ZINC OXIDE 229
QUESTIONS
- What is the reaction between Cu++ and OH~ ions? To
a little 1N CuSO* add 6 N NaOH drop by drop, until it is present
in excess.
- Explain what successive reactions occur when ammonium
hydroxide is added instead of sodium hydroxide.
- To 5 cc. of IN CUSO4 add 10 cc. of a molal solution of
tartaric acid; then add sodium hydroxide solution, as in (1),
and compare the results with those in (1) and (2), but do not
attempt to ascribe a definite formula to the complex compound
formed.
PREPARATION 31
ZINC OXIDE, ZnO
Zinc oxide is used as a white pigment, for which purpose it has
the advantage of not turning black under the action of hydrogen
sulphide. It may be obtained directly by burning metallic zinc,
or from a soluble zinc salt by precipitating first a basic carbonate
and then heating this to convert it into the oxide. Both zinc
carbonate and zinc hydroxide are insoluble in water, but the basic
carbonate is of still greater insolubility, and therefore precipitates
more readily than either of the former when the ions necessary
for its formation are brought together. The simplest formula for
the basic carbonate is
„ .OH
Zn<In < CO^3
but the precipitate may be of varying composition according to the
conditions of its formation.
If zinc sulphate in solution is treated with sodium bicarbonate,
pure zinc carbonate is precipitated, because a sodium bicarbonate
solution contains but a minute quantity of OH~ ions. On the
other hand, a sodium carbonate solution, in consequence of hydroly-
sis, contains a considerable quantity of OH~ ions, and thus it
furnishes both the CO 3 ~~ and OH~ ions necessary for the forma-
tion of basic zinc carbonate.
Basic zinc carbonate is decomposed by heat into zinc oxide and
carbon dioxide.
