What Is Aluminium Sulfate? Structure, Preparation, and Applications
Properties of Aluminium Sulfate Al₂(SO₄)₃
The chemical formula of Aluminium Sulfate is Al2(SO4)3. It is a chemical compound typically used as a coagulating agent for purification of drinking water, paper manufacturing, and wastewater treatment plants. It is also known as filter alum or dialuminium trisulfate. It is a white crystalline solid in its anhydrous form. However, in its liquid form, it is colourless. Both these forms are non-combustible and non-toxic.
Aluminium Sulfate is water-soluble but insoluble in ethanol. It does not have any odour and has a mildly astringent and sweet taste. When it decomposes it emits highly toxic fumes of Sulphur oxides. Also, the Aluminium Sulfate solution is corrosive to Aluminium. This is produced in the laboratory when Aluminium Hydroxide is added to Sulphuric Acid. Here, we will learn about what is Aluminium Sulfate Al2(SO4)3, the properties of Aluminium Sulfate Al2(SO4)3, its structure and uses.
Aluminium Sulfate Structure Al2(SO4)3 Structure
Now that you have learned what is Aluminium Sulfate, let us take a look at its structure and how it looks. Given below is its structure.
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Aluminium Sulfate Formula
Let us now learn the formula of Aluminium Sulfate and how is this formula derived chemically.
Aluminium Sulfate is known to be an ionic compound, which means that it is a combination of both positive and negative ions. An atom or molecule turns into an ion when it loses or gains electrons. Molecules can be either monatomic, which means that they contain one atom, or polyatomic, which means that they contain more than one atom. Most ionic compounds consist of a one metal atom that is a positive ion, and a radical or negative ion that is a negative ion.
If you consider Aluminium Sulfate, the positive ion in it is Aluminium ion, Al3+. The radical is the sulfate ion, (SO4)2- that has a negative charge.
Since all ionic compounds are electrically neutral, there is a ratio of molecular weight by which the aluminum and sulfate ions react, so their electrical charges are cancelled out. You can determine this ratio by the criss-cross method of deriving a chemical formula. Let us see how this method works.
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Here, the sign on the sulfate ion is reversed. Then the charges are put as subscripts on the opposite ion.
Properties of Aluminium Sulfate Al2(SO4)3
Let us now learn about the properties of Aluminium Sulfate Al2(SO4)3. Take a look at the below table.
Aluminium Sulfate Al2(SO4)3 Uses
Let us now learn about the common uses of Aluminium Sulfate.
Aluminium Sulfate is used in the purification of water.
It is used to prepare baking soda.
It is used for dyeing clothes.
It is used for gardening purposes and helps to balance the soil pH.
Aluminium Sulfate is used in making paper.
It is also used for making different prints on cloth.
It is used in concrete as a waterproofing agent and an accelerator.
It is used in making the firefighting foam.
It is also used in the treatment of sewage in water treatment plants.
Aluminium Sulfate is used as a fireproofing agent.
FAQs on Aluminium Sulfate (Al₂(SO₄)₃): Properties & Uses
1. What is Aluminium Sulfate and what is its correct chemical formula?
Aluminium Sulfate is an inorganic chemical compound, a type of salt widely used in various industrial processes. It is often referred to as 'papermaker's alum' or 'filter alum'. The correct chemical formula for Aluminium Sulfate is Al₂(SO₄)₃. It consists of two aluminium cations (Al³⁺) and three sulfate anions (SO₄²⁻).
2. What are the main physical and chemical properties of Aluminium Sulfate?
Aluminium Sulfate exhibits several distinct properties:
Appearance: It is a white crystalline solid in its anhydrous form and a colourless liquid in solution.
Odour and Taste: It is odourless and has a mildly astringent, slightly sweet taste.
Solubility: It is highly soluble in water but is not soluble in ethanol.
Hygroscopic Nature: It readily absorbs moisture from the air.
Acidity: When dissolved in water, it forms an acidic solution due to hydrolysis.
3. What are the most important uses of Aluminium Sulfate (Al₂(SO₄)₃)?
Aluminium Sulfate has many significant industrial and commercial applications. Its primary uses include:
Water Purification: It acts as a coagulating agent (flocculant) to remove suspended impurities and purify drinking water.
Paper Manufacturing: It is used for sizing paper, which improves its strength and printability.
Textile Industry: It serves as a mordant, helping dyes to adhere firmly to cloth fibres.
Gardening: It is used to lower the pH of highly alkaline soils to make them suitable for acid-loving plants.
4. How is Aluminium Sulfate prepared in a laboratory or industrial setting?
Aluminium Sulfate is typically prepared by reacting aluminium hydroxide or bauxite ore with sulfuric acid. In a laboratory, it is formed by adding a calculated amount of sulfuric acid (H₂SO₄) to freshly precipitated aluminium hydroxide (Al(OH)₃). The resulting solution is then evaporated to obtain crystals. The balanced chemical reaction is:
2Al(OH)₃ + 3H₂SO₄ → Al₂(SO₄)₃ + 6H₂O
5. Why is Aluminium Sulfate so effective for water purification?
Aluminium Sulfate's effectiveness in water purification stems from its ability to act as a coagulant. When added to water, it reacts to form a sticky, gelatinous precipitate of aluminium hydroxide, Al(OH)₃. This sticky substance attracts and traps fine suspended particles, dust, and certain microorganisms. These particles clump together to form larger, heavier masses called 'flocs,' which can be easily removed by sedimentation and filtration, leaving the water clear.
6. What happens when Aluminium Sulfate is dissolved in water, and why is the resulting solution acidic?
When Aluminium Sulfate, a salt of a strong acid (H₂SO₄) and a weak base (Al(OH)₃), is dissolved in water, it undergoes a process called salt hydrolysis. The Al³⁺ ions react with water molecules to form aluminium hydroxide and release hydrogen ions (H⁺). This increase in the concentration of H⁺ ions in the solution causes it to become acidic. This is a key chemical property that explains its use in lowering soil pH.
7. What are the key safety precautions for handling Aluminium Sulfate?
While not highly toxic, Aluminium Sulfate is a skin and eye irritant. Proper safety measures are important:
Wear safety goggles and protective gloves to prevent contact with eyes and skin.
Use it in a well-ventilated area to avoid inhaling the fine powder, which can irritate the respiratory tract.
In case of accidental ingestion, seek medical attention, as it can be harmful to the stomach and intestinal lining by forming sulfuric acid.
