deformation by out-of-plane vibrations.

Dilute nitric acid (6%) gives bands at 3400 and 1642 cm

-1

which should be

assigned to the stretching and deformation vibrations, respectively of the OH

group in water. Two more bands at 1385 and 770 cm-1 have the same origin as

in concentrated acid.

Vapour pressure measurements

The exceptionally low vapour pressure of HNO 3 in admixture with sulphuric

acid is a proof that nitric acid is present, in such a mixture, not in the free state,

but as a compound. Gelfman [69] measured the vapour pressures of mixtures of

HNO 3 + H 2 SO 4 + SO 3 and drew some conclusions from the values obtained. He

stated that the vapour pressure minimum corresponds to the weight ratio SO 3 : HNO 3

=2.5. The vapour pressure rises as this ratio increases-at first slowly, then more

rapidly, especially when the ratio SO 3 : HNO 3 = 6.5. Near the minimum, the vapour

consists chiefly of HNO 3 and near the maximum mainly of SO 3. Gelfman suggested

on the basis of Sapozhnikov’s work (p. 10) and that of Chédin on the Raman

spectra of nitric and sulphuric acid mixtures, that the vapour pressure minimum

is due to the formation of a stable compound N 2 O 5 .4SO 3 , and the less stable com-

pounds N 2 O 5 .3SO 3 and N 2 O 5 .5½SO 3.

This view has been criticized by Spasokukotskii [59] who explains Gelfman’s

observations in the light of Brand’s equilibrium equations.

Spasokukotskii points out that undissociated compounds of the N 2 O 5 .nSO 3

type cannot exist in solution in sulphuric acid since both nitric acid and nitric

anhydride undergo dissociation, yielding the NO 2

+

ion. At the weight ratio SO 3 :HNO 3

= 2.5, the molar ratio is 2 which would be consistent with the equation:

HNO 3 + 2SO 3 <-> NO 2 + + HS 2 O 7 - (38)

Besides, it would just correspond to Gelfman’s compound N 2 O 5 .4SO 3. In

the case of smaller amounts of SO 3 the equation would be:

2HNO 3 + 3SO 3 <-> 2NO 2 + + S 2 O 7 2- + H 2 SO 4 (39)

which again would correspond to another compound suggested by Gelfman, viz.

N 2 O 5 .3SO 3.