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Hydrogen Peroxide : Hydrogen Peroxide, Oxidising Reactions



Hydrogen peroxide is an active oxidising agent, the labile oxygen atom being easily split _ off, with formation of water. Arsenious and sulphurous acids are oxidised to arsenic and sulphuric acids: H3AsO3 + H2O2 = H3AsO4 + H2O; H2SO3 + H2O2 = H2SO4 + H2O. Black lead sulphide is oxidised to white lead sulphate: PbS + 4H2O2 = PbSO4 + 4H2O, a reaction utilised in restoring discoloured oil-paintings in which the white-lead pigment (basic lead carbonate) has become converted into black PbS by atmospheric hydrogen sulphide. Ferrous salts in acid solution are converted into ferric salts: 2FeSO4 + H2O2 + H2SO4 = Fe2(SO4)3 + 2H2O.

The oxidising action of hydrogen peroxide is used in bleaching delicate materials (wool, silk, ivory, feathers) which would be injured by chlorine: the solution of the peroxide is made faintly alkaline with ammonia, or added to a 10 per cent, solution of sodium acetate, Hydrogen peroxide bleaches hair to a golden-yellow colour: it is called an auricome when used for this purpose. It is also a powerful antiseptic, and as it leaves no injurious products after its action it is largely used as a gargle, etc.

Platinum black, and especially colloidal platinum (prepared by striking electric arcs between platinum wires under distilled water), bring about a rapid catalytic decomposition of hydrogen peroxide: 2H2O2 = 2H2O + O2

Expt. 4. - Add a little colloidal platinum to a solution of H2O2. There is a brisk evolution of oxygen. Stirring the liquid with a glass rod accelerates the reaction.
Liebermann (1904) considered that the platinum first absorbs atmospheric oxygen, rendering this "active," and the activated oxygen, probably in the atomic condition, then reacts with the labile oxygen atom of the peroxide: = H2O + O2. Finely divided silver, manganese dioxide, and other substances also cause catalytic decomposition.


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