Chemical Bonding and Molecular Structure

Aluminium Sulfate

Aluminium Sulfate is a chemical compound with the chemical formula \(Al_2(SO_4)_3\). Aluminium Sulfate is also known as Filter Alum or Dialuminium trisulphate. In its anhydrous form, Aluminium Sulfate is a white crystalline solid. Aluminium Sulfate in its solution form appears like a colourless liquid. In both the forms of Aluminium Sulfate, it is non-toxic and non-combustible.

Aluminium Sulfate

Introduction to Aluminium Sulfate

Aluminium Sulfate is soluble in water but insoluble in ethanol. Aluminium Sulfate is odourless and has a mildly astringent taste with a sweet taste. On decomposing, Aluminium Sulfate emits toxic fumes of sulphur oxides. The solution of it is corrosive to aluminium. Aluminium Sulfate is produced in the laboratory by adding aluminium hydroxide with sulphuric acid.

Aluminium Sulfate is in use as a coagulating agent. It promotes particle collision by neutralizing charge in the purification of drinking water and wastewater treatment plants.

The anhydrous form of Aluminium Sulfate occurs naturally as a rare mineral millosevichite. For instance, in volcanic environments and on burning coal-mining waste dumps.

Aluminium Sulfate is also called alum or papermaker’s alum in some industries. Basically, the name “alum” is properly in use for any double sulphate salt. It has a generic formula \(XAl(SO_4)_2·12H_2O,\) where X is a monovalent cation like potassium or ammonium.

Properties of Aluminium Sulfate

  • The molecular mass or molar mass of Aluminium Sulfate is 342.15 gram per mole.
  • The density of Aluminium Sulfate is 2.672 gram per centimetre cube.
  • While the boiling point and the melting point of Aluminium Sulfate are \(214^oF\) and \(770^oC\) respectively.

Production of Aluminium Sulfate

In the laboratory

Aluminium Sulfate is made by adding aluminium hydroxide i.e., \(Al(OH)_3\), to sulfuric acid i.e., \(H_2SO_4\).

\(2 Al(OH)_3 + 3 H_2SO_4 \rightarrow Al_2(SO_4)_3 + 6 H_2O\)

or it is done by heating aluminium metal in a sulfuric acid solution.

\(2 Al + 3 H_2SO_4 \rightarrow Al_2(SO_4)_3 + 3 H_2\uparrow\)

From alum schists

The alum is a mixture of iron pyrite, aluminium silicate and various bituminous substances. These are found in upper Bavaria, Bohemia, Belgium, and Scotland. These are roasted or exposed to the action of the air. In the roasting process, sulfuric acid is formed. It acts on the clay to form Aluminium Sulfate a similar condition of affairs being produced during weathering action of the air. Later, extracted with water and a solution of Aluminium Sulfate of specific gravity 1.16. This solution is allowed to stand for some time and is then evaporated until iron (II) sulphate crystallizes on cooling. It is then drawn off and evaporated until it attains a specific gravity of 1.40 and decanted from any sediment.

From clays or bauxite

From clays or bauxite, the material is calcined and then mixed with sulfuric acid and water. Then it is heated gradually to boiling. If concentrated acid is in use then no external heat is required as the formation of Aluminium Sulfate is exothermic. Later, it is allowed to stand for some time and then the clear solution is drawn off.

From cryolite

When cryolite is in use as ore, it is mixed with calcium carbonate and heated and sodium aluminate is formed. Sodium aluminate is then extracted with water and precipitated by sodium bicarbonate or by passing a current of carbon dioxide through the solution. The precipitate is then dissolved in sulfuric acid.

Uses of Aluminium Sulfate

  • In baking soda.
  • For gardening to balance the soil PH.
  • In the purification of water.
  • In the dyeing of cloth.
  • For the printing on cloth.
  • In making paper.
  • In concrete as an accelerator and waterproofing agent.
  • As the fireproofing agent.
  • In firefighting foam.
  • In use in sewage treatment.

FAQs on Aluminium Sulfate

Question 1: What type of compound is Aluminium Sulfate?

Answer: Aluminium Sulfate is an ionic compound. Aluminium Sulfate is a combination of positive and negative ions. Ions are charged atoms, that either be monatomic ions i.e., single atoms or polyatomic i.e., multiple atoms combined to form a charged part. Aluminium forms a + 3 ion i.e., \(Al^{+3}\), and sulphate is the -2 polyatomic ion, \((SO_4)^{-2}\).

Question 2: How dangerous is Aluminium Sulfate?

Answer: Aluminium Sulfate is an irritant to the skin and eyes. One should wear gloves and eye protection while dealing with Aluminium Sulfate. Aluminium Sulfate is mildly dangerous if it is swallowed in any way because when the salt is swallowed and can form extremely corrosive sulphuric acid.

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