58 OXIDES.
FIG. 12.end. This tube is prepared, cleaned, and dried before beginning
the distillation; care must be taken to round the lower end and to
make the constriction without lessening
the thickness of the walls at any point.
In order to distil the sulphur dioxide,
transfer the wash bottle from the freezing
mixture to a bath of water at room tem-
perature, and place the thick-walled tube,
which now serves as the receiver, in the
freezing mixture. The sulphur dioxide
soon begins to distil. When the receiver
is about half filled, stop the distillation
and seal the tube at the constriction,
taking great care that the strength of the
walls is not lessened thereby. While the
tube is being sealed it must be kept in
the freezing mixture; the tube and bathtogether may be held by a second person at a proper distance
from the blast lamp. The tube must be allowed to cool while
resting in a perpendicular position so that the liquid sulphur diox-
ide does not come in contact with the hot parts of the glass.
- Sulphur Trioxide by the Contact Process.
The fact that sulphur dioxide will combine with oxygen when in the presence
of finely divided platinum was known in the first half of the last century. Cl.
Winkler showed as early as 1875 that, by means of such a contact-process,
sulphuric acid could be made on an industrial scale from mixtures of sulphur
dioxide and oxygen. It was not until nearly the close of the century, how-
ever, that the doubts with regard to the feasibility of manufacturing sulphuric
anhydride on a large scale from the gases evolved in the roasting of pyrite
were overcome; then the Badische Aniline- und Sodafabrik made public the
most favorable temperature for this process and directed the attention of a
wide circle of chemists to the problem of freeing the gases from catalyzer
poisons (cf. No. 20), particularly arsenic compounds.
Sulphur trioxide exists in two allotropic modifications: as a mobile liquid
(boiling-point 46°) which forms crystals on being sufficiently cooled (freezing-
point 15°), and as a white asbestos-like mass which on warming volatilizes
without previously melting. The latter is the more stable modification;
liquid sulphur trioxide on standing goes over slowly of itself, or more rapidly
in the presence of a trace of sulphuric acid which acts as catalyzer, into this
asbestos-like condition.
The asbestos-like form when dissolved in phosphorus oxychloride is bi-