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Aluminum Sulfate: Uses & Toxicity

Instructor: Sarah Pierce

Sarah has taught high school chemistry and biology, as well as college level chemistry(general, organic, analytical, biochemistry), and has a doctorate in chemistry.

The uses of aluminum sulfate, or alum as it is commonly known, are discussed in this lesson. Alum is used to dye fibers, tan leather, purify water, and in firefighting foam. The toxicity of aluminum sulfate is also covered.

Winter Wear

Winter is approaching, so you go to the closet and pull out your favorite hat and scarf knitted by your grandmother. You look at the yarn and marvel at the bright colors. Did you know that a chemical salt called aluminum sulfate helped produce the vibrant colors on the yarn? Like all chemicals, it can be very useful, but also toxic in high doses. Let's find out more about the uses and toxicity of aluminum sulfate.

Aluminum sulfate is used to produce vibrant colors when dying yarn
colorful yarn dyed using alum

Uses of Aluminum Sulfate

Aluminum sulfate is a white crystal that is sometimes called alum, which is a broad term to describe aluminum salts. It has many uses including dyeing fabric, tanning leather, and water purification. It's even used in the foam that firefighters use! Let's learn a little bit more about each of these applications.

Aluminum sulfate
aluminum sulfate crystals

Mordant

Alum is used in the dying industry to produce bright colors on fabrics. It is called a mordant or a substance that helps dye stick to fibers. To use aluminum sulfate as a mordant, the salt is dissolved in hot water with the dye. Then the fabric is submerged into the dye for a period of time. When the fabric is washed, the color is retained. If a mordant was not used, the color would look very faded.

The fabric strip on the right was dyed using a mordant, while the fabric strip on the left had no mordant
one faded fabric dyed without a mordant and the other fabric dyed with a mordant that is vibrant

Leather Tanning

Aluminum sulfate could also have been utilized in the tanning process used to make the leather in your sofa, gloves, or belt. When hides of animals are processed to make leather, they are treated with chemicals in a process called tanning. During this process, aluminum sulfate is mixed with sodium chloride (common table salt), flour, and another salt called sodium carbonate and rubbed on the hides. The mixture sits on the hides for 24 hours before it is removed. This entire process is repeated several times. The mixture containing aluminum sulfate prevents the hides from decaying.

Aluminum sulfate is used to tan the leather used to make many products such as this couch
leather sofa

Water Purification

Aluminum sulfate is added to water to remove impurities. It makes the impurities clump together and fall to the bottom of the container holding the water in a process called flocculation. The impurities are removed when the water is filtered. This is commonly used in ponds to remove algae and in sewers to remove pollutants. Unfortunately, aluminum sulfate has caused some sewer systems to corrode.

Fire Extinguisher

Did you know that some fires can't be put out with water? Think about an oil fire. If you add water to the oil, the fire just gets worse! So, what do you do? You could use a foam to extinguish the fire. If you did that, you would most likely be using aluminum sulfate! Aluminum sulfate reacts with sodium bicarbonate (better known as baking soda), producing foam with carbon dioxide in it. The carbon dioxide extinguishes the fire.

Aluminum sulfate is used to produce the foam used to extinguish fires
foam spray

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