60 OXIDES.
and in addition through a tube loosely filled with glass wool in
order to free it from the spray mechanically carried along from the
generating flask. Regulate the flow of the two gases so that
a little more oxygen than sulphur dioxide passes into the con-
tact tube. Keep the receiver immersed in ice water; sulphur
trioxide collects abundantly either in the liquid modification or
in the asbestos-like form. The experiment takes about three hours.
For the following experiments use the liquid form. In case
the asbestos-like form has been obtained, loosen it with a glass
stirring rod and place a layer of it about 1 cm. deep in each of
three test-tubes; add (under the hood) a drop of concentrated
sulphuric acid to each tube, heat just to the melting-point, and
then allow to cool. To one test-tube add flowers of sulphur
from the point of a knife blade; to the second, powdered selenium;
and to the third, iodine. In the first test-tube an indigo-blue
solution is formed, in the second a bluish green, and in the third
likewise a bluish green, or, if considerable iodine has been added,
a brown solution. The compounds, S 2 O 3 and SSeO 3 , are pro-
duced in the first and second tubes respectively.Oxidation of Naphthalin with Sulphuric A aid.
The dissociation of sulphur trioxide, SO3 = SO2 + O, in opposition to the
reaction of its synthesis by the contact process, is frequently utilized for
technical and analytical purposes; by an increase of temperature, and by the
presence of catalyzers, such as mercury or copper salts, the oxidizing action
is considerably accelerated. Instead of sulphur trioxide, fuming sulphuric
acid or even ordinary concentrated sulphuric acid may be employed, although
the effect is not then so readily obtained.
The most important technical utilization of this oxidizing power of fuming
sulphuric acid is in the transformation of naphthalin into phthalic acid, the
latter being used in the preparation of artificial indigo:
CI0H 8 +90= C 8 H 6 O 4 + H 2 O + 2 CO 2.
Naphthalin Phthalic AcidPlace 5 g. naphthalin, 1 g. mercury, and 80 g. concentrated
sulphuric acid in which the remainder of the above sulphur
trioxide has been dissolved (or 80 g. of commercial, fuming sul-
phuric acid) in a 300 c.c. retort, and heat the mixture slowly,
almost to boiling, on a Babo funnel. Insert the neck of the retort
into a small flask, which serves as a receiver, and cool the latter
with water. A white sublimate of phthalic anhydride soon appears
in the upper part and neck of the retort, and the odor of sulphur