LEAD DIOXIDE 279nitrate solution. Then add an excess of the reagent. Repeat,
using sodium hydroxide instead of ammonium hydroxide. Write
equations and explain the amphoteric character of lead hydroxide.- Are lead salts (nitrate or chloride) appreciably hydrolyzed
in aqueous solution? Compare the basic strength of the hydroxide
of divalent lead with that of aluminum hydroxide. - Precipitate a little lead chloride by adding hydrochloric acid
to a solution of lead nitrate. Describe its properties and compare
them with those of lead tetrachloride (reference books). To what
oxide of lead does lead tetrachloride correspond?
PREPARATION 47
LEAD DIOXIDE, PbO2
The compounds of lead in which its valence is 2 are the most
stable, but with strong oxidizing agents the valence may be raised
to 4. The oxide PbO 2 is very much less basic than the lower
oxide, and, furthermore, it is very insoluble — either the anhy-
drous oxide or its hydrated forms, Pb(0H)4 or PbO(OH) 2. The
effect usually observed when a salt of divalent lead is oxidized is
a precipitation of dark brown lead dioxide. This precipitate can
be obtained by oxidizing an alkaline solution containing a lead
salt with chlorine, but it cannot be obtained in an acid solution
with this oxidizing agent because hydrochloric acid reduces lead
dioxide (see Experiments 10 and 6, Chapter IV, pages 162 and
164). In an acid solution containing no reducing agent, that is
in a nitric acid or a sulphuric acid solution, lead dioxide can be
formed by the action of a very strong oxidizing agent, as for
example,^ by the electrolytic oxidizing action at the anode of a
lead storage battery. It is to be noted that nitric acid does not
oxidize lead to the tetravalent condition.
In this preparation we shall make use of bleaching powder in
a slightly alkaline solution as the oxidizing agent. This is chosen
in preference to chlorine because it is easier to handle and no
precautions need be taken to avoid escape of objectionable chlorine
into the laboratory. It should be recalled that the effect of bleach-
ing powder is the same as that which would be obtained by passing
chlorine into an alkali. The precipitate finally obtained after
the bleaching powder has acted contains the greater part of the
lead in the form of lead dioxide; but it may also contain a small