176 ALUM.— Chemistry of.
Fahr. (22-°2 to 27°-2 C.), and adding an aqueous solu-tion of potassium hydrate in alcohol in successivesmall portions, till the subsalt of alumina, throwndown by each addition, was no longer dissolved uponthe agitation of the liquid. The form assumed bythe salt after adding a quantity of potassium equalto about one and a half equivalents aluminium (thatis, on taking from the aluminium sulphate containedin the alum nearly half its acid), was that of trun-cated and cubical octahedrons; but on the furtheraddition of alkali, cubes were formed on spontaneousevaporation of the liquid.
Cubical ammonium alum he procured by dis-solving the pure octahedral salt in three to fourtimes its weight of water at 113° Fahr. (45° C.), andadding ammonia till the small quantity of subsaltceased to dissolve in the solution.
When some of the solutions thus prepared hadgiven a certain quantity of cubic crystals the solu-tion became cloudy, and deposited a gelatinous tri-basic aluminium sulphate (A1 2 0 2 (S0 4 ) + 9II 2 0).
The potassium and ammonium cubic alums resistthe action of air dried by sulphuric acid in a bell-jarfor weeks without efflorescing. Loewel suggeststhat it would be more correct to state that no efflor-escence takes place, lie placed crystals of purealum under bell-jars, in which the air was dried bysulphuric acid; at first the crystals lost some thou-sandths of their weight of interposed water, butafterwards remained for several months, even for awhole year, at a temperature rising to 77° Fahr.(25°-0 C.).
According to De IIauep. a crystal of cubic alumwhen placed in a solution of alum rendered basicby addition of an alkali, continues to grow, pre-serving at the same time its cubical form; and if itsfaces be cut so as to make it an octahedron beforeplacing it in such a solution, the cubical shape isin a short time restored. The opposite phenomenonis observed when an octahedral crystal, cut into acube, is immersed in a solution of ordinary alum,
When the cubical crystals of Roman alum aredissolved in water, at a temperature ranging be-tween 96° and 101° Fahr. (35°-5 to 40° C,), and thesolution set aside to crystallize by spontaneous evapo-ration, the alum is again produced in cubes. At ahigher temperature the solution becomes turbid fromthe separation of basic alum, and the clear liquordeposits alum in octahedral crystals.
The solubility of potassium alum is, accordingto Poggiai.e, as follows :—
Alum melts at 197°-6 Fahr, (92° C.),
Falir
100 parts of water at'32 s , dissolve 3-29 parts of alum.
“ “ 50 “ 0-52
“ “ 86 t‘ 22 00 “■
“ 140 “ 31-00
“ “ 158 “ 90 00
“ 112 0 357-00 O
At a white heat all tlie sulphuric acid and the waterof crystallization pass off as sulphurous acid, oxygen,and steam, leaving a mixture of alumina and potas-sium sulphate. At a still higher temperature alumi-nate of potassium is obtained. Potassium alum, on
being heated to redness with sugar out of contactwith air, is converted into alumina, carbon, andpotassium sulphide, in a fine state of division, andconstitutes the pyropliorus of IIombeeg, whichignites spontaneously on exposure to air.
Alum, when heated with alkaline chlorides, liber-ates hydrochloric acid; again, if a concentratedsolution of this salt is boiled with sodium or potas-sium chlorides, the same acid is given off, and asparingly soluble basic alum precipitated. Richterstates that a solution containing alum, chloride ofsodium, and nitrate of soda, is a solvent for gold.
When alum is dissolved in twenty parts of water,and ammonia dropped slowly into the solution tillthe liquid is nearly saturated, a bulky white precipi-tate appears, which, when thoroughly washed withdistilled water, is pure alumina. If dried andweighed, it will be found to be 10-82 per cent, ofthe weight of the alum taken. If this precipitatewhile moist be dissolved in dilute sulphuric acid, itconstitutes, when as neutral as possible, the normalaluminium sulphate of alumina (Al 0 (SO 4 ) 3 ), whichrequires only 2 parts of cold water for solution.If this solution be now decomposed by pouring intoit liquid ammonia, there appears an insoluble whitepowder, the tribasic sulphate or basic alum. It con-tains three times as much earth as the neutral sul-phate. However, by adding a strong solution ofpotassium sulphate to that of the neutral sulphate,a white powder will fall, which is true alum. Whenrecently-precipitated alumina is boiled in a solutionof alum, a portion of the earth enters into combina-tion with the salt, constituting the insoluble com-pound which falls as a white amorphous powder:the same combination occurs if a boiling solution ofalum be decomposed by potassium hydrate.
Ammonium alum, ammonio - aluminic sulphate(A1(NII 4 )S0 4 ,12H 9 0) behaves in all respects asdoes potassium alum, except that it is more solublein water, 100 parts dissolving 5‘22 parts of ammo-nium alum at 32° Fahr. (0° C.), and 421-9 parts at212° Fahr. (100° C.). When heated to redness itloses both its sulphuric acid and ammonia, and be-comes alumina. Addition of ammonia to a boilingsolution of ammonium alum gives rise to a basicalum as in the case of potassium alum,
Argento-aluminic sulphate, silver alum, lias beenprepared by Ciiurch by heating silver and aluminiumsulphates, together with water in an oil bath. Thissalt crystallizes in octahedra, and is decomposed bywater. Its fonnula is AlAg(S0 4 ) 2 ,12II 2 0,
Sodio -aluminic sulphate, AlNa(S0 4 ) 2 12II 2 0, sod-ium alum, crystallizes in large octahedras, but withgreat difficulty, Rut for this cause it would belargely used on account of its great solubility. Itis much mqre soluble than potassium alum; 10parts of water dissolve 11 parts of this alum atordinary temperature. In all other characteristicsit resembles potassium alum.
The relative solubilities of the three common alumsat 60° Fahr. (15°-5 C.) in 100 parts of water are—am-monium alum, 9-37; potassium alum, 14-79; sodiumalum, 327-6.