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Preparation of Hydrogen Peroxide

Hydrogen peroxide is largely manufactured by the electrolysis of 50% Sulphuric acid at

0^{\circ} \textrm{} C between the platinum anode and lead cathode separated by a porous diaphragm. 50% Sulphuric acid solution is taken in a stoneware vessel. A platinum rod is dipped in the solution which acts as an anode. A lead wire acts as the cathode. These two electrodes are separated by a porous diaphragm.

Laboratory Methods of Hydrogen Peroxide Preparation

When barium peroxide is acidified and the excess water is removed by the process of evaporation under reduced pressure, we obtain hydrogen peroxide. The following reaction will clarify this:

BaO2.8H2O(s) + H2SO4(aq) → BaSO4(s) + H2O2(aq) + 8H2O(l)

Industrial Method of Hydrogen Peroxide Preparation

Hydrogen peroxide is prepared by the electrolysis of 30% ice-cold H2SO4. When acidified sulfate solution is electrolyzed at a high current density, peroxodisulphate is obtained. Peroxodisulphate is then hydrolyzed to get hydrogen peroxide.

2HSO–4(aq) [Electrolysis] → HO3SOOSO3H(aq) [Hydrolysis] → 2HSO–4(aq)+2H+(aq)+H2O2(aq)

Reaction Mechanism

  • Electrolyte: 30% dilute H2SO4
  • Cathode: Pb wire
  • Anode: Pt rod

2H2SO4 → 2H+ + 2HSO–4

At Cathode: 2H+ + 2e– → H2

At Anode: 

2HSO–4 → H2S2O8 + 2e– ⇒ Peroxodi Sulphuric Acid [Marshall’s acid]

H2S2O8 + H2O → H2SO5 + H2SO4 ⇒ Peroxomono Sulphuric Acid [Caro’s acid]

H2SO5 + H2O → H2SO4 + H2O2

Properties of Hydrogen Peroxide

Physical Properties:

  • In its pure state, hydrogen peroxide is an almost colorless (very pale blue) liquid.
  • It melts at 272.4 K and has a boiling point of 423 K (extrapolated).
  • It is miscible in water in all proportions and forms hydrates.

Chemical Properties:

Hydrogen peroxide in both acidic and basic mediums acts as an oxidizing as well as a reducing agent. The following reactions will give a clear picture:

Oxidation Reactions of H2O2

  • Oxidizes black Pbs to white PbSO4. Reaction: Pbs + 4H2O2 → PbSO4 + 4H2O
  • Oxidises KI to Iodine. Reaction: 2KI + H2O2 → 2KOH + I2
  • Oxidizes nitrites to nitrates. Reaction: NaNO2 + H2O2 → NaNO3 + H2O
  • Oxidizes acidified Potassium ferrocyanide. Reaction: 2K4Fe(CN)6 + H2SO4 + H2O2 → 2K3Fe(CN)6 + K2SO4 + 2H2O