LIQUID SULPHUR DIOXIDE. 57
The copper oxide, as fast as it is formed, dissolves in the sulphuric acid,
forming copper sulphate. It is essential for the decomposition that the
sulphuric acid should be hot and that it should be concentrated, whereby its
content of SO 3 is increased; the nature of the reducing agent is less important,
for the copper may be replaced by other metals or even by carbon.
Sulphur dioxide can be condensed to a liquid (boiling-point — 10°) by
cooling the gas in a mixture of ice and common salt.
Heat 50 g. of copper turnings in a round-bottomed flask with
200 g. of concentrated, commercial sulphuric acid until the boiling-
point of the latter is nearly reached. Lower the flame as soon
as gas is given off freely. Pass the gas through a wash bottle
containing concentrated sulphuric acid, and into a second empty
wash bottle which is surrounded with a mixture of three parts
of ice to one of salt, and in which the sulphur dioxide condenses
to a liquid.
When the evolution of the sulphur dioxide slackens, pour the
liquid from the evolution flask into an evaporating dish before
it has a chance to solidify, and allow it to cool by standing over
night. In the morning decant the liquor from the mass of crystals
which have separated, dissolve the crystals in as little boil-
ing water as possible, and filter off any insoluble black
powder on a large plaited filter. Copper vitriol, CuSC>4 • 5 H 2 O,
separates from the nitrate in well-defined crystals; collect them
in a filter funnel and evaporate the mother liquor to obtain a
second, and finally a third, crop of crystals. When the product
has been dried as much as possible by suction, place it in an
evaporating dish, which is covered with filter paper, and allow it
to dry for several days at the room temperature.
The sulphur dioxide, as prepared above, always contains some
sulphuric acid, fumes of which are carried over mechanically
by the gas from the evolution flask, and are not entirely kept
back by the first wash-bottle; immediately after being pre-
pared, therefore, the sulphur dioxide should be purified by distilla-
tion. Close one tube of the wash bottle which contains it, and
connect the other tube by means of a short piece of rubber with a
glass tube which is bent at right angles, and whose vertical arm is
drawn out to an internal diameter of about 0.2 cm. (Fig. 12). In-
troduce this narrow tube nearly to the bottom of a thick-walled
sealing tube, 0.4 to 0.5 cm. wide, which is sealed at the bottom,
and drawn out a little at a point about 18 cm. above the lower