A type of double salt. Alums are
double sulfates obtained by crystallizing
mixtures in the correct proportions. They
have the general formula:
M2SO4.M';2(SO4)3.24H2O
Where M is a univalent metal or ion,
and M′ is a trivalent metal. Thus, aluminum
potassium sulfate (called potash
alum, or simply alum) is
K2SO4.Al2(SO4)3.24H2O
Aluminum ammonium sulfate (called
ammonium alum) is
(NH4)2SO4.Al2(SO4)3.24H2O
The name ‘alum’ originally came from
the presence of Al3+ as the trivalent ion, but
is also applied to other salts containing
trivalent ions, thus, Chromium(III) potassium
sulfate (chrome alum) is
K2SO4.Cr2(SO4)3.24H2O.
Alum is a double sulphate of a monovalent and trivalent
salt.
Alum acts as a flocculating agent. It is used in sewage
treatment and in purification of drinking water. Alum is
also used in the preparation of mordant and as a
fireproofing agent.
Aluminum sulphate is sometimes erroneously called
alum in some industries (such as in the paper industry).
