Hydrogen Peroxide : Hydrogen Peroxide

Barium monoxide or baryta, BaO, can absorb oxygen, forming a higher oxide, BaO₂, called barium peroxide, which is produced: (a) by passing a stream of oxygen over baryta heated to dull redness: 2BaO + O₂ <=> 2BaO₂; (b) by adding baryta to fused potassium chlorate, and washing out the soluble potassium chloride from the residue with water: 3BaO + KClO₃ = 3BaO₂ + KCl.

If barium peroxide is added to cold dilute hydrochloric acid, no oxygen is evolved; the solution contains a new substance, hydrogen peroxide: BaO₂ + 2HCl = BaCl₂ + H₂O₂, which Thenard, its discoverer ( 1818), called oxygenated water.

It is an oxidising agent, liberating iodine from a neutral or acid solution of potassium iodide: 2KI + H₂O₂ = 2KOH + I₂. From the amount of iodine liberated, the proportion of hydrogen peroxide may be calculated.

In order to obtain hydrogen peroxide free from soluble salts, barium peroxide may be treated with an acid, such as sulphuric, carbonic, phosphoric or hydrofluosilicic (H₂SiF₆), which forms an insoluble barium salt. The filtrate is an aqueous solution ofhydrogen peroxide.

Expt. 1. - Stir barium peroxide with distilled water and pass a rapid stream of carbon dioxide through the suspension. After a few minutes add a solution of potassium iodide and starch: a blue colour is produced. According to Merck, the reaction should be carried out as described, not by adding the barium peroxide in small quantities at a time, when the particles become coated with insoluble barium carbonate. If excess of barium peroxide is used at once the decomposition is complete. An unstable barium percarbonate, BaCO₄, is first produced, which is then decomposed by water, producing barium carbonate and hydrogen peroxide: BaO₂ + CO₂ = BaCO₄; BaCO₄ + H₂O = BaCO₃ + H₂O₂.

Anhydrous barium peroxide is not easily decomposed by dilute sulphuric acid, on account of the formation of a coating of insoluble sulphate on the particles of peroxide. A hydrated barium peroxide, BaO₂,8H₂O, is readily decomposed by this acid, and is prepared as follows. Commercial barium peroxide is finely powdered and added a little at a time to a cold mixture of equal volumes of water and concentrated hydrochloric acid until the latter is neutralised. A little baryta solution is then added, which precipitates iron and aluminium impurities as hydroxides. These, together with the silica originally contained in the barium peroxide, are filtered off, and the nitrate is added to a saturated solution of barium hydroxide. A white, crystalline precipitate of hydrated barium peroxide is formed, which is filtered off, washed with cold water free from carbon dioxide, and kept moist in a stoppered bottle:

BaO₂ + 2HCl = BaCl₂ + H₂O₂;

H₂O₂ + Ba(OH)₂ + 6H₂O = BaO₂,8H₂O.

If this hydrated peroxide is treated with cold dilute sulphuric acid (1 vol. of acid: 5 vols. of H₂O), or with hydrofluosilicic acid, insoluble barium salts and a solution of hydrogen peroxide are produced:

BaO₂ + H₂SO₄ = BaSO₄ + H₂O₂; BaO₂ + H₂SiF₆ = BaSiF₆ + H₂O₂.

Barium peroxide also hydrates slowly when stirred with water and can then be decomposed by adding phosphoric acid, followed by sulphuric acid. This method is used on the large scale.

Metallic sodium burns in oxygen with a yellow flame, and a yellow mass of sodium peroxide, Na₂O₂, is left.

Expt. 2. - Burn a small piece of sodium in a deflagrating spoon in a jar of dry oxygen. When the spoon is cold, dissolve the sodium peroxide in it by placing the spoon in water. Add dilute HCl, and a solution of KI and starch. A blue colour is produced: Na₂O₂ + 2HCl = 2NaCl+H₂O₂.

Sodium peroxide is manufactured by heating sodium in a current of dry air, purified from carbon dioxide. The calculated amount is added in small quantities at a time to 20 per cent, sulphuric acid cooled in ice: Na₂O₂ + H₂SO₄ = Na₂SO₄ + H₂O₂. Two-thirds of the sodium sulphate separate as crystals of Glauber's salt, Na₂SO₄,10H₂O, and the liquid is then decanted and distilled in vacua. Hydrogen peroxide is less volatile than water, so that the later fractions are collected. Hydrogen peroxide is also manufactured by electrolysis.

More dilute solutions of hydrogen peroxide are prepared (usually from barium peroxide) for use in pharmacy. The strength of these solutions is stated in terms of the volume of oxygen evolved on heating, when the peroxide decomposes: 2H₂O₂ = 2H₂O + O₂. Commercial peroxide is usually "10 volumes," or "20 volumes," according as it gives off 10, or 20, times its volume of oxygen. Merck's 30 per cent. preparation evolves 100 volumes of oxygen. From the equation it is seen that 2 x 34 gm. of hydrogen peroxide evolve 32 gm. of oxygen, occupying 22.4 litres at S.T.P. Thus each gram of peroxide evolves 329.4 c.c. of O₂. A 1 per cent, solution evolves 3.294 times its volume of oxygen; "10 vol." peroxide is 3.04 per cent, strength.

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Hydrogen Peroxide
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