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Potassium alum

Potassium aluminium sulfate
Other names potassium alum, potash alum
CAS number 7784-24-9
Molecular formula KAl(SO4)2
Molar mass 258.207 g/mol
Density 1.76 g/cm3
Melting point

92-93 °C

Boiling point

200 °C

Solubility in other solvents 36.80 grams of alum/100 grams of water at 50 °C
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Potassium alum or potash alum is the potassium double sulfate of aluminium. Its chemical formula is KAl(SO4)2 and it is commonly found in its dodecahydrate form as KAl(SO4)2·12(H2O). It is commonly used in water purification, leather tanning, fireproof textiles, and baking powder. It also has cosmetic uses as a deodorant and as an aftershave treatment. Its mineral form is known as kalunite (rock form) and kalinite (fiber form).


Potassium alum crystallizes in regular octahedra with flattened corners, 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. Alum finds application as a mordant, in the preparation of lakes for sizing hand-made paper and in the clarifying of turbid liquids.

Mineral form and occurrence

Potassium alum is a naturally occurring sulfate mineral which typically occurs as encrustations on rocks in areas of weathering and oxidation of sulfide minerals and potassium-bearing minerals. Alunite is an associate and likely potassium and aluminium source. Found at Vesuvius, Italy,East of Springsure,Queensland, Alum Cave, Tennessee, and Alum Gulch, Arizona in the United States.


    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Potassium_alum". A list of authors is available in Wikipedia.
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