HYDROGEN. 83 



certain solid substances to absorb moisture from the air, thereby 

 becoming damp or even liquid, as, for instance, potassium hydroxide, 

 calcium chloride, etc. Such substances are spoken of also as being 

 hygroscopic, and are used for drying gases. 



Hydrogen dioxide, Hydrogen peroxide, H 2 O 2 . This compound T 

 may be obtained in aqueous solution by the action of carbonic acid 

 (or other acids) on barium dioxide suspended in water, when barium 

 carbonate and hydrogen dioxide are formed : 



BaO 2 + H 2 O + CO 2 = BaCO 3 + H 2 O 2 . 



The liquid, separated by decantation from the insoluble carbonate, 

 may be concentrated under the receiver of an air-pump, and is, when 

 thus obtained, a colorless liquid of a specific gravity 1.45, possessing 

 remarkable bleaching and antiseptic properties. By higher temper- 

 atures, as well as by the action of many substances, it is readily de- 

 composed into water and oxygen. 



Solution of hydrogen dioxide, Aqua hydrogen!! dioxidi is 

 made according to the U. S. P. by adding to barium dioxide sus- 

 pended in water of a temperature not above 10 C. (50 F.), diluted 

 phosphoric acid until the reaction, which is first alkaline, has become 

 neutral. Insoluble barium phosphate is formed, which is removed 

 by filtration. From the clear filtrate traces of barium yet remaining 

 in solution are precipitated by the addition of a few drops of sul- 

 phuric acid. The filtered solution is then diluted until it contains 

 about 3 per cent., by weight, of pure dioxide, corresponding to about 

 10 volumes of available oxygen in 1 volume of the solution. 



The solution is colorless and without odor, and has a slight acid 

 reaction due to a trace of free sulphuric acid present ; it is liable to 

 deteriorate by age, especially on exposure to heat and light. 



Glycozone is hydrogen dioxide dissolved in glycerin instead of in 

 water. 



Hydrogen dioxide may be recognized by the following reactions : 



1 . Silver oxide causes the decomposition of hydrogen dioxide with 

 evolution of oxygen, metallic silver and water being formed at the 



same time : 



Ag 2 + H 2 2 = 2Ag + 20 + H 2 0. 



2. Mixed solutions of ferric chloride and potassium ferricyanide 

 (which should have no blue tinge) assume an intense blue color on 

 addition of even a very small quantity of hydrogen peroxide. 



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