Aluminium sulphate is an acid salt that is manufactured through the reaction of aluminium hydroxide, water and sulphuric acid. The combination of an acid (H2SO4) and a base (Al(OH)3 forms a salt (Al2(SO4)3 that plays an important role in industry where it is used as a flocculant, precipitating solids from water to clarify it. Other uses are in antiperspirants and to alter the pH (acidity) of soil. The salt forms as a white crystalline product from the reaction, and is ground to a fine powder and sold under the name of alum.
- Skill level:
- Moderately Challenging
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Things you need
- Aluminium Hydroxide (Al(OH)3
- Sulphuric Acid (H2SO4) 20 per cent
- Litmus paper
- Glass measuring container
- Glass mixing container
- Safety glasses
- Lab coat
- Glass stirring stick
Wear a lab coat and a pair of safety goggles.
Prepare a clear bench to carry out the experiment and allow for ventilation by opening a door or window.
Measure out 100 millilitres of aluminium hydroxide powder and place into a glass container. Slowly add 10 per cent sulphuric acid solution to the aluminium hydroxide while stirring. The temperature of the mixture will start to climb. Therefore take this step slowly and allow the mixture to cool before adding more acid.
Dip a strip of litmus paper into the mixture after stirring each addition of sulphuric acid.
If the litmus paper turns blue, continue to add acid. If it has turned red then add a small amount of aluminium hydroxide to the mixture until the litmus paper has reached a neutral colour - the colour of litmus paper before dipping.
Allow the mixture to sit and cool, and the aluminium sulphate should begin to precipitate out; this may take a few days. Without stirring up the precipitate, pour off the liquid leaving behind the white crystalline aluminium sulphate.
Pour the aluminium sulphate on to some paper towels and spread it out to speed up the drying process. Place the dried aluminium sulphate into a glass container and label it.
Tips and warnings
- This experiment should be carried out under the supervision of someone with experience in handling sulphuric acid, such as a lab instructor.
- Sulphuric acid can cause severe burns if it comes in contact with the skin, and can burn holes into clothing.
- Flush eyes or skin with water if any acid makes contact.
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