Modern inorganic chemistry

THE ELEMENTS OF GROUP III 151

the position of the equilibrium. The equilibria involved have been

discussed on p. 45; essentially they involve the species

[A1(H 2 0) 6 ]3+ ... [A1(OH) 3 (H 2 0) 3 ] ... [A1(OH) 6 ]^3 -

_____ »

„ acid

Therefore, when an anhydrous aluminium salt is dissolved in water

initially, the octahedral ion [A1(H 2 O) 6 ]3+ is formed by hydration

of the Al^3 ion. However, since some hydrolysis occurs, the solution

will contain H 3 O^ and be acidic. Addition of any molecule or ion

which removes H 3 O* for example alkali, or even sodium carbonate;

will cause the equilibrium to be displaced to the right and hydrated

aluminium hydroxide is precipitated.

H 3 O+ + COl~ -> H 2 O + CO 2

Addition of an excess of alkali displaces the equilibrium further
and finally the hexahydroxoaluminate(III) ion [A1(OH) 6 ]^3 ~ is
formed. Addition of H 3 O+ causes the displacement of equilibrium
to the left.
On standing, gelatinous aluminium hydroxide, which may initially
have even more water occluded than indicated above, is converted
into a form insoluble in both acids and alkalis, which is probably a
hydrated form of the oxide A1 2 O 3. Both forms, however, have strong
absorptive power and will absorb dyes, a property long used by the
textile trade to dye rayon. The cloth is first impregnated with an
aluminium salt (for example sulphate or acetate) when addition of a
little alkali, such as sodium carbonate, causes aluminium hydroxide
to deposit in the pores of the material. The presence of this
aluminium hydroxide in the cloth helps the dye to^4 bite' by ad
sorbing it—hence the name mordant (Latin mordere = to bite) dye
process.
Sheet aluminium can be given a colour by a similar process. The
aluminium is first made the anode in a bath of "chromic acid' (p. 377)
when, instead of oxygen being evolved, the aluminium becomes
coated with a very adherent film of aluminium oxide which is very
absorbent. If a dye is added to the bath the oxide film is coloured,
this colour being incorporated in a film which also makes the remain-
ing aluminium resistant to corrosion. This process is called "anodis-
ing' aluminium.
Salts containing the hydroxoaluminate ions [A1(OH) 4 (H 2 O) 2 ]~
and [A1(OH) 6 ]^3 ~ are known in solution but on heating they behave
rather like aluminium hydroxide and form hydrated aluminates.