AMMONIUM CHROMATE AND AMMONIUM DICHROMATE 327Procedure: Place the chromic anhydride in a 500-cc. flask and add
70 cc. of water. Shake the mixture until solution is complete and
then cool the flask in a pan of water. Add the ammonium hy-
droxide in 5-cc. portions. Cool after each addition and shake the
mixture to prevent the formation of a solid cake. After 100 cc. of
the hydroxide have been added, break up any lumps with a glass
rod and add the remaining 60 cc. in four portions. Shake the mix-
ture thoroughly; cool the flask in a pan of ice water and shake for
10 minutes. Break up the solid cake by shaking and transfer the
material to a suction filter. Use the filtrate to rinse out the flask
and transfer all the solid to the filter. Remove the liquor as
completely as possible by pressing the solid on the filter. Cover
the crystals with 10 cc. of 15N ammonium hydroxide, and after it
has stood a few minutes, apply suction again. Evaporate the
filtrate, under the hood, to half the original volume. Place the
flask in a pan of ice water, and when the liquid has cooled to about
60°, add 10 cc. of 152V NH 4 0H and shake for 10 minutes. Filter
off the second crop of crystals and treat them as before. If
the two crops are the same color, combine them and dry them
together. If, however, the second crop is much darker than the
first, it should be kept separate. Dry the crystals at room tem-
perature and weigh them. One half of the preparation should be
preserved in a 4-ounce cork-stoppered bottle, and the other half
used to prepare ammonium dichromate.
To convert the ammonium chromate to dichromate it is dissolved
in acetic acid. Calculate the volume of 62V acetic acid required
to convert the weight taken of dry ammonium chromate according
to the equation:
2(NH 4 ) 2 Cr04 + 2HC2H3O2 = (NH 4 )2Cr 207 + 2NH4C2H3O2 + H 2 O
Actually use 100 cc. of 62V acetic acid for each 45 grams of ammo-
nium chromate. Pour this volume of 6 2V acetic acid into a 500-cc.
flask and add the solid ammonium chromate. Insert a funnel in
the neck of the flask and heat the mixture until it just boils. Keep
it below the boiling temperature until solution is complete. If
solution is not complete in 10 minutes, add a little more 6 2V acetic
acid through the funnel. When solution is complete, place the
flask containing the solution in a pan of ice water and shake for
10 minutes. Filter off the ammonium dichromate, using suction.
Remove the liquor as completely as possible by pressing the solid