338 HEAVY METALS OF GROUPS VI, VII, AND VIII
QUESTIONS
- Explain the action of manganese dioxide in the generation
of chlorine gas from hydrochloric acid. In what state of oxidation
does manganese exist in the salt manganous chloride?
- If iron were in the ferrous condition, it would not be removed
frbm the solution by the above procedure. Explain why iron is
necessarily in the ferric condition in the liquors used.
- Dissolve a small grain of manganous chloride in a half test
tube of water. Test the solution with hydrogen sulphide; then
add a few drops of ammonia, and if necessary add a little more
hydrogen sulphide. Then add acetic acid (a weak acid) until the
solution is again faintly acid. Does the manganous sulphide
dissolve? Compare the solubility of manganous sulphide with
that of copper sulphide; of zinc sulphide.
- Explain how facts involved in the foregoing preparation
show that Mn(OH) 2 is more strongly basic than Fe(OH^».
PREPARATION 71
POTASSIUM PERMANGANATE, KMnO 4
Although manganese dioxide is a powerful oxidizing agent, it
is nevertheless capable of being itself oxidized when it is fused
with a basic flux. The trioxide of manganese is acidic in nature
and combines with the base to form a salt. Thus it is evident that
the presence of a base favors the oxidation.
The dioxide of manganese is neither strongly basic nor acidic in
nature and shows no marked tendency to form salts. The mon-
oxide is distinctly basic and the trioxide is distinctly acidic, so
that the former forms salts with acids and the latter with bases.
It follows, therefore, that in the presence of acids the dioxide has
a tendency to produce salts of manganous oxide whereby an atom
of oxygen is set free, and that in the presence of bases manganese
dioxide has a tendency to take on another atom of oxygen in order
to produce a salt of the trioxide.
Thus, when manganese dioxide is fused with potassium hy-
droxide and an oxidizing agent, the salt potassium manganate is
formed. This salt is soluble in water and is fairly stable as long as
a considerable excess of potassium hydroxide is present; but in
presence of an acid — even as weak a one as carbonic acid — the
