CHROMIC ALUM 329chromium. Alcohol may be used as the reducing agent, it being
itself oxidized to acetaldehyde, C2H4O, a substance whose presence
is made very evident by its penetrating odor.
Chromic alum is isomorphous with common alum and can easily
be obtained in large and beautiful deep purple crystals. Chromic
salts present a phenomenon which complicates the task of crystal-
lizing out definite salts. In cold solution the simple chromic ion
probably has the composition [Cr-6H2O]+++. At higher tempera-
tures this ion becomes altered, through loss of H2O and substitu-
tion of OH~ ions or of the anions of the salt. For example,
[Cr-4H 2 OS04]+ is a possible composition of the chromium ion
in a hot solution of chromic sulphate. The ion [Cr-6H 2 O]+++ is
purple, and with the SC>4~~ ion the crystallizable purple chromic
sulphate, or the chromic alum, can be obtained. The altered ion
in the hot solution is of a very deep green color, and it does not give
crystallizable compounds.
Probably similar conditions hold in the solutions of the salts of
most heavy metals, but in most cases the change back and forth is
BO rapid that the crystallizable form of the salt is at once obtained
when the solution is cooled. With chromium, however, once the
green form is obtained when the solution is heated, the change
back to the purple form, which is the stable form at lower tempera-
ture, is very slow. In this preparation we have the option of
carrying but the reduction at a low temperature and obtaining
the crystallizable chromic alum at once, or of letting the tempera-
ture rise and then leaving the green non-crystallizable form to
change very slowly back to the purple form and crystallize slowly
as it does so. We shall choose the latter option particularly as it
offers the advantage of a very slow crystallization which of course
favors the formation of large, perfectly formed crystals.
At 25°, 24 grams of KaSO^CraCSOOs^I^O will dissolve in
100 grams of water, and the solubility increases very rapidly with
the temperature.
Materials: potassium dichromate, K 2 Cr 2 07, 98 grams = 0.33
F.W.
36 N H2SO4, 76 cc.
95 per cent ethyl alcohol, 63 cc.
Apparatus: 8-inch porcelain dish.
8-inch glass plate.