SODIUM PEROXIDE. 101Within the class of compounds possessing complex anions that sub-class
will first be treated which comprises the compounds containing homogeneous
complexes. Homogeneous complex anions arise, as the name implies, when one
or more additional atoms of the same element are joined to the negative con-
stituent of a binary compound,
K 2 S + 3 S = K 2 S 4.
Homogeneous complex compounds can be distinguished from simple com-
pounds of the general formula Am Bn by the fact that the greater number of
negative atoms is not due to an increased valence of the positive component,
but rather to an increased combining power of the negative part originally
present in the mother-substance. FeCl 2 differs from FeCl 3 in the valence of
the iron. Aqueous solutions of FeCl 2 and FeCl 3 are identical with regard to
their anions but different with regard to the cations; in aqueous solutions of KI
and KIs, however, the exact opposite is true.
Only a few compounds with homogeneous complex cations are known.
The part played by the valence theory in the interpretation of complex
compounds is varied. In some cases, as with sulphuric acid, it offers certain
advantages in its original form. In other cases an enlargement of our valence
ideas seems necessary (see Complex Halogen Salts, p. 140).COMPOUNDS WITH HOMOGENEOUS COMPLEXES
(a) Peroxides.- Sodium Peroxide.
Arrange a train of apparatus so that air may be drawn by
means of a suction pump through one wash-bottle containing
sodium hydroxide and two more containing concentrated sul-
phuric acid, and then into a wide, 30 cm. long, combustion tube
which is to be heated on a row burner with an asbestos cover
(Fig. 3). The other end of the combustion tube is made nar-
rower and fitted to an air filter which serves to hold back sodium
peroxide dust. This filter consists of a 30 cm. long and 3 cm.
wide glass tube loosely filled with asbestos fibers. Between
the air filter and the pump, place an empty suction flask to serve
as a safety bottle. Into the combustion tube introduce an
aluminium boat, about 16 cm. long and as deep as possible,
which can be prepared from thin aluminium foil; in the boat place
2 to 3 g. of sodium. Since, during the progress of the experi-
ment, the sodium will tend to flow in a direction opposite to
that of the air current, the combustion tube together with the
burner should be placed in a slanting position so that the end
at which the air enters is uppermost.