Synthetic Inorganic Chemistry

324 HEAVY METALS OF GROUPS VI, VII, AND VIII

QUESTIONS

1. Name at least three oxidizing agents which might have
   been used instead of potassium nitrate in this preparation, and
   write equations.


2. To 5 cc. of a chromic sulphate solution add NaOH in excess;
   cool, add about 1 gram of sodium peroxide, agitate for a few min-
   utes, and then boil until effervescence ceases. Describe observa-
   tions and write equations.


3. To a solution of potassium dichromate add K 2 CO 3 until no
   more effervescence takes place. Explain the effervescence and
   the change in color. Write equation.


4. To the solution from Experiment 3 add 6 N H2SO4; observe
   and explain as before. Explain fully the difference between chro-
   mates and dichromates.


5. Show that potassium acid sulphate, KHSO4, and potassium
   dichromate, are very similar, differing mainly in the degree of
   hydration.

PREPARATION 62
CHROMIC ANHYDRIDE, CrO 3
The addition of sulphuric acid to a solution of either a chromate
or a dichromate liberates chromic acid which is very soluble and
can exist in solution in the different forms, H 2 Cr04, H 2 Cr 2 07 and
CrO 3 , in equilibrium with each other. With the addition of a
large excess of concentrated H2SO4, water is withdrawn from the
hydrated forms and the anhydride separates in the shape of red
needles.

Materials: sodium dichromate, Na2Cr2O?-2H2O, 100 grams =

0.33 F.W.

36.ZVH2SO4, 400 cc

Apparatus: 8-inch porcelain dish.

glass plate to cover the 8-inch dish,

suction filter with glass marble,

glass-stoppered sample bottle,

tripod.

Bunsen burner.

Procedure: Dissolve the 100 grams of sodium dichromate in 250

cc. of water and filter from any sediment. Add rather slowly with