ARSENIC. 209 



pbides to form sulpho-salts, as, for instance, K 2 S.As 2 S 3 , or (NH 4 ) 2 S. 

 As 2 S 3 . These compounds are known as sulph-arsenides. 



Arsenous iodide, Arseni iodidum, AsI 3 = 454.5 (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 

 official preparation, Solution of arsenic and mercuric iodide, Donovan's 

 solution, which is made by dissolving one part each of arsenous iodide 

 and mercuric iodide in 98 parts of water. 



Analytical reactions. 

 (Use arsenous oxide, As. 2 O 3 , and sodium arsenate, NajHAsO^ respectively.) 



1. Add hydrogen sulphide to an aqueous solution of arsenous oxide : 

 a yellow coloration but no precipitate is formed until some hydro- 

 chloric acid is added, when yellow arsenic trisulphide, As 2 S 3 (Plate 

 V., 1) is precipitated : 



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

 or 



2H 3 AsO 3 -f 3H 2 S = 6H 2 O + As 2 S 3 . 



When hydrogen sulphide is added to a cold solution of arsenic 

 oxide or of an arsenate, acidified with hydrochloric acid, a yellow 

 mixture of arsenic trisulphide, As 2 S 3 , and sulphur is slowly pre- 

 cipitated : 



2H 3 As0 4 + 5H 2 S = 8H 2 + As 2 S 3 + 2S. 



When the same substances act upon one another in hot solution, and 

 when also an excess of hydrogen sulphide (preferably a current of the 

 gas) is used, yellow arsenic pentasulphide is precipitated : 



As 2 O 5 + 5H 2 S = 5H 2 O + As 2 S 5 ; 

 or 



2H 3 AsO 4 + 5H 2 S = 8H 2 O + As 2 S 5 . 



2. Add ammonium sulphide or any alkali hydroxide to the yellow 

 precipitate of arsenous or arsenic sulphide : the precipitates are 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 at first 

 formed) produces in neutral solutions of arsenous acid a yellow precipi- 

 tate of silver arsenite, Ag 3 AsO 3 (Plate V., 3) ; in arsenic acid solu- 

 tions a reddish-brown precipitate of silver arsenate, Ag 3 AsO 4 (Plate 

 V., 4). The two precipitates are soluble in both alkalies and acids. 



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