Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

62 OXIDES.

After the evolution of gas from the flask has ceased, pass a cur-

rent of oxygen through the condensed liquid in the wash-bottle,

still keeping it surrounded with ice, until the color becomes a
pure yellowish brown; only a small amount of the nitrous gases

are lost during this operation. If it is desired to preserve the

preparation, heat the flask containing the liquid nitrogen dioxide

cautiously by means of lukewarm water and distil the liquid
into a sealing tube, in exactly the same manner as with sulphur

dioxide (p. 57). Boiling-point 22° C.

Cork stoppers and rubber tubing are energetically attacked

by the oxides of nitrogen. Therefore, in fitting up the above
apparatus, select corks which fit tightly and protect them with

a coating of vaseline; where the use of rubber connections is

unavoidable, bring the ends of the glass tubing close to-

gether.

Gaseous nitrogen dioxide is always mixed with considerable nitrogen tetrox-
ide, N 2 O 4 , formed by association of the dioxide molecules. The mixture is
light brown at room temperature; on being heated it becomes darker as a
result of progressive dissociation: N»O 4 —> 2 NO 2 , but on being heated more
strongly (above 130°) it becomes lighter in color owing to the decomposition
of the dioxide into nitric oxide, NO, and oxygen. These changes in color can
be shown by a simple experiment. Take a 25-30 cm. long glass tube of
difficultly-fusible glass, with about 1 cm. outside diameter, and introduce a
small amount of liquid nitrogen dioxide by the method described for sulphur
dioxide in Preparation No. 27. After all the liquid has been allowed to evapo-
rate, seal the tube filled with the vapor. For the nitrogen dioxide to be com-
pletely dry at this point, phosphorous pentoxide should have been added to the
liquid in the wash bottle before distilling it into the experimental tube. Clamp
the tube in a horizontal position and heat it in the middle with a broad flame
from a Bunsen burner until the gas in this part of the tube appears color-
less (NO + O2); the gas will be dark brown on both sides of the colorless
zone (NO2), while the outer portions of the gas will appear light brown
(NO 2 + N 2 O 4 ).

Arsenic acid may be obtained from the residue in the evolution

flask. Complete the oxidation by further heating with con-

centrated nitric acid, and evaporate the solution to a fairly thick

sirup. If exactly the right concentration of the sirupy solution

is obtained, it will form a nearly solid mass of crystals of arsenic

acid on standing in the ice chest.