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Hydrogen Peroxide : Hydrogen Peroxide, Reducing Actions |
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In certain reactions hydrogen peroxide appears to function as a reducing agent. Thenard (1819) found that gold and silver oxides are reduced by it to the metals: H2O2 + Ag2O = H2O +O2 2 Ag. Expt. 5. - Add caustic soda solution to a solution of silver nitrate: a brown precipitate of silver oxide is formed: 2AgNO3 + 2NaOH = Ag2O + 2NaNO3 + H2O. Add H2O2 to this: it is at once converted into black metallic silver, with brisk evolution of oxygen. If a further quantity of H2O2 is added, it is catalytically decomposed by the finely divided silver. Brodie (1850) showed that when hydrogen peroxide acts as a reducing agent, the labile oxygen atom withdraws another oxygen atom from the compound reduced, to produce a molecule of gaseous oxygen. It reacts (rather slowly) with ozone:O2  H2O = O2 + O2 + H2O.Hydrogen peroxide is used as an antichlor to remove excess of chlorine from bleached fabrics: Cl2 + H2O2 = 2HCl + O2. A solution of potassium permanganate acidified with sulphuric acid is readily reduced by hydrogen peroxide, with evolution of oxygen: 2KMnO4 + 3H2SO4 + 5H2O2 = K2SO4 + 2MnSO4 + 8H2O + 5O2. Manganese dioxide brings about an evolution of oxygen from a neutral solution of hydrogen peroxide, the action being apparently catalytic, but in acid solution the manganese dioxide is reduced to a manganous salt: MnO2 + H2O2 + H2SO4 = MnSO4 + 2H2O + O2. Solutions of bleaching powder and sodium hypobromite evolve oxygen: NaOBr + H2O2 = NaBr + H2O + O2. Iodine is liberated from acidified potassium iodide: 2KI + H2O2 + H2SO4 = K2SO4 + 2H2O + I2. All these reactions are applied in the estimation of hydrogen peroxide. An interesting case of the oxidising and reducing action of hydrogen peroxide was discovered by Brodie. An acid solution of potassium ferrocyanide is oxidised by hydrogen peroxide to potassium ferricyanide: 2K4FeC6N6 + H2O2 = 2K3FeC6N6 + 2KOH. An alkaline solution of potassium ferricyanide, however, is reduced to potassium ferrocyanide by hydrogen peroxide, with evolution of oxygen: 2K3FeC6N6 + 2KOH + H2O2 = 2K4FeC6N6 + 2H2O + O2. Hydrogen peroxide acts powerfully on a photographic plate. The effects of traces of this substance have often been attributed to "rays." |
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