C 261 3 



XL. On Bromo-arsenious Acid. 

 By William Wallace, Ph.D., F.C.S.* 



TN former papers f I liave described chloro-arsenious acid and 

 A iodo-arsenious acid, compounds which consist of arsenious 

 acid in which an equivalent of oxygen is replaced by chlorine 

 or iodine. In the present communication I purpose to notice 

 another member of the same series of compounds — bromo-arse- 

 nious, or arsenious acid with one equivalent of oxygen replaced 

 by bromine. This compound is intermediate in its properties 

 between the corresponding acids containing chlorine and iodine 

 respectively. 



Terbromide of arsenic is readily formed by very slowly intro- 

 ducing an excess of powdered metallic arsenic into bromine con- 

 tained in a tubulated retort. The arsenic takes fire as soon as 

 it comes in contact with the bromine. By distilling twice, the 

 bromide is obtained perfectly pure, as a white, fibrous, crystalline 

 mass. When a considerable quantity of fused bromide of arsenic 

 is cooled very slowly, crystals of more than an inch in length 

 are readily obtained. From these the still liquid portion may 

 be poured ofi", and thus the bromide is prepared in a state of 

 perfect purity. I intend to employ the bromide purified in this 

 manner for the redetermination of the equivalent number of 

 bromine. 



Bromo-arsenious Acid. 

 Fused terbromide of arsenic readily dissolves a considerable 

 quantity of arsenious acid, forming a slightly viscid, dark- 

 coloured fluid, which does not solidify so readily as the pure 

 bromide. When this liquid is gradually distilled until it becomes 

 rather thick, and allowed to cool to about 150°, it separates into 

 two fluids, the heavier being very viscid. The upper liquid 

 consists of bromo-arsenious acid, while the lower is a compound 

 of that body with arsenious acid. Bromo-arsenious acid, thus 

 formed, is a soft, unctuous, semisolid mass, having a dark colour, 

 which docs not appear to be owing to the presence of any im- 

 purity. Analysis gave — 



Arsenic 1 = 75 43-86 



Bromine. . . 4710 1 z= 80 46-78 

 Oxygen 2 = J^ 9-36 



171 10000 

 As BrO' or As Br^ -f- 2 AsO'. The more viscid mass which sepa- 

 rates from the above appears to consist of SAsBrO^-t-AsO^, or 



* Communicated by the Author. 



t Phil. Mag. vol. xvi. p. 358, and vol. xvii. p. 122. 



