Q. What is Peroxodisulphuric Acid?

It is also known as perdisulphuric acid or simply persulphuric acid. The acid can be obtained  by the action of chlorosulphonic acid on H₂0₂:

H₂O₂+ 2ClSO₂ OH -----------------> H₂S₂O₈ + 2HCl

This acid is obtained by the electrolysis of 50% sulphuric acid in cold using high current density and a platinum anode. The following reactions are supposed to take place during the process:

2HSO^-₄--------------> H₂S₂O₈ + 2e at anode

2H₊ + 2e-----------> H₂

The peroxodisulphuric acid is a colourless solid, m. p. 338 K. The acid is highly soluble in water. In aqueous solution it slowly changes to peroxomonosulphuric acid.

H₂S₂O₈ + H₂O ---------------------> H₂SO₅ + H₂O₂

Potassium and ammonium peroxodisulphate are the most important salts of this acid. These are freely soluble in water. These salts are, in fact, easier to prepare than the acid and both are made on an industrial scale by anodic oxidation of the corresponding sulphates under controlled conditions.

Peroxodisulphuric acid and its salts are powerful oxidising agents. They liberate iodine from Kl slowly. This distinguishes H₂S₂O₈ from H₂ SO₅ which liberates iodine immediately.

2KI + K₂S₂O₈ ----------------------> 2K₂SO₂ + I₂

Both fie acids are distinguished from H₂, 02by their failure to react with KMnO₄,.

The peroxodisulphates also oxidise ferrous salts to ferric, manganous salts to permanganates and chromic salts to dichromate in presence of a trace of silver nitrate:

2MnSO₄+ 5K₂S₂O₈ + 8H₂O -------> 2KMnO₄+ 4K₂SO₄ + 8H₂SO₄

Cr₂ (SO₄) ₃+ K₂S₂O₈ + 7H₂O------> K₂Cr₂O₇ + 7H₂SO₄

Some metals, e.g., copper and zinc dissolve in an aqueous solution of persulphates giving metal sulphates:

Cu + K₂₂ O₈---------> CuSo₄ + K₂SO₄

Zn + K₂S₂O₈---------> ZnSO₄+ K₂SO₄