130 OXY-ACIDS AND SALTS OF SULPHUR.
suction or by evaporation. Washing with anhydrous acetic acid is effective,
however, because this solvent does not react appreciably with either sulphuric
acid or antimony sulphate, and the excess can be easily removed.Add 10 g. of finely powdered antimony (from No. 7 or 10) in
small portions to 100 c.c. of hot, concentrated sulphuric acid, and
heat the mixture in a beaker nearly to boiling until all the anti-
mony has dissolved. On cooling, antimony sulphate crystallizes
in long, colorless, lustrous needles. Allow the solution to cool
completely, dilute it with 80 c.c. of anhydrous acetic acid, and
again permit the mass to cool. Drain the crystals by suction on
a hardened filter, and wash them rapidly with a little anhydrous
acetic acid and then with ether; allow the product to dry for one
or two days in a vacuum desiccator over sulphuric acid. Yield,
13 g.
If a sample of the salt is treated with water, it undergoes
hydrolysis, and free sulphuric acid is found in the solution decanted
from the insoluble basic salt.
For the quantitative analysis of the salt, dissolve a weighed
sample in a little concentrated hydrochloric acid, add tartaric
acid, dilute well, and precipitate with barium chloride. From a
second solution prepared in the same way, precipitate the antimony
with hydrogen sulphide and weigh it in a Gooch crucible as Sb 2 S 3
after drying it in an atmosphere of carbon dioxide at 280°.- Alum from Kaolin.
Potassium aluminium sulphate, K 2 SO 4. A1 2 (SO 4 ) 3 .24 H 2 O, is distin-
guished from other aluminium salts by its marked ability to crystallize, and
on this account it can readily be obtained pure. Thus it has long been the
most important salt of aluminium in spite of the fact that it contains but
5.71% of that element. More recently, however, aluminium sulphate has
been used a great deal instead of alum; it is prepared by neutralizing with sul-
phuric acid the hydrated aluminium oxide, which can now be prepared pure
from the mineral bauxite.
Stir 50 g. of powdered kaolin with 75 g. of concentrated sul-
phuric acid in an evaporating dish; heat the mixture for 2J hours
on a Babo funnel, at first gently and then more strongly, until
white fumes escape. Triturate the mass when cold, and extract
once with 300 c.c. and then three times with 100 c.c. of boiling
water; the last filtrate should show no test for aluminium with