precipitated by seeding with material
from a previous batch and this is
then roasted at 1150–1200°C to give
pure α-alumina, or at 500–800°C to
give γ-alumina. The bonding in alu-
minium hydroxide is not purely ionic
due to polarization of the oxide ion.
Although the compound might be
expected to be amphoteric, α-alu-
mina is weakly acidic, dissolving in
alkalis to give solutions containing
aluminate ions; it is resistant to acid
attack. In contrast γ-alumina is typi-
cally amphoteric dissolving both in
acids to give aluminium salts and in
bases to give aluminates. α-alumina
is one of the hardest materials
known (silicon carbide and diamond
are harder) and is widely used as an
abrasive in both natural (corundum)
and synthetic forms. Its refractory
nature makes alumina brick an ideal
material for furnace linings and alu-
mina is also used in cements for
high-temperature conditions. See also
aluminium hydroxide.aluminium potassium sulphate
(potash alum; alum)A white or
colourless crystalline compound,
Al 2 (SO 4 ) 3 .K 2 SO 4 .24H 2 O; r.d. 1.757;
loses 18H 2 O at 92.5°C; becomes anhy-
drous at 200°C. It forms cubic or oc-
tahedral crystals that are soluble in
cold water, very soluble in hot water,
and insoluble in ethanol and acetone.
The compound occurs naturally as
the mineral kalinite. It is a double
salt and can be prepared by recrystal-
lization from a solution containing
equimolar quantities of potassium
sulphate and aluminium sulphate. It
is used as a mordant for dyeing and
in the tanning andÜnishing of
leather goods (for white leather). See
also alums.aluminium sulphateA white or
colourless crystalline compound,
Al 2 (SO 4 ) 3 , known as the anhydrous
compound (r.d. 2.71; decomposes at770 °C) or as the hydrate Al 2 (SO) 3.
18H 2 O (monoclinic; r.d. 1.69; loses
water at 86.5°C). The anhydrous salt
is soluble in water and slightly solu-
ble in ethanol; the hydrate is very
soluble in water and insoluble in
ethanol. The compound occurs natu-
rally in the rare mineral alunogenite
(Al 2 (SO) 3 .18H 2 O). It may be prepared
by dissolving aluminium hydroxide
or china clays (aluminosilicates) in
sulphuric acid. It decomposes on
heating to sulphur dioxide, sulphur
trioxide, and aluminium oxide. Its so-
lutions are acidic because of hydroly-
sis.
Aluminium sulphate is commer-
cially one of the most important alu-
minium compounds; it is used in
sewage treatment (as aÛocculating
agent) and in the puriÜcation of
drinking water, the paper industry,
and in the preparation of mordants.
It is also aÜre-prooÜng agent. Alu-
minium sulphate is often wrongly
called alumin these industries.aluminium trimethylSee
trimethylaluminium.alumsA group of double salts with
the formula A 2 SO 4 .B 2 (SO 4 ) 3 .24H 2 O,
where A is a monovalent metal and B
a trivalent metal. The original exam-
ple contains potassium and alu-
minium (called potash alum or
simply alum); its formula is often
written AlK(SO 4 ) 2 .12H 2 O (aluminium
potassium sulphate-12-water). Ammo-
nium alum is AlNH 4 (SO 4 ) 2 .12H 2 O,
chrome alum is KCr(SO 4 ) 2 .12H 2 O (see
potassium chromium sulphate), etc.
The alums are isomorphous and can
be made by dissolving equivalent
amounts of the two salts in water
and recrystallizing. See also alu-
minium sulphate.alunogeniteA mineral form of
hydrated *aluminium sulphate,
Al 2 (SO 4 ) 3 .18H 2 O.aluminium potassium sulphate 26a