0/ONE AND HYDROGEN PEROXIDE D ALTON'S LA\V 209 



barium peroxide remains in solution. 23 The reaction is expressed 

 by the equation BaO2 + H 2 SO 4 =H 2 O 2 + BaSO 4 . It is best to take a 

 weak cold solution of sulphuric acid and to almost saturate it with 

 barium peroxide, so that a small excess of acid remains; insoluble 

 barium sulphate is formed. A more or less dilute aqueous solution 

 of hydrogen peroxide is obtained. This solution may be concentrated 

 in a vacuum over sulphuric acid. In this way the water may even be 

 entirely evaporated from the solution of the hydrogen peroxide ; only 

 in this case it is necessary to work at a low temperature, and not to 

 keep the peroxide for long in the rarefied atmosphere, as otherwise it 

 decomposes. 24 



When pure, hydrogen peroxide is a colourless liquid, without smell, 

 and having a very unpleasant taste such as belongs to the salts of 

 many metals the so-called ' metallic ' taste. Water held in zinc vessels 

 has this taste, which is probably due to its containing hydrogen peroxide. 

 The tension of the vapour of hydrogen peroxide is less than that of 

 aqueous vapour ; this enables its solutions to be concentrated in a 

 vacuum. The specific gravity of anhydrous hydrogen peroxide is 1'455. _^ , 

 Pure hydrogen peroxide decomposes, with the evolution of oxygen, when 

 heated even to 20 (by the action of light ?). But the more dilute its 

 aqueous solution the more stable it is. Very weak solutions may be 

 distilled without the hydrogen peroxide decomposing. It decolorises 

 solutions of litmus and turmeric, and acts in a similar manner on many 

 colouring matters of organic origin (for which reason it is employed for 

 bleaching tissues). 



Many substances decompose hydrogen peroxide, forming water and 

 oxygen, without apparently suffering any change. In this case sub- 

 stances in a state of fine division evince an incomparably quicker action 



23 With the majority of acids, that salt of barium which is formed remains in solution ; 

 thus, for instance, by employing hydrochloric acid, hydrogen peroxide and barium chloride 

 remain in solution. Complicated processes would be required to obtain pure hydrogen 

 peroxide from such a solution. It is much more convenient to take advantage of the 

 action of carbonic anhydride on the pure hydrate of barium peroxide. For this purpose 

 the hydrate is stirred up in water, and a rapid stream of carbonic anhydride is passed 

 through the water. Barium carbonate, insoluble in water, is formed, and the hydrogen 

 peroxide remains in solution, so that it may be separated from the carbonate by filtering 

 only. On a large scale hydrofluosilicic acid is employed, because its barium salt is also 

 insoluble in water. 



24 Hydrogen peroxide may be extracted from very dilute solutions by means of ether, 

 which dissolves it, and when mixed with it the hydrogen peroxide may even be distilled. 

 A solution of hydrogen peroxide in water may be enriched by cooling it to a low tempera- 

 ture, when the water crystallises out that is, is converted into ice whilst the hydrogen 

 peroxide remains in solution, as it only freezes at very low temperatures. It must be 

 observed that hydrogen peroxide, in a strong solution in a pure state, is exceedingly 

 unstable even at the ordinary temperature, and therefore it must be preserved in vessels 

 always kept cold, as otherwise it evolves oxygen and forms water. 



VOL. I. P 



