330 HEAVY METALS OF GROUPS VI, VII, AND VIIIProcedure: Pulverize 98 grams of potassium dichromate, and
cover it in an 8-inch porcelain dish with 400 cc. of water. Add
76 cc. of concentrated sulphuric acid, and stir until all the salt is
dissolved. Adding the sulphuric acid should produce enough heat
to dissolve the dichromate, but, if it is necessary, heat the mixture
a little more. After all the salt has dissolved, filter the solution if
it is not absolutely clear. Add the alcohol rather cautiously,
stirring after each addition. Allow the heat of the reaction to
raise the temperature of the solution to the boiling point. Cover
the hot solution with a glass plate and set it away in a cupboard for
a week or longer. Remove the crystals from the liquid, and if
possible get a further crop of crystals. Dry the crystals by leaving
them wrapped in paper towels over night (Note 9 (6), page 15), and
then stopper them at once in a bottle, since they are efflorescent.
QUESTIONS- Write equations for the following steps in the reaction of
potassium dichromate in acid solution with alcohol, (a) Resolve
the salt into its basic and acidic anhydrides. (6) Let the acid
anhydride, which is the higher oxide of chromium, be reduced by
the alcohol to the lower oxide, Cr 2 O 3. (c) Let the sulphuric acid
form salts with the basic anhydride and the lower oxide of chro-
mium, which is also a basic oxide. Add the steps together to give
the complete equation. - Sulphur dioxide might serve as the reducing agent. Write
equations for the reaction in steps and add the equations. - Describe the effect observed when hydrogen sulphide is
passed into a hot acidified solution of K 2 Cr2O7. Write the equa-
tion in one line marking the valences of sulphur and chromium
and showing that the algebraic sum of the valence changes is zero.
PREPARATION 65
BASIC LEAD CHEOMATE, PbCrCVPbO
Basic salts may be considered as bases partially neutralized by
acids, or as compounds of a base and a salt of that base.
Bi(OH) 3 + HC1 -+ Bi(OH) 2 Cl
Pb(OH) 2 + PbCrO -• Pb(OH) 2 -PbCrO 4
Such basic salts lose water very readily, but are still considered
as basic salts in the dehydrated condition, e.g., BiOCl, PbO-PbCrO.