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A dilute solution of hydrogen peroxide may be concentrated in several ways. If it is frozen, ice separates, and the residual liquid is enriched in peroxide. It may be concentrated by evaporation in a dish on a water-bath: hydrogen peroxide is appreciably less volatile than water. At a certain point, however, decomposition begins. The solution may then be placed in a flat dish in an exhausted desiccator containing concentrated sulphuric acid. When a certain concentration of peroxide is reached, the latter begins to volatilise, but by working at low temperatures Thenard was able to obtain a liquid (sp. gr. 1.452) giving off 475 vols. of O2 at 14°, i.e., containing 95 per cent, of H2O2.
These concentrated solutions easily decompose on heating, or even at the ordinary temperature. They are rendered more stable by a trace of acid. Dilute aqueous solutions are stable if acidified.
More concentrated hydrogen peroxide may be obtained by distillation under reduced pressure; this method was also used by Thenard.
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