Encyclopedia
Alum [aluminium potassium sulfate], in
chemistry, is a term given to the crystallized double
sulfates of the typical formula M
+2SO
4·M
3+23·24H
2O, where M
+ is the sign of an alkali metal , and M
3+ denotes one of the trivalent metal ions . The
ammonium ion also occurs in the M
+ position.
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 is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains.
Potassium alum is the common alum of commerce, although both soda alum and ammonium alum are manufactured.
Alchemical and later discoveries and uses
The presence of
sulfuric acid in potassium 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 sulfuric 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 sulfuric 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 sulfuric 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 Vauquelin.
Pliny's writings
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-colored alumen being useful in brilliant dyes, the dark-colored only in dyeing black or very dark colors. 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 alum would possess no such property. Pliny says that there is another kind of alum that 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 colors. 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 as 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, and 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.
Early uses in industry
Alum was imported into England mainly from the Middle East, and, from the late
15th Century onwards, the Papal States for hundreds of years. Its use there was as a
dye-fixer for
wool , the value of which increased significantly if dyed. These sources were unreliable, however, and there was a push to develop a source in England. With state financing, attempts were made throughout the
16th Century, but without success until early on in the
17th Century. An industry was founded in
Yorkshire to process the shale which contained the key ingredient,
aluminium sulfate, and made an important contribution to the
Industrial Revolution.
Alum from alunite
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
pyrite, 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, sulfuric 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 , 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 , 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.
Alum from clays or bauxite
In the preparation of alum from
clays or from
bauxite, the material is gently calcined, then mixed with sulfuric 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 sulfuric acid, the requisite amount of potassium sulfate added and the solution allowed to crystallize.
Types of alum
Potassium alum, K
2SO
4·Al
23·24H
2O, 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 92 °C 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. See the mineral
alunite.
Soda alum
Sodium alum, Na
2SO
4·Al
23·24H
2O, 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 60 °C. 100 parts of water dissolve 110 parts of sodium alum at 0 °C, and 51 parts at 16 °C.
Chrome alum
Chrome alum, K
2SO
4·Cr
23·24H
2O, appears chiefly as a by-product in the manufacture of
alizarin, and as a product of the reaction in dichromate batteries. Below 60
oC it dissolves in water to give a purple solution, while above that temperature it gives a green solution, due to the hydrolysis of the chromium ion.
Ammonia alum, NH4Al2·12H2O, a white crystalline double sulfate of aluminium, is used in water purification, in vegetable glues, in porcelain cements, in natural deodorants, in tanning, dyeing and in fireproofing textiles.
Alum solubility
The solubility of the various alums in water varies greatly, sodium alum being readily soluble in water, while 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 |
Potassium Alum |
Rubidium Alum |
|---|
0 °C |
2.62 |
0.19 |
3.90 |
0.71 |
10 °C |
4.50 |
0.29 |
9.52 |
1.09 |
50 °C |
15.9 |
1.235 |
44.11 |
4.98 |
80 °C |
35.20 |
5.29 |
134.47 |
21.60 |
100 °C |
70.83 |
|
357.48 |
|
Uses
Shaving alum is a powdered form of alum used as an astringent to prevent bleeding from small shaving cuts. The styptic pencils sold for this purpose contain aluminium sulfate or potassium aluminium sulfate. Similar products are also used on animals to prevent bleeding after nail-clipping.
Crystal deodorant: Alum was used in the past as a natural underarm deodorant in Mexico, Thailand, and the Far East and in the Philippines where it is called Tawas. It is now commercially sold for this purpose in many countries, often in a plastic case that protects the crystal and makes it resemble other non-liquid deodorants.
Alum powder, found amongst spices at most grocery stores, is used in pickling recipes as a preservative, to maintain crispness, and as an ingredient in some play dough recipes. It is also commonly cited as a home remedy or pain relief for canker sores.
Water treatment: Alum is used in water treatment. The addition of alum to raw water causes small particles and colloids to stick together form heavier particles which will settle in water. This process is called coagulation or flocculation.
By soaking and then drying cloth and paper materials they can be made fireproof.
Wax: Alum is used in the Middle East as a component in wax, compounded with other ingredients to create a hair-removal substance.
And many more.
See also
Alum is used regularly as an adjuvant in human immunizations.
References
External links
