'

124 ABSORPTION OF NITROUS GASES

This fact has led to the introduction of a number of patents
covering the process. One such1 states that liquid nitrogen
tetroxide and oxygen at atmospheric pressure and temperature
yield concentrated nitric acids with water. By employing a
large excess of N204 (5-7 times the theoretical quantity
required by the water present) nitric acids containing 90-95 per
cent, HN03 can be obtained. Maxted2 recommends the frac-
tional liquefaction of the oxides of nitrogen from the oxidation

-20

8-40

-100

-no

700

90

80 70 60

Volume, Per Cent NOZ

FIG. 37.—Freezing-point Curves of N2O4/N2O3 Mixtures (Wittorf).
From Abegg's Handbuch-der awrganische Chemie.
of ammonia. The bulk of the water is removed between
50° and 100° C.? and the further separation of the oxides of
nitrogen carried out at appropriate temperatures below 0° C.
Another suggestion3 is the addition of liquid nitrogen
tetroxide to water, or dilute nitric acid, and subsequent
addition of oxygen. To keep the water saturated, the pro-
1 Eng. Pat. 126,704, 1919. 2 Eng. Pat. 127,343, 1917.
s Eng. Pat. 131,336, 1916.