BASIC LEAD CHROMATE 331It is often true that a basic salt is less soluble than either the
base itself or the salt, and in such cases the basic salt is especially
easy to prepare. In general, basic salts do not have definite
composition but are indefinite mixtures of base and salt. This is
true of basic lead chromate in which the ratio of PbO to PbCrO*
varies with the method of preparation. Lead chromate, PbCrO4,
is used as a yellow pigment for paints. Basic lead chromate is
bright red and is also used as a pigment.Materials: lead acetate, Pb(C 2 H 3 O2)2-3H2O, 38 grams = 0.1
F.W.
sodium dichromate, Na 2 Cr2O7-2H 2 O, 8 grams,
sodium hydroxide, NaOH, 12 grams.
Apparatus: 8-inch porcelain dish.
5-inch funnel.
2-liter common bottle,
iron ring and ring stand.
Bunsen burner.Procedure: Dissolve 38 grams of lead acetate in 400 cc. of water
in an 8-inch porcelain dish. (The solution will not be clear.)
Dissolve 8 grams of sodium dichromate in 50 cc. of water, and
add the solution to the lead acetate. Yellow lead chromate will
precipitate. Dissolve 12 grams of sodium hydroxide in 50 cc. of
water and add this solution while stirring. Heat the mixture
to boiling. The color of the solid will slowly change to a reddish
orange. Boil the mixture gently until no further change in color
takes place. Pour the mixture into a liter of cold water in a
large bottle. Allow the precipitate to settle and wash it three
times by decantation. Transfer the basic lead chromate to a filter
and let it drain without suction. When the cake is dry enough to
hold its shape, unfold the filter paper from it, spread it on a watch
glass, and dry it on the hot plate. Pulverize the dry product and
preserve it in a 2-ounce cork-stoppered bottle.
QUESTIONS- Treat about 0.5 gram of your product with 5 cc. of 6 N acetic
acid. Warm the mixture until it boils. Note the change in color.
Filter and add a few drops of sodium sulphide solution to the