Inorganic Chemicals

Formula: Al2(SO4)3; MW 342.14

Occurrence and Uses
It occurs in nature in minerals; alunite, KAl3(SO4)2(OH)6 and natroalunite, NaAl3(SO4)2(OH)6. The anhydrous salt is used in food applications.

Physical Properties
White powder; refractive index 1.47; density 2.71 g/cm3; mp 770°C (decomposes); hygroscopic; readily soluble in water (31% at 0°C; solubility increases with temperature 98% in boiling water); soluble in dilute mineral acids; slightly soluble in alcohol.

Preparation
The anhydrous salt may be obtained by slow and progressive heating of commercial hydrated salt, Al2(SO4)3 •18H2O. Most water molecules are lost at heating between 250 to 420°C. The last three water molecules are lost between 250 to 420°C at a heating rate of 10°C/min.

Chemical Reactions
The compound decomposes to γ−alumina and SO3 when heated between 580 and 900°C. It combines with water forming hydrated salts of various compositions. Produces calcium aluminate with evolution of SO3 when calcined with CaCO4, (Bayliss, N. S. 1945. J and Proc. Austral. Chem. Inst., 12, 127)

Al2(SO4)3 + CaSO4 ——› Ca(AlO2)2 + 4SO3

Reacts with NaHCO3 in aqueous solution, forming fire-extinguishing foams, producing CO2, (Albert K. 1937. French Pat. 820,492, November 12, 1937)

Al2(SO4)3 + 6NaHCO3 ——› 2 Al(OH)3 + 3Na2 SO4 + 6CO2

Reaction with ammonium phosphate yields AlPO4 (see Aluminum phosphate, preparation)

Analysis
Elemental analysis: Al 15.77%; O 56.12%; S 28.11%. Al may be determined by colorimetric method or by atomic absorption or emission spectrophotometry; sulfate may be determined by BaCl2 precipitation method in the aqueous solution of the salt.

Formula: Al2(SO4)3 18H2O; MW 648.41

Synonyms: alum, cake alum (the term alum also refers to aqueous solutions of this substance, as well as other hydrate salts containing varying number of waters of crystallization; also the term alum applies to a whole class of sulfate double salts, such as potassium aluminum sulfate or ammonium aluminum sulfate.)

Uses
The compound is used heavily in the paper industry. In its acidic solution form, which contains a slight excess of H2SO4, it is used for pH control of pulp slurries, treatment of process waters, setting of dyes and also for precipitating dissolved resin for setting the size on the fibers. In slight basic form (containing a slight excess of Al2O3), it is used for treatment of drinking and waste waters (e.g., for reducing phosphorus content). Other major applications include dyeing, tanning, catalysts, modification of concrete, and in the manufacture of various chemicals and pharmaceutical products.

Physical Properties
White crystal; sweet taste; density 1.62 g/cm3: decomposes at 86.5°C; soluble in water.

Preparation
Prepared from bauxite, kaolin or aluminum compounds on reaction with H2SO4. The insoluble silicic acid is filtered out; the hydrate salt forms on crystallization.