Synthetic Inorganic Chemistry

328 HEAVY METALS OF GROUPS VI, VII, AND VIII

on the filter. Wash the crystals with 10 cc. of ice water and drain

again. Dry the product in a warm place until it does not smell of

acetic acid. Preserve the preparation in a 4-ounce cork-stoppered

bottle.

QUESTIONS

1. Heap up 10 grams of ammonium dichromate in a small
   mound on a porcelain plate, and apply the flame to the top of the
   mound until a reaction starts. Write the equation. What ele-
   ment is oxidized and what one reduced? Show that the algebraic
   sum of the valence changes is zero.


2. Note the color of the dry crystals of ammonium chromate
   and ammonium dichromate. Do either have the odor of am-
   monia? Dissolve 1 gram of the chromate in as little water as
   possible. Note whether the solution has an odor of ammonia.
   What is its color (yellow like K 2 CrC>4 or orange like K 2 Cr 2 07)?
   Write equation for the hydrolysis of (NH^CrO* and explain why
   it should hydrolyze more than K 2 CrC>4.


3. Heat 10 grams of ammonium chromate and compare its
   action with that of ammonium dichromate and with that of
   chromic anhydride when heated.

PREPARATION 64
CHROMIC ALUM, K 2 SO 4 -Cr 2 (SO 4 )3-24H 2 O
The preparation of potassium dichromate (Preparation 61)
illustrated how chromic oxide, Cr2O3, can be oxidized to a chromate
in which chromium exists as CrC>3. For the preparation of chromic
alum, it might seem as if chromic oxide or the natural chromite
should yield chromic sulphate directly on treatment with sulphuric
acid. This is impossible, however, because both of these sub-
stances are very resistant to the action of acids. Practically, they
yield only to the action of alkaline oxidizing agents, which convert
them into a chromate. Therefore potassium, or sodium, dichro-
mates are always the products made directly from the mineral,
and these serve as the materials from which other compounds of
chromium are prepared. To make chromic alum from potassium
dichromate it is necessary to reduce the chromium to the state of
oxidation in which it originally existed in the mineral, and to add
sufficient sulphuric acid to form the sulphates of potassium and