STANNOUS CHLORIDE 269Apparatus: shredded asbestos suspended in water,
suction filter and trap bottle.
8-inch porcelain dish,
iron ring and ring stand.
Bunsen burner.Procedure: Place 100 grams of feathered tin in a 750-cc. casse-
role, cover with 175 cc. of 12 iV HC1, and add (at the hood) 25 cc. of
6 N HN0 3 , a little at a time, during a period of 10 minutes. Then
concentrate the solution, by boiling over a free flame, to a volume
of 90-100 cc, at which point a crystal scum will form on blowing
on the surface of the hot liquid. There should still be left a small
amount of undissolved metal. If at any time during the evapo-
ration all the tin becomes used up, add a little more. Prepare an
asbestos filter (Note 4 (d), page 8), moisten it with concentrated
hydrochloric acid, and filter the concentrated stannous chloride
solution before it has cooled to below 60-70°. Finally, rinse out
the casserole with 15 cc. of concentrated hydrochloric acid and
pour this liquid through the filter, letting it mix with the main
part of the solution. If during the filtration the liquid stops flow-
ing, due to crystals separating in the filter, add 5-10 cc. of boiling
water. Pour the solution into an 8-inch porcelain dish, and
leave it to evaporate slowly at room temperature in a place ex-
posed to the air but protected from dust. (The solubility of
stannous chloride decreases very rapidly with decreasing temper-
ature. Hence it is advantageous to carry out the crystallization
in as cool a place as possible.) When a good crop of crystals has
formed, pour off the liquid into another dish; spread the crystals
on paper towels and allow them to dry. It is to be remembered
that stannous chloride is extremely soluble in water and that the
composition of the mother liquor is not far different from that of
the crystals of SnCl 2 -2H 2 O which separate. Heat the remaining
solution carefully just to the boiling temperature, but do not allow
it to boil more than a moment. In this way sufficient water and
hydrochloric acid are expelled to allow another crop of crystals
to form. If too much hydrochloric acid is expelled by the evap-
oration and an indistinctly crystalline precipitate of basic salt
separates on cooling, add a few drops of hydrochloric acid and
redissolve the salt by warming. Set the solution aside to cool and
evaporate, as before, and collect another crop of crystals. By