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In 1894 Wolffenstein obtained practically pure hydrogen peroxide by the fractional distillation of a concentrated aqueous solution under reduced pressure. He found that, under special conditions, hydrogen peroxide is fairly stable towards heat, viz., when it is free from (a) all alkaline substances, (b) every trace of heavy metal compounds, (c) all kinds of solid bodies, even of otherwise indifferent chemical character, e.g., silica, alumina, etc. The sodium sulphate in Merck's method of preparation is indifferent towards hydrogen peroxide. By evaporating a 4.5 per cent, solution of the peroxide in a porcelain dish on a water-bath at 75°, it is concentrated to 66.6 per cent. Some peroxide was lost, not by decomposition, but by evaporation, since it is volatile. This solution was shaken with ether to precipitate alumina, and the ether evaporated from the filtered liquid on a water-bath. The strong hydrogen peroxide was then distilled at a pressure of 65 mm.; the fraction coming over between 81° and 85° contained 90.5 per cent, of H2O2. This was again fractionated under reduced pressure, and the fraction between 84° and 85° contained 99.1 per cent, of H2O2, and was free from all impurities.
The apparatus used for distillation under reduced pressure consists
 | Fig: Distillation under reduced pressure |
of a distilling flask, containing the solution of hydrogen peroxide, fitted with a thermometer, and placed on a water-bath. The side tube is fitted by a rubber stopper to the inside of a second distilling flask, which serves as a receiver, and is cooled by a stream of cold water. The side tube of this flask communicates by pressure tubing with a large empty bottle, which is connected with a good water pump. A pressure gauge is connected with this bottle, and a three-way stopcock allows air to be admitted to the apparatus when the experiment is finished so that the different parts may be disconnected, or when the receiver is changed during the operation. In the experiment there is some danger of explosion, when the apparatus is shattered. This appears to be due to the formation of unstable ethyl hydroperoxide, C2H5HO2, discovered by Brodie, which is produced from the ether remaining in the peroxide. It is safer to begin the experiment directly with Merck's 30 per cent, perhydrol, which has not been treated with ether.
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