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    Hydrogen peroxide
      Formation
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Concentration of hydrogen peroxide






In dilute solutions such as are obtained as the first product in most of the methods of preparation, hydrogen peroxide is fairly stable. Concentration of hydrogen peroxide from very dilute solutions can be performed by careful evaporation on a water bath, but it is not usually advantageous to concentrate in this way beyond 50 per cent, because of the increase in the rate of decomposition. Another method of concentrating a dilute solution is to submit it to partial freezing, when the mother liquors are enriched in hydrogen peroxide; by repeating the treatment with the mother liquors several times it is possible to attain a concentration of 30 per cent, with little trouble. The low temperature involved in this method introduces the advantage of reducing the tendency of the solution to undergo spontaneous decomposition. For the preparation of pure hydrogen peroxide, fractional distillation is the most convenient process, the distillation being effected under reduced pressure in order that the temperature may be as low as possible; water, being the more volatile, distils first. The danger of explosive decomposition during distillation appears to be much greater with solutions which have already been concentrated by extraction with ether.

Exceptionally pure hydrogen peroxide was obtained by Maass and Hatcher from a 3 per cent, solution prepared in the usual way from commercial barium hydroxide. This was first concentrated to 30 per cent., using a special type of sulphuric acid concentrator, but this product required very careful handling as all the non-volatile impurities had also been concentrated in the process. The liquid was now distilled at 65° C. under a pressure of 10 mm. of mercury maintained with a sulphuric acid pump. Qualitative analysis of the distillate showed the absence of all non-volatile impurities; sulphuric acid was also absent, but in those cases where the original peroxide solution contained large amounts of chloride, some hydrochloric acid was present in the distillate unless the original solution was first made alkaline. The distillate was now exceedingly pure and contained on the average 85 per cent, of peroxide, the remainder being water. This was concentrated to 90 per cent, peroxide over sulphuric acid in the special concentrator referred to above. It now remained to remove, in so far as possible, the remaining 10 per cent, of water. This was effected by systematic fractional crystallisation, a product containing 99.93 per cent. of peroxide being ultimately obtained. Concentration of hydrogen peroxide in the commercial solution containing about 30 per cent. H2O2 is known as perhydrol.


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