344 HEAVY METALS OF GROUPS VI, VII, AND VIII
QUESTIONS- What tests can be applied to show whether this preparation
dissociates in solution into the ions of the two simple salts? - Recall instances in which complex salts of heavy metals do
not give the simple ions of the heavy metals.
FERRIC AMMONIUM ALUM
Ferrous sulphate in solution can be oxidized by nitric acid
to a ferric compound, but to obtain the full yield of ferric sulphate
the presence of sulphuric acid is necessary according to the equation
6FeSO 4 + 2HNO 3 + 3H2SO4 -> 3Fe2(SO 4 ) 3 + 4H 2 O + 2NOAfter adding (NH^SC^ to the solution so obtained, the double salt,
ammonium ferric sulphate, or ferric alum, may be crystallized out-
In order to obtain a satisfactory product it is very important to
adjust the amount of acid very precisely. Too little acid will
allow a brown basic salt to form (mFe(OH)3-nFe 2 (SO4) 3 ), which
will discolor the preparation. Too much acid has the obvious
disadvantage that it will cling to the crystals of the product from
which it cannot be removed by evaporation or by washing
(because of the great solubility of the product).
Procedure: Transfer the second portion of the ferrous sulphate
solution to an 8-inch porcelain evaporating dish, and add 61 cc.
of 6N H2SO4. The volume of the mixture should be not less
than 165 cc. or more than 185 cc. Carry the dish to the hood,
add 14 cc. of 16 N HNO 3 , and heat the mixture slowly to the
boiling point. Continue heating, and stir until a very vigorous
reaction begins to take place with the evolution of oxides of
nitrogen. Remove the flame, and let the reaction proceed.
This reaction may take place when the nitric acid is added but
usually it is necessary to heat the solution for several minutes.
When the reaction is complete the color of the solution will
change from black to light brown. The solution may be tested
for ferrous iron at this point with potassium ferricyanide. When
the oxidation is complete heat the mixture for a few minutes,
and then add 25 grams of ammonium sulphate. Cool to room
temperature, and add enough distilled water to make the total
volume 250 cc. Allow it to stand in the evaporating dish until
the alum has crystallized. Decant the mother liquor from the