326 HEAVY METALS OF GROUPS VI, VII, AND VIII- Without the recrystallization the weight of the product of
this preparation is often greater than that calculated. To what
impurity is this excess weight due?
RECRYSTALLIZATION OF CHROMIC ANHYDRIDE
Dissolve about 300 grams of the student's preparation in 0.6 its
weight of water. Decant carefully from any undissolved residue
and add very slowly with stirring such an amount of concentrated
sulphuric acid as will make 100 cc. for every 100 grams of the
crude chromic anhydride. When the suspension has cooled to
room temperature, collect and wash crystals in the same manner
as in the original preparation using 60 cc. of 16 N HNO3 for the
first washing and 40 cc. each for the second and third washings.
Drain the crystals very thoroughly with suction, transfer them to
an 8-inch porcelain dish and place it on the hot plate. Break up
and turn over the mass of crystals frequently with a spatula. After
several hours, when nitric acid vapors cease to come off, weigh and
bottle the product.
PREPARATION 63
AMMONIUM CHROMATE AND AMMONIUM DICHEOMATE,
(NH 4 ) 2 Cr04 AND (NH 4 )2Cr 207
The raw material for this preparation is the chromic anhydride
from the preceding preparation, which contains at least a small
amount of sodium acid sulphate. This dissolves with the chromic
anhydride, and it should remain in the mother liquor (as neutral
sulphate) from which the ammonium chromate is crystallized.
The second and third crop of crystals of the latter, however, are
likely to be contaminated with sodium sulphate. An excess of
ammonium hydroxide is used to insure the formation of the
neutral chromate.
Materials: chromic anhydride, CrO3, 100 grams = 1 F.W.
15 N ammonium hydroxide, 160 cc.
6 N acetic acid.
Apparatus: 500-cc. flask.
5-inch funnel.
suction filter and trap bottle,
iron ring and ring stand.
Bunsen burner.