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12 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

esterifying agent. The addition of water shifts the equilibrium towards the for-

mation of the dissociated form of the true acid, which has no nitrating properties:

NO 2 .OH <-> NO 3 - H+

H 2 O

(6)

Thus, the addition of sulphuric acid to nitric acid increases the concentration

of the pseudo-acid. The sulphuric acid, being the stronger, gives off a proton to

pseudo-nitric acid and causes the formation of the following mono- and bivalent

ions :

NO 2 .OH + H 2 SO 4 -> NO(OH) 2 + + HSO 4 - (7)

NO 2 .OH + 2H 2 SO 4 -> N(OH) 3 2+ + 2HSO 4 - (8)

Hantzsch named the ion H 2 NO 3 + nitracidium, and the ion H 2 NO 3 2+ hydro-

nitrucidium. In particular, the eqn. (8), according to which the H 3 NO 3 2+ ion is

formed, appeared to be correct in the light of Hantzsch’s cryometric studies [21]

published in 1909. He found that the freezing point of sulphuric acid fell very con-

siderably when nitric acid was added, indicating that the value of the van’t Hoff

i-factor (in the equation PV = inRT) ranges from 3.4 to 4.0. Among other cryometric

investigations, measurements by Oddo and Scandola [25], Hammett, et al. [26-28],

Robles and Moles [29] should be mentioned. On the whole they are in agreement

with Hantzsch’s data.

To obtain further arguments for the confirmation of his theory in 1925 Hantzsch

[21] examined the ultra-violet absorption spectra of nitric acid. On dilution with

water a band appears which may be ascribed to the NO 3 - ion. The band is not

present in solutions of nitric acid in sulphuric acid, since, as the author says, the

ions H 2 NO 3

+

and H 3 NO 3

2+

must appear in such circumstances.

Halban [18] reported, in the paper already referred to, that the difference

between the absorption spectrum of pure nitric acid and that of its mixture with

sulphuric acid is due to the presence of N 2 O 5 in the mixture, while Walden [17]

believed instead that it was the nitronium ion, NO 2 +, which will be discussed later,

that was present in the mixture.

Hantzsch tried to obtain further evidence for the existence of the above men-

tioned ions, using the electric conductivity method but without success. Finally

he tried to isolate the sulphates of the two hypothetical ions nitracidium and hydro-

nitracidium:

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

Although he did not succeed in isolating these salts he postulated, for confirmation

of his theory, that the compound with the empirical formula N 2 O 5 .4SO 4 .3H 2 O

isolated by Weber as early as in 1871 is a salt of pyrosulphuric acid :

(H 2 NO 3 )+ (HS 2 O 7 )- or (H 3 NO 3 )2+(S 2 O 7 )2-

An observation by Holmes [30] seemed to confirm Hantzsch’s hypothesis; Holmes
demonstrated by thermal analysis the existence of the molecular compound