142 COMPLEX HALOGEN ACIDS AND SALTS.
The secondary valences have nothing to do with the state of oxidation of
the central atom, which is determined by the primary valence alone. Thus
in tervalent cobaltic salts, the element cobalt acting as a central atom tends to
unite with six other atoms, radicals or molecules. In the [Co(NH 3 )6]+ + + ion,
therefore, the 6 molecules of NH 3 are held in place by secondary valence
bonds and the ion has the triple charge because none of the principle valences
are satisfied in the complex cobalt-ammonia ion. In [Co(N02)o] , the NO2
radicals have a negative valence of one; the 6 NO2~, therefore, neutralize all
of the primary valences of the cobalt and since there are only three of these,
the anion has a negative valence of 6 — 3 = 3. The primary valences are
characteristic of the element and its position in the periodic classification of
the elements; the secondary valences are residual "affinities" which remain
when the principal valences have been satisfied.
This theory of Werner, which started with the study of complex inorganic
compounds of a very special nature, has gradually become of increasing im-
portance as a theoretical basis of inorganic chemistry and the application of
the theory is constantly being broadened. The electronic conception has
helped to interpret the theory. Many compounds such as potassium ferro-
cyanide and ferricyanide, to which very complicated structural formulas were
formerly assigned, are now much easier to understand. In the later chapters
of this book this theory will be mentioned repeatedly.
103. Hydrofluosilicic Acid
The reaction, SiO 2 + 4 HF = SiF 4 + 2 H 2 O, is reversible. When warm
and in the presence of sulphuric acid, which has a dehydrating action, it
proceeds from left to right. If, however, silicon tetrafiuoride is brought in
contact with a large amount of water, then silicic acid and hydrofluoric acid
are formed, but the hydrofluoric acid combines in a secondary reaction with
undecomposed silicon fluoride to form hydrofluosilicic acid:
2 HF + SiF 4 = H 2 [SiF 6 ].Place a mixture of 100 g. of powdered fluorspar and 40 g. of pre-
cipitated silicic acid, or 80 g. of sand, in a round-bottomed flask,
and, while shaking, add 500 g. of concentrated sulphuric acid in
small portions. Heat the mixture on a Babo funnel and conduct
the escaping silicon fluoride into 1 liter of water by means of a dry
delivery tube that dips directly into mercury which one-third fills
a small beaker standing in the bottom of a large beaker containing
the water. The lower end of the delivery tube should be made
wider by sealing on a short piece of tube 1.5 cm. in diameter.
The mercury keeps the opening of this tube dry and thus pre-
vents it from becoming clogged with silicic acid. If the water
becomes too much thickened with silicic acid, remove a part of
the liquid without interrupting the process, filter it through a