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20 CHEMISTRY AND TECHNOLOGY OF EXPLOSIVES

vestigations [24] between 1910 and 1916 referred to above, and also to the work

of Halban and Eisenbrand [18].

Halban and Eisenbrand investigated the absorption spectrum of nitric acid

at concentrations up to 68% (15.3 N) and noted the decrease in the intensity

of the NO 3

• ion band with increasing concentration. They also observed a max-
  imum at 265 mμ in the spectrum of solutions of anhydrous acid in anhydrous
  ether and attributed this to the presence of the unionized NO 2 -OH molecule
  (Fig. 6). However, Hantzsch did not confirm this part of their work He found

FIG. 6. Ultra-violet absorption spectra of nitric acid (Halban and

Eisenbrand [18]).

no maximum in the absorption curve of 100% nitric acid, nor in the curve of

a solution of nitric acid in anhydrous ether. He did note, however, that a band

appeared on addition of a trace of water to such an ethereal solution.

Further experiments on ultra-violet absorption spectra of nitric acid in organic

solvents have been carried out by a number of authors (Dalmon and Bellin [56],

Kortiim [57]. Eventually R. N. Jones, Thorn et al. [58] expressed the view that

nitrogen pentoxide is present in nitric acid of 95-96% and higher concentrations.

This was based on the following observations. At concentrations between 80
and 95%, an undissociated NO 2 OH predominates which would be responsible

for the absorption maximum near 260 mμ. This molecule or ion (“260 mμ
chromogen”) is present in maximal concentration at 91 to 92%. At 95 to 96%,

changes occur in the spectrum which can be attributed to the presence of free