ARSENIC. 349 



The official Solution of arsenous acid, Liquor acidi arxcnosi, is a 



1 per cent, solution of arsenous oxide in water to which 5 per cent, 

 of diluted hydrochloric acid has been added. 



The official Solution of potassium arsenite, Liquor potassii arsenitis, or 

 Fowler's solution, is made by dissolving 1 part of arsenous oxide and 



2 parts of potassium bicarbonate in 94 parts of water and adding 



3 parts of compound tincture of lavender ; the solution contains the 

 arsenic as potassium met-arsenite. 



Arsenic oxide, As 2 O 5 (Arsenic pentoxide, Arsenic acid anhydride). 

 When arsenous oxide is heated with nitric acid, it becomes oxidized 

 and is converted into arsenic acid, H 3 AsO 4 , from which the water 

 may be expelled by further heating, when arsenic oxide is left : 

 2H 3 AsO 4 = As 2 O 5 + 3H 2 O. 



Arsenic oxide is a heavy, white, solid substance which, in contact 

 with water, is converted into arsenic acid. This acid resembles phos- 

 phoric acid in composition, but diifers from it in not forming pyro- 

 and metarsenic acids. Arsenic acid when moderately heated loses all 

 its water and leaves the pentoxide, which at higher heat is decom- 

 posed into the trioxide and oxygen. Phosphoric acid, however, when 

 heated is converted into metaphosphoric which can be volatilized, 

 and phosphorus pentoxide does not decompose by heating. Sodium 

 pyroarsenate, perhaps, is formed when sodium arsenate is heated, 

 but when the mass is dissolved in water arsenate is formed at once, 

 whereas the pyrophosphate can be crystallized from water. 



Arsenic oxide and arsenic acid are used largely as oxidizing agents 

 in the manufacture of aniline colors. 



Disodium hydrogen arsenate, Sodii arsenas, Na 2 HAsO 4 .7H 2 O 

 = 3O9.84 (Sodium arsenate). This salt is made by fusing arsenous 

 oxide with carbonate and nitrate of sodium. 



As 2 O 3 -f 2NaNO 3 + Na 2 CO 3 = Na 4 As 2 O 7 + N 2 O 3 + CO,. 



Sodium pyroarsenate is formed, nitrogen trioxide and carbon dioxide 

 escaping. By dissolving in water and crystallizing, the official salt is 

 obtained in colorless, transparent crystals : 



Na 4 As 2 7 -f 15H 2 O = 2(Na 2 HAsO 4 .7H 2 O). 



Exsiccated sodium arsenate, Sodii arsenas exsiceatus, Na 2 HAsO 4 , is 

 the product obtained by driving off all the water of crystallization at 

 150 C. (302 F.). Liquor sodii arsenatis is a 1 per cent, solution of 



