ALUM , inchemistry, a
See also:term given to the crystallized
See also:double sulphates of the typical
See also:formula M2SO4.M2n•(SO4),24H2O, where M is the sign of an
See also:metal (potassium, sodium, rubidium, caesium),
See also:silver or ammonium, and M" denotes one of the trivalent metals, aluminium, chromium or ferric iron . These salts are employed in dyeing and various other
See also:industrial processes . They are soluble in
See also:water, have an astringent, acid, and sweetish taste, react acid to
See also:litmus, and crystallize in
See also:regular octahedra . When heated they liquefy; and if the
See also:heating be continued, the water of
See also:crystallization is driven off, the
See also:salt froths and swells, and at last an amorphous powder remains . Potash alum is the
See also: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
See also: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
See also:earth possesses
See also:peculiar properties, and is one of the ingredients in common
See also:clay (Experiences faites sur la terre de l'alun, Marggraf's Opusc. ii.111) . 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
See also:ammonia is dropped into this liquid, it immediately deposits perfect crystals of alum (Sur la regeneration de l'alun, Marggraf's Opusc. ii . 86) . T . 0 .
See also: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 (De confectione aluminus, Bergman's Opusc. i . 225), and that potassium sulphate is frequently found in alum . After M . H . Klaproth had discovered the presence of potassium in
See also:leucite and
See also:lepidolite, it occurred to Li N .
See also: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 (Annales de chimie, xxii . 258) . Soon after, J . A . Chaptal published the analysis of four different kinds of alum, namely,
See also:Roman alum,
See also:Levant alum,
See also:British alum and alum manufactured by himself . This analysis led to the same result as that of Vauquelin (
See also:Ann. de chim. xxii . 280) . The word alumen, which we translate alum, occurs in Pliny's Natural
See also:History . In the 15th
See also:chapter of his 35th
See also:book he gives a detailed description of it . By comparing this with the account of vrrvnrrrtpia 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
See also:medicine, the
See also:light-coloured alumen being useful in brilliant dyes, the dark-coloured only in dyeing black or very dark
See also:colours . One
See also:species was a liquid, which was
See also:apt to be adulterated; but when pure it had the
See also:property of blackening when added to
See also:pomegranate juice . This property seems to characterize a solution of iron sulphate 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
See also:call schistos . It forms in
See also:white threads upon the
See also: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
See also:slate and bituminous shale, and which consists chiefly of the sulphates of iron and aluminium . Possibly in certain places the iron sulphate 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 sulphate, sometimes probably an aluminium sulphate, and usually a mixture of the two . But the ancients were unacquainted with our alum .
They were acquainted with a crystallized iron sulphate, and distinguished it by the names of misy, sory, chalcanthum (Pliny xxxiv . 12) . As alum and
See also: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
See also: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 sulphate; and the name at; amentum 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
See also:alunite (q.v.) or alum-
See also:stone, alum schist,
See also:bauxite and
See also:cryolite . In
See also:order to obtain alum from alunite, it is calcined and then exposed to the
See also:action of air for a considerable
See also:time . During this exposure it is kept continually moistened with water, so that it ultimately falls to a very
See also:fine powder . This powder is then lixiviated with hot water, the liquor decanted, and the alum allowed to crystallize . The alum
See also:schists employed in the manufacture of alum are mixtures of iron
See also:pyrites, aluminium silicate and various bituminous substances, and are found in upper
See also:Bavaria, Bohemia, Belgium and Scotland . These are either roasted or exposed to the weathering action of the air . In the roasting
See also:process, sulphuric acid is formed and acts on the clay to
See also:form aluminium sulphate, a similar
See also:condition of affairs being produced during weathering . The mass is now systematically extracted with water, and a solution of aluminium sulphate of specific gravity 1.16 is prepared .
This solution is allowed to stand for some time (in order that anycalcium sulphate and basic ferric sulphate may
See also:separate), and is then evaporated until ferrous sulphate crystallizes on cooling; it is then
See also:drawn off and evaporated until it attains a specific gravity of 1.4o . It is now allowed to stand for some time, decanted from any sediment, and finally mixed with the calculated quantity of potassium sulphate (or if ammonium alum is required, with ammonium sulphate), well agitated, and the alum is thrown down as a finely-divided precipitate of alum
See also:meal . If much iron should be
See also:present in the shale then it is preferable to use potassium chloride in place of potassium sulphate . In the preparation of alum from
See also: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 sulphate 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
See also:carbon dioxide through the solution . The precipitate is then dissolved in sulphuric acid, the requisite amount of potassium sulphate added and the solution allowed to crystallize . Potash alum, K2SO4•Al2(SO4)s.24H20, crystallizes in regularoctahedra and is very soluble in water . The solution reddens litmus and is an astringent . When heated to nearly a red
See also: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
See also:mordant, in the preparation of lakes for sizing
See also:hand-made paper and in the clarifying of turbid liquids .
Sodium alum, Na2SO4•Al2(SO4),.24H2O, occurs in nature as the
See also: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
See also:cold, and to evaporate them at a temperature not exceeding 6o° C. too parts of water dissolve
See also:Ito parts of sodium alum at o° C . (W . A .
See also:Tilden, Jour . Chem .
See also:Soc., 1884, 45, p . 409), and 51 parts at 16° C . (E . Auge, Comptes rendus, 189o, to, p . 1134) .
Chrome alum, K2SO4•Cr2(SO4)3.24H20, appears chiefly as a by-product in the manufacture of
See also: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 .
ALUMINIUM (symbol Al; atomic weight 27.0)
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