I
24 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVESno 1050 and 1400 cm-1 lines, whereas solid N 2 O 5 showed the presence of both
the 1050 and 1400 cm-1 lines.
On the grounds of Walden’s observations (in his cryometric studies), that nitric
anhydride exists in POCl 3 solution in a non-associated form, and that according
to Russ and Pokorny [64] it is associated when in solid form, Chédin has come
to a conclusion that the Raman 1050 and 1400 cm-1
lines correspond to associated
N 2 O 5 , while the 707, 860, 1033, 1240, and 1335 cm-1
lines correspond to the non-
associated, monomolecular form of N 2 O 5. Consequently N 2 O 5 in nitric acid or in
nitric and sulphuric acids solution would appear to exist in an associated form.
As the presence of the nitronium ion NO 2 + in nitric acid and in its mixtures
with sulphuric acid had again been suggested, the view has been expressed that
the 1400 cm-1 line should be assigned to that ion, and the 1050 cm-1 line to
the nitrate ion NO 3 - (Chedin drew attention at an earlier date to that). It was also
revealed that the hydrosulphate ion (HSO 4- ) present in sulphuric acid and in
mixtures of sulphuric and nitric acids also produces the 1050 cm
-1
line.
Nitric anhydride produces a spectrum composed of the two lines, since it under-
goes dissociation according to the eqn. (24) given on p. 15.
According to the eqn. (18) on p. 15, nitric acid also yields the nitronium
ion with sulphuric acid.
This interpretation of the Raman spectrum was verified in 1946 by Ingold and
his co-workers [36] for mixtures of nitric acid with other concentrated acids. Thus,
for example, mixtures of nitric and perchloric or selenic acids produce a high in-
tensity 1400 cm-1 line, but not a trace of the 1050 cm-1 line. Instead of the latter,
lines corresponding to the perchlorate and selenate ions appear.
On the basis of these experiments it has been established that the 1400 and
1500 cm-1 lines correspond to completely different molecules. It has also been
established decisively that the 1400 cm-1
line appears on the addition of another
strong acid to nitric acid and that most likely it corresponds to the NO 2 + ion, which
is formed in such a solution.
In 1936 Chédin [60a] also examined the Raman spectrum of the solution of
nitric acid in oleum. He observed, of course, the 1400 cm-1 line, and also another
one, between 1075 and 1095 cm-1. At first the two lines had been assigned to the
N 2 O 5 (SO 3 )n complex. Yet the work of Millen and Ingold [36,37] has shown that
KHSO 4 also produces a line ranging from 1075 to 1095 cm-1; the frequency varied
with concentration. At present the line is assigned to the hydropyrosulphate ion
HS 2 O 7
which is probably formed according to the equation:
HSO 4 - + H 2 S 2 O 7 <-> H 2 SO 4 + HS 2 O 7 - (36)Since in this system the HSO 4 - ion is also present, giving the 1050 cm-1 line,
the two frequencies 1095 and 1050 cm-1 are superimposed and the resultant
frequency is 1095-1075 cm-1, depending on the intensity of the 1050 cm-1 frequency.
The Raman spectra of solutions N 2 O 5 + nH 2 O, with n>1, confirm the view
that nitric anhydride dissociates according to the equation presented on p. 19.