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What Is Alum and How Is It Used?
From deodorant to cooking, this mineral is commonly used
everyday
By Anne Marie Helmenstine, Ph.D.
Usually, when you hear about alum it is in reference to potassium alum,
which is the hydrated form of potassium aluminum sulfate and has the chemical
formula KAl(SO4)2·12H2O.
However, any of the compounds with the empirical formula AB(SO4)2·12H2O are
considered to be an alum.
Sometimes alum is seen in its crystalline form, although it is
most often sold as a powder.
Potassium alum is a fine white powder that you can find sold
with kitchen spices or pickling ingredients.
It is also sold as a large crystal as a "deodorant
rock" for underarm use.
Types of Alum
o Potassium
Alum: Potassium alum is also known as potash alum or tawas.
It is aluminum potassium sulfate. This is the type of alum that
you find in the grocery store for pickling and in baking powder.
It is also used in leather tanning, as a flocculant in water
purification, as an ingredient in aftershave and as a treatment to fireproof
textiles. Its chemical formula is
KAl(SO4)2.
o Soda
Alum: Soda alum has the formula NaAl(S O4)2·12H2O. It
is used in baking powder and as an acidulant in food.
o Ammonium
Alum: Ammonium alum has the formula N4Al(SO4)2·12H2O.
Ammonium alum is used for many of the same purposes as potassium
alum and soda alum.
Ammonium alum finds applications in tanning, dyeing textiles,
making textiles flame retardant,
in the manufacture of porcelain cement and vegetable glues, in water
purification and in some deodorants.
o Chrome
Alum: Chrome alum or chromium alum has the formula KCr(S O4)2·12H2O.
This deep violet compound is used in tanning and can be added to
other alum to grow lavender or purple crystals.
o Selenate
Alums: Selenate alums occur when selenium takes the place of
sulfur so that instead of a sulfate you get a selenate, (SeO42-).
The selenium-containing alums are strong oxidizing agents, so
they can be used as antiseptics, among other uses.
o Aluminum
Sulfate: This compound is also known as papermaker's alum. However, it is
not technically an alum.
Uses of Alum
Alum has several household and industrial uses.
Potassium alum is used most often, although ammonium alum,
ferric alum, and soda alum may be used for many of the same purposes.
· purification of drinking water as a chemical flocculant
· in styptic pencil to stop bleeding from minor cuts
· the adjuvant in vaccines (
a chemical that enhances the immune response)
· deodorant "rock"
· pickling agent to help keep pickles crisp
· flame retardant
· the acidic component of some types of baking powder
· an ingredient in some homemade and commercial modeling clay
· an ingredient in some depilatory (hair removal) waxes
· skin whitener
· ingredient in some brands of toothpaste
Alum Projects
There are several interesting science projects that use alum. In
particular, it is used to grow stunning
non-toxic crystals.
Clear crystals result from potassium alum, while purple crystals grow
from chrome alum.
Alum Sources and Production
Several minerals are used as the source material to produce
alum, including alum schist, alunite, bauxite, and cryolite.
The specific process used to obtain the alum depends on the
original mineral.
When alum is obtained from alunite, the alunite is calcined.
The resulting material is kept moist and exposed to air until it
turns to a powder, which is lixiviated with sulfuric acid and
hot water. The liquid is decanted and the alum
crystallizes out of solution.
Anne
Marie Helmenstine, Ph.D.
Chemistry
Expert
Education
Ph.D.,
Biomedical Sciences, University of Tennessee at Knoxville
B.A.,
Physics and Mathematics, Hastings College
Introduction
Ph.D.
in biomedical sciences from the University of Tennessee at Knoxville - Oak
Ridge National Laboratory.
Science
educator with experience teaching chemistry, biology, astronomy, and
physics at the high school, college, and graduate levels.
ThoughtCo
and About Education chemistry expert since 2001.
Widely-published
graphic artist, responsible for printable periodic tables and other
illustrations used in science.
Experience
Anne
Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education
since 2001, and other sciences since 2013. She taught chemistry, biology,
astronomy, and physics at the high school, college, and graduate levels.
She has worked as a research scientist and also abstracting and indexing
diverse scientific literature for the Department of Energy.
In
addition to her work as a science writer, Dr. Helmenstine currently serves as a
scientific consultant, specializing in problems requiring an interdisciplinary
approach. Previously, she worked as a research scientist and college
professor.
Education
Dr.
Helmenstine holds a Ph.D. in biomedical sciences from the University of
Tennessee at Knoxville and a B.A. in physics and mathematics with a minor
in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine
developed ultra-sensitive chemical detection and medical diagnostic tests.
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