combination with aluminium sul-
phate (known as red liquor); in the
paper and board industry for sizing
and hardening; and in tanning. It
was previously used as an antiseptic
and astringent.
aluminium hydroxide A white
crystalline compound, Al(OH) 3 ; r.d.
2.42–2.52. The compound occurs nat-
urally as the mineral gibbsite(mono-
clinic). In the laboratory it can be
prepared by precipitation from
solutions of aluminium salts. Such
solutions contain the hexaquo-
aluminium(III) ion with six water
molecules coordinated, [Al(H 2 O) 6 ]3+.
In neutral solution this ionizes:
[Al(H 2 O) 6 ]3+ˆH++ [Al(H 2 O) 5 OH]2+The presence of a weak base such as
S2–or CO 3 2–(by bubbling hydrogen
sulphide or carbon dioxide through
the solution) causes further ioniza-
tion with precipitation of aluminium
hydroxide
[Al(H 2 O) 6 ]3+(aq) →Al(H 2 O) 3 (OH) 3 (s) +
3H+(aq)The substance contains coordinated
water molecules and is more cor-
rectly termed hydrated aluminium
hydroxide. In addition, the precipi-
tate has water molecules trapped in
it and has a characteristic gelatinous
form. The substance is amphoteric.
In strong bases the *aluminate ion is
produced by loss of a further proton:
Al(H 2 O) 3 (OH) 3 (s) + OH–(aq) ˆ
[Al(H 2 O) 2 (OH) 4 ]–(aq) + H 2 O(l)On heating, the hydroxide trans-
forms to a mixed oxide hydroxide,
AlO.OH (rhombic; r.d. 3.01). This sub-
stance occurs naturally as diaspore
and boehmite. Above 450°C it trans-
forms to γ-alumina.
In practice various substances can
be produced that are mixed crys-
talline forms of Al(OH) 3 , AlO.OH, and
aluminium oxide (Al 2 O 3 ) with water
molecules. These are known as hy-
drated alumina. Heating the hydrated
hydroxide causes loss of water, and
produces various activated aluminas,
which differ in porosity, number of
remaining –OH groups, and particle
size. These are used as catalysts (par-
ticularly for organic dehydration re-
actions), as catalyst supports, and in
chromatography. Gelatinous freshly
precipitated aluminium hydroxide
was formerly widely used as a mor-
dant for dyeing and calico printing
because of its ability to form insolu-
ble coloured *lakes with vegetable
dyes. See also aluminium oxide.aluminium oxide (alumina)A
white or colourless oxide of alu-
minium occurring in two main
forms. The stable form α-alumina
(r.d. 3.97; m.p. 2015°C; b.p. 2980 ±
60 °C) has colourless hexagonal or
rhombic crystals; γ-alumina (r.d.
3.5–3.9) transforms to the α-form on
heating and is a white microcrys-
talline solid. The compound occurs
naturally as corundumor emeryin
the α-form with a hexagonal-close-
packed structure of oxide ions with
aluminium ions in the octahedral in-
terstices. The gemstones ruby and
sapphire are aluminium oxide
coloured by minute traces of
chromium and cobalt respectively. A
number of other forms of aluminium
oxide have been described (β-, δ-, and
ζ-alumina) but these contain alkali-
metal ions. There is also a short-lived
spectroscopic suboxide AlO. The
highly protectiveÜlm of oxide
formed on the surface of aluminium
metal is yet another structural varia-
tion, being a defective rock-salt form
(every third Al missing).
Pure aluminium oxide is obtained
by dissolving the ore bauxite in
sodium hydroxide solution; impuri-
ties such as iron oxides remain in-
soluble because they are not
amphoteric. The hydrated oxide is25 aluminium oxide
a