ALUMINUM SULFATE
Sunday, October 12th, 2008Formula: 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.
