102 PEROXIDES.
Heat the sodium to above its melting-point and draw air over
it; at about 300°, it takes fire. Then lower the flame, for
from that point on the combustion proceeds almost without any
application of external heat. Continue to pass a vigorous cur-
rent of air until the reaction has ceased. The yield is 4 to 5 g.
of bright yellow sodium peroxide, which must be protected from
moisture.
- Barium Peroxide.
At a dull red heat, dry barium oxide is changed by the oxygen of the air to
barium peroxide. In carrying out the reaction, the air must be free from
moisture and from carbon dioxide. Since the barium peroxide gives off its
oxygen again at the same temperature and a reduced oxygen pressure, or at
the same pressure and a higher temperature, a technical process for obtaining
pure oxygen from the air according to the reversible reaction
BaO + 0 <=> BaO,has been developed. Barium oxide cannot be prepared conveniently from
barium carbonate, at least not on a small scale, because the decomposition
temperature of the latter lies too high.
Crude Barium Peroxide. Place 130 g. of barium nitrate in a
large clay crucible and heat it in a charcoal furnace with slowly
rising temperature, to a dull red heat. After cooling break
the hard, porous, gray contents of the crucible into small
lumps and transfer it, before it can attract moisture, into a
weighed combustion tube. Determine the quantity of barium
oxide by weighing the tube and contents. Heat the tube in a
combustion furnace to a barely perceptible redness, and then
draw through it a current of air which has passed through caustic
soda solution and then through concentrated sulphuric acid. After
two or three hours, let the tube cool and determine the gain in
weight, which should be one-tenth the original weight of the
barium oxide; if it is less, the preparation must be heated again
in the current of air.
Pure Barium Peroxide. Dissolve the barium peroxide, a
little at a time, in the calculated amount of ice-cold \°j 0 hydro-
chloric acid and add barium hydroxide solution to the cloudy
liquid until a precipitate consisting chiefly of metal hydroxides,
which are present as impurities, just forms. Filter, and precipi-
tate the nitrate completely with barium hydroxide, whereby a fine,
crystalline powder of barium-peroxide-hydrate, BaO 2 • 8 H 2 O, is
formed. Drain the precipitate and dry it in the steam closet.