ARSENIC. 187 



sulphuric acid. Both sulphides of arsenic are sulphur-acids, 

 uniting with other metallic sulphides to form sulphur-salts, as, 

 for instance, K^S.As^, or (NH 4 ) 2 S.As 2 S 3 . These compounds 

 are known as sulph-arsenides. 



Arsenious iodide, Arsenii iodidum, AsI 3 = 454.7 (Iodide of arsenic), 

 may be obtained by direct combination of the elements, and 

 forms orange-red crystalline masses, soluble in water and alcohol, 

 but decomposed by boiling with either of these liquids. It is 

 used in the officinal preparation, Solution of iodide of arsenic and 

 mercury, Donovan's solution, which is made by dissolving equal 

 parts of arseuious iodide and mercuric iodide in 100 parts of 

 water. 



Analytical reactions. 



1. Add hydrosulphuric acid to a slightly acid solution of 

 arsenic : a yellow precipitate of arsenious sulphide is produced : 



As 2 O 3 + 3H 2 S = 3H 2 + As 2 S 3 . 



If arsenic is present as arsenic acid, this compound is reduced 

 to arsenious acid previous to its precipitation ; heating of the 

 liquid facilitates this reaction : 



As 2 O 5 -f 2H 2 S = As 2 O 3 + 2H 2 + 2S. 



2. Add ammonium sulphide or any alkaline hydrate to the 

 yellow precipitate of arsenious sulphide : the latter is readily 

 dissolved but may be reprecipitated by neutralizing with an 

 acid. 



3. Ammonio-nitrate of silver (silver nitrate to which enough 

 of water of ammonia has been added to redissolve the precipitate 

 formed at first) produces in neutral solutions of arsenious acid 

 a yellow precipitate of arsenite of silver (Ag 3 As0 3 ), in arsenic 

 acid solutions a chocolate-colored precipitate of arsenite of silver 

 (Ag 3 AsO 4 ). The two precipitates are soluble, both in alkalies 

 and acids. 



4. Ammonio-sulphate of copper (made similarly to am- 

 monio-nitrate of silver from cupric sulphate) added to neutral 

 arsenious solutions produces a green precipitate of cupric 

 arsenite (CuHAs0 3 ), known as Scheele's green. (Arsenite of 

 copper mixed with cupric acetate is known as Schweinfurth 



