332 HEAVY METALS OF GROUPS VI, VII, AND VIIInitrate, (a) Write equation. (6) Explain the change in color,
(c) Compare the reactions of normal lead chromate and basic
lead chromate with acetic acid.PREPARATION 66
CHROMIUM METAL BY THE GOLDSCHMIDT PROCESS
The readiest method of obtaining the metal chromium from its
oxide, and one which yields it in a high state of purity, is the so-
called Goldschmidt, or alumino-thermic, process, in which use is
made of metallic aluminum as the reducing agent according to the
reaction
2A1 + Cr 2 O 3 = A1 2 O 3 + 2CrThe heat produced by the oxidation of aluminum is so great that
it is sufficient to effect the decomposition of the chromic oxide with
still enough surplus heat to melt the metallic chromium and the
slag of AI2O3. It is evident that before this reaction can be made
to progress spontaneously a sufficient temperature must be
developed to decompose the chromium oxide. This necessary
temperature is a good deal higher than that of a flame or of a
common furnace, but can be obtained by use of the fuse powder
described below. When once started in this way the reaction
itself produces a temperature high enough to insure its continu-
ance.
When the reaction is carried out on the small scale of a laboratory
preparation, the heat produced is not quite sufficient to melt the
metal and slag so thoroughly that the metal can settle out to form a
compact regulus at the bottom of the crucible. By adding a small
amount of potassium dichromate to the charge, however, the
reaction becomes more energetic, owing to the more available
supply of oxygen.
Materials: chromic oxide, Cr2O3, 210 grams,
potassium dichromate, 60 grams,
granulated aluminum, 96 grams,
barium peroxide, 20 grams,
magnesium ribbon,
mortar and pestle.