POTASSIUM CHROMATE AND DICHROMATE 323minutes' heating the mixture becomes quite thick and is likely to
froth over. To prevent this, remove the cover and stir, protecting
the hand against spattering. When the mass becomes solid, heat
it again until it froths very little. Finally heat this solid for 20
minutes with the full flame of one burner and with the cover on.
Stir at frequent intervals and scrape any solid from the sides of the
crucible into the hot mass. When the heating is finished and the
solid has partially cooled, stir it and, while still hot, scrape it into
an 8-inch porcelain evaporating dish. When the crucible has
cooled to room temperature, heat in it 300 cc. of water to boiling
and add the hot water, with what it has extracted from the crucible,
to the solid in the evaporating dish. Heat this until all the lumps
have disintegrated. Allow any sediment to settle and decant
the yellow solution through a gravity filter. Boil an additional
50 cc. of water with the sediment in the dish, and pour this extract
through the same gravity filter. Transfer the entire yellow fil-
trate to the porcelain evaporating dish, and evaporate it over a
flame until crystals of potassium chromate begin to separate on the
surface of the liquid. (If a small amount of dark-colored solid
separates during the evaporation, the solution should be filtered
a second time.) Add 50 cc. of water to the hot solution and allow
it to stand until a satisfactory crop of crystals has separated.
Remove the crystals from the solution, using suction filtration.
Press the solid on the filter plate to remove as much of the mother
liquor as possible. Spread the yellow crystals of K 2 Cr04 on a
white paper towel and allow them to dry at room temperature.
Pour the filtrate into the evaporating dish and add 30 cc. of glacial
acetic acid. Stir the mixture, and if it is not orange-red add small
portions of glacial acetic acid until it is. Cool the mixture in a
pan of ice water. Filter off the meal of potassium dichromate on
a suction filter and press it as dry as possible. Dissolve the crystal
meal in boiling water, adding small portions of water until solution
is complete. If a clear solution is not obtained, filter hot, using
suction. Allow the K 2 Cr 2 07 to crystallize from the solution.
Dry the potassium dichromate on a white paper towel at room
temperature. Put the potassium chromate and dichromate into
separate 2-ounce cork-stoppered bottles. Calculate the theo-
retical yield of potassium chromate from 50 grams of chromic oxide.
Deduct your yield of chromate from this result, and calculate the
theoretical yield of potassium dichromate.