Alum
Alum, in chemistry, is a term given to the crystallized double sulfates of the typical formula M2SO4·MIII2(SO4)3·24H2O, where M is the sign of an alkali metal (potassium, sodium, rubidium, caesium), silver or ammonium), and MIII denotes one of the trivalent metals (aluminium, chromium, or ferric iron). These salts are employed in dyeing and various other industrial processes. They are soluble in water, have an astringent, acid, and sweetish taste, react acid to litmus, and crystallize in regular octahedra. When heated they liquefy; and if the heating be continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.
Potash alum is the common alum of commerce, although both
soda alum and ammonium alum are manufactured. The presence
of sulphuric acid in potash alum was known to the alchemists.
J. H. Pott and A. S. Marggraf demonstrated that alumina was
another constituent. Pott in his Lithogeognosia showed
that the precipitate obtained when an alkali is poured into
a solution of alum is quite different from lime and chalk,
with which it had been confounded by G.E. Stahl. Marggraf
showed that alumina is one of the constituents of alum, but
that this earth possesses peculiar properties, and is one of
the ingredients in common clay. He also
showed that crystals of alum cannot be obtained by dissolving
alumina in sulphuric acid and evaporating the solutions, but
when a solution of potash or ammonia is dropped into this
liquid, it immediately deposits perfect crystals of alum.
Torbern Bergman also observed that the addition of potash or ammonia
made the solution of alumina in sulphuric acid crystallize, but
that the same effect was not produced by the addition of soda
or of lime, and that potassium sulfate is frequently found in alum.
After M.H. Klaproth had discovered the presence of potassium
in leucite and lepidolite, it occurred to L.N. Vauquelin
that it was probably an ingredient likewise in many other
minerals. Knowing that alum cannot be obtained in crystals
without the addition of potash, he began to suspect that
this alkali constituted an essential ingredient in the salt,
and in 1797 he published a dissertation demonstrating that
alum is a double salt, composed of sulphuric acid, alumina, and potash. Soon after, J.A. Chaptal published the analysis of four different kinds of alum, namely, Roman alum, Levant alum, British alum and alum manufactured by himself. This analysis led to the same result as that of Vauquelin.
The word "alumen," which we translate "alum," occurs in Pliny's Natural History. In the 15th chapter of his 35th book he gives a detailed description of it. By comparing this with the account of stupteria given by Dioscorides in the 123rd chapter of his 5th book, it is obvious that the two are identical. Pliny informs us that alumen was
found naturally in the earth. He calls it salsugoterrae.
Different substances were distinguished by the name of
"alumen"; but they were all characterized by a certain
degree of astringency, and were all employed in dyeing and
medicine, the light-coloured alumen being useful in brilliant
dyes, the dark-coloured only in dyeing black or very dark
colours. One species was a liquid, which was apt to be
adulterated; but when pure it had the property of blackening
when added to pomegranate juice. This property seems
to characterize a solution of iron sulfate in water; a
solution of ordinary (potash) alum would possess no such
property. Pliny says that there is another kind of alum which
the Greeks call schistos. It forms in white threads upon
the surface of certain stones. From the name schistos, and
the mode of formation, there can be little doubt that this
species was the salt which forms spontaneously on certain
slaty minerals, as alum slate and bituminous shale, and
which consists chiefly of sulfates of iron and aluminium.
Possibly in certain places the iron sulfate may have been
nearly wanting, and then the salt would be white, and would
answer, as Pliny says it did, for dyeing bright colours.
Several other species of alumen are described by Pliny, but
we are unable to make out to what minerals he alludes.
The alumen of the ancients, then, was not the same with the
alum of the moderns. It was most commonly an iron sulfate,
sometimes probably an aluminium sulfate, and usually a
mixture of the two. But the ancients were unacquainted with
our alum. They were acquainted with a crystallized iron
sulfate, and distinguished it by the names of misy, sory, chalcanthum. As alum and green vitriol were applied to a variety of substances in common, and as both are distinguished by a sweetish and astringent taste,
writers, even after the discovery of alum, do not seem
to have discriminated the two salts accurately from each
other. In the writings of the alchemists we find the words
misy, sory, chalcanthum applied to alum as well as to iron
sulfate; and the name atramentum sutorium, which ought
to belong, one would suppose, exclusively to green vitriol,
applied indifferently to both. Various minerals are employed
in the manufacture of alum, the most important being alunite
or alum-stone, alum schist, bauxite and cryolite.
In order to obtain alum from alunite, it is calcined and
then exposed to the action of air for a considerable
time. During this exposure it is kept continually moistened
with water, so that it ultimately falls to a very fine
powder. This powder is then lixiviated with hot water, the
liquor decanted, and the alum allowed to crystallize. The
alum schists employed in the manufacture of alum are mixtures
of iron pyrites, aluminium silicate and various bituminous
substances, and are found in upper Bavaria, Bohemia, Belgium,
and Scotland. These are either roasted or exposed to the
weathering action of the air. In the roasting process,
sulphuric acid is formed and acts on the clay to form aluminium
sulfate, a similar condition of affairs being produced during
weathering. The mass is now systematically extracted with
water, and a solution of aluminium sulfate of specific
gravity 1.16 is prepared. This solution is allowed to stand
for some time (in order that any calcium sulfate and basic
ferric sulfate may separate), and is then evaporated until
ferrous sulfate crystallizes on cooling; it is then drawn
off and evaporated until it attains a specific gravity of
1.40. It is now allowed to stand for some time, decanted from
any sediment, and finally mixed with the calculated quantity
of potassium sulfate (or if ammonium alum is required, with
ammonium sulfate), well agitated, and the alum is thrown
down as a finely-divided precipitate of alum meal. If much
iron should be present in the shale then it is preferable
to use potassium chloride in place of potassium sulfate.
In the preparation of alum from clays or from bauxite, the
material is gently calcined, then mixed with sulphuric acid
and heated gradually to boiling; it is allowed to stand for
some time, the clear solution drawn off and mixed with acid
potassium sulfate and allowed to crystallize. When cryolite
is used for the preparation of alum, it is mixed with calcium
carbonate and heated. By this means, sodium aluminate is
formed; it is then extracted with water and precipitated
either by sodium bicarbonate or by passing a current of
carbon dioxide through the solution. The precipitate is then
dissolved in sulphuric acid, the requisite amount of potassium
sulfate added and the solution allowed to crystallize.
Potash alum, K2SO4·Al2(SO4)3·24H2O, crystallizes in regular octahedra and is very soluble in water. The solution reddens litmus and is an astringent. When heated to nearly a red
heat it gives a porous friable mass which is known as "burnt
alum." It fuses at 92C in its own water of crystallization.
"Neutral alum" is obtained by the addition of as much sodium
carbonate to a solution of alum as will begin to cause the
separation of alumina; it is much used in mordanting. Alum
finds application as a mordant, in the preparation of lakes for
sizing hand-made paper and in the clarifying of turbid liquids.
Sodium alum, Na2SO4·Al2(SO4)3·24H2O, occurs in nature as the mineral mendozite. It is very soluble in water, and is extremely difficult to purify. In the preparation of
this salt, it is preferable to mix the component solutions in the cold, and to evaporate them at a temperature not exceeding 60C. 100 parts of water dissolve 110 parts of sodium alum at 0C, and 51 parts at 16C.
Chrome alum, K2SO4·Cr2(SO4)3·24H2O, appears chiefly as a by-product in the manufacture of alizarin, and
as a product of the reaction in bichromate batteries.
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, whilst caesium and rubidium alums are only sparingly soluble. The various solubilities are shown in the following table.
At temperature T, 100 parts water dissolve:
T |
Ammonium Alum |
Caesium Alum |
Potash Alum |
Rubidium Alum |
|---|
0C |
2.62 |
0.19 |
3.90 |
0.71 |
10C |
4.50 |
0.29 |
9.52 |
1.09 |
50C |
15.9 |
1.235 |
44.11 |
4.98 |
80C |
35.20 |
5.29 |
134.47 |
21.60 |
100C |
70.83 |
|
357.48 |
|
- (from an old encyclopedia)
See also: List of minerals
Referenced By
List of chemistry topics | List of minerals
|
