(i) Laboratory method : In laboratory, H₂O₂ is prepared by Merck’s process. It is prepared by adding calculated amounts of sodium peroxide to ice cold dilute (20%) solution of H₂SO₄.

Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

(ii) By the action of sulphuric acid or phosphoric acid on hydrated barium peroxide BaO2.8H2O

(a) BaO₂.8H₂O + H₂SO₄ → BaSO₄ ↓+ H₂O₂ + 8H₂O

It must be noted that anhydrous barium peroxide does not react readily with sulphuric acid (because a coating of insoluble barium sulphate is formed on its surface which stops further action of the acid). Therefore, hydrated barium peroxide, BaO₂.8H₂O must be used.

(b) 3BaO₂ + 2H₃PO₄ → Ba₃(PO₄)₂ + 3H₂O₂

Ba₃(PO₄)₂ + 3H₂SO₄ → 3BaSO₄ + 2H₃PO₄

Phosphoric acid is preferred to H₂SO₄ because soluble impurities like barium persulphate (from BaO₂.8H₂O + H₂SO₄) tends to decompose H₂O₂ while H₃PO₄ acts as preservative (negative catalyst) for H₂O₂.

(iii) Industrial method: On a commercial scale, H₂O₂ can be prepared by the electrolysis of 50% H₂SO₄solution. In a cell, peroxy disulphuric acid is formed at the anode.

2H₂SO₄ ——————→ H₂S₂O8 (aq.) + H₂(g)

Electrolysis Peroxy disulphuric acid

This is drawn off from the cell and hydrolysed with water to give H₂O₂.

H₂S₂O₈ + 2H₂O ——→ 2H₂SO₄ + H₂O₂ The resulting solution is distilled under reduced pressure when H₂O₂ gets distilled while H₂SO₄ with high boiling point, remains undistilled.

(iv) By redox process: Industrially H₂O₂ is prepared by the auto-oxidation of 2-alkylanthraquinols. The process involves a cycle of reactions. The net reaction is the catalytic union of and to give

The H₂O₂ formed (about 1%) is extracted with water and concentrated.

(i) Laboratory method : In laboratory, H₂O₂ is prepared by Merck’s process. It is prepared by adding calculated amounts of sodium peroxide to ice cold dilute (20%) solution of H₂SO₄.

Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂

(ii) By the action of sulphuric acid or phosphoric acid on hydrated barium peroxide BaO2.8H2O

(a) BaO₂.8H₂O + H₂SO₄ → BaSO₄ ↓+ H₂O₂ + 8H₂O

It must be noted that anhydrous barium peroxide does not react readily with sulphuric acid (because a coating of insoluble barium sulphate is formed on its surface which stops further action of the acid). Therefore, hydrated barium peroxide, BaO₂.8H₂O must be used.

(b) 3BaO₂ + 2H₃PO₄ → Ba₃(PO₄)₂ + 3H₂O₂

Ba₃(PO₄)₂ + 3H₂SO₄ → 3BaSO₄ + 2H₃PO₄

Phosphoric acid is preferred to H₂SO₄ because soluble impurities like barium persulphate (from BaO₂.8H₂O + H₂SO₄) tends to decompose H₂O₂ while H₃PO₄ acts as preservative (negative catalyst) for H₂O₂.

(iii) Industrial method: On a commercial scale, H₂O₂ can be prepared by the electrolysis of 50% H₂SO₄solution. In a cell, peroxy disulphuric acid is formed at the anode.

2H₂SO₄ ——————→ H₂S₂O8 (aq.) + H₂(g)

Electrolysis Peroxy disulphuric acid

This is drawn off from the cell and hydrolysed with water to give H₂O₂.

H₂S₂O₈ + 2H₂O ——→ 2H₂SO₄ + H₂O₂ The resulting solution is distilled under reduced pressure when H₂O₂ gets distilled while H₂SO₄ with high boiling point, remains undistilled.

(iv) By redox process: Industrially H₂O₂ is prepared by the auto-oxidation of 2-alkylanthraquinols. The process involves a cycle of reactions. The net reaction is the catalytic union of and to give

The H₂O₂ formed (about 1%) is extracted with water and concentrated.