230 HEAVY METALS OF GROUPS I AND II
Commercial zinc sulphate invariably contains a small amount of
iron as an impurity. Since FeSO4-7H 2 O crystallizes isomor-
phously with ZnS(V7H 2 O a preparation of the latter cannot be
freed of the former by recrystallization. By addition of chlorine,
or its equivalent, to the solution of zinc sulphate, the iron is oxidized
to ferric salt; the ferric salt hydrolyzes somewhat, and, if the acid
produced by the hydrolysis is neutralized as fast as formed, the
hydrolysis proceeds to completion and all the iron is precipitated
as Fe(OH) 3. In this case, the reagent used to bring about the
exact neutrality of the solution is a suspension of basic zinc car-
bonate. (Compare the similar procedure for removing traces of
iron in the preparation of strontium chloride, Preparation 21.)
To obtain the pure white, impalpable powder which is sought it
is necessary: to remove the iron salts, which cause a yellow or
brown color; to wash entirely free of soluble sodium sulphate,
which would give a lumpy product; to avoid getting filter fibers
mixed with the material, for these would char and cause darken-
ing; and to avoid overheating the product, which might cause it
to react with the glaze of the dish.
Materials: zinc sulphate, ZnSO4-7H 2 O, 57 grams = 0.2 F.W.
anhydrous sodium carbonate, Na2CO3, 24 grams,
bleaching powder or calcium hypochlorite
(" H. T. H."), 1 gram.
Apparatus: two 2-liter common bottles.
5-inch filter funnel.
8-inch porcelain dish.
4-inch porcelain dish,
iron ring and ring stand.
Bunsen burner.Procedure: Dissolve the ZnS(V7H 2 O in 700 cc. water. Pre-
pare a NaOCl solution as follows: Add 1 gram of calcium hypo-
chlorite or bleaching powder and 1 gram of Na 2 CO3 to 100 cc. cold
water. Stir thoroughly and pour through a filter. Add the cold
nitrate to the cold solution of ZnSO* in 700 cc. water in an 8-inch
porcelain dish. Heat with a small flame during 10 minutes to a
temperature of 100°, when the iron will be completely precipitated
as Fe(OH) 3. Pour the solution through a filter, collecting the
filtrate in a 2-liter common bottle. Dissolve 23 grams of sodium