352 METALS AND THEIR COMBINATIONS. 



When a rapid stream of hydrogen sulphide is passed into a hot acid-. 

 ified solution of an arsenate, a yellow precipitate of arsenic pentasul- 

 phide is gradually formed : 



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



Solution of ammonium sulphide or caustic alkali readily dissolves 

 both sulphides of arsenic. Addition of acid reprecipitates the 

 sulphides. 



2. Add 1 or 2 c.c. of silver nitrate solution to about 5 c.c. of the 

 arsenousacid solution ; no precipitate results. Now pour carefully upon 

 the surface of the mixture a little very dilute ammonia water ; a yellow 

 precipitate of silver arsenite (Plate V., 3) is formed at the line of con- 

 tact of the two liquids, which may be increased by cautiously mixing 

 the liquids. The precipitate is soluble in excess of ammonia or in 

 nitric acid. When solution of an arsenite instead of free arsenous 

 acid is used, silver nitrate gives a precipitate at once, without addition 

 of ammonia water : 



H 3 As0 3 + 3AgN0 3 + 3NH 4 OH == Ag 3 AsO 3 + 3NH 4 NO 8 + 3H 2 O. 



Na 3 AsO 3 + 3AgNO 3 == Ag 3 AsO 3 + 3NaNO 3 . 



Dissolve the precipitate in a slight excess of ammonia water, add a 

 few drops of caustic soda, and apply heat ; a mirror of metallic silver 

 is formed, due to the reducing action of the arsenite, which becomes 

 arsenate. 



When silver nitrate is added to the sodium arsenate solution (about 

 3 c.c.), a reddish-brown precipitate of silver arsenate is formed, which 

 is soluble in ammonia water or nitric acid (Plate V., 4) : 



Na 2 HAsO 4 4- 3AgNO 3 = Ag 3 AsO 4 + 2NaNO 3 + HNO 3 . 



3. Add 2 or 3 drops (avoid excess) of copper sulphate solution to 

 about 5 c.c. of the arsenous acid, and overlay the mixture with very 

 dilute ammonia water as in test 2 (if an arsenite is used, ammonia 

 water is unnecessary) ; a green precipitate of copper arsenite, Scheele's 

 green, CuHAsO 3 , is produced (Plate V., 2). Add some caustic alkali 

 solution to the precipitate and boil ; red cuprous oxide is formed, due to 

 reduction by the arsenite, which becomes arsenate. (Schweinfurt green, 

 copper aceto-arsenite, 3Cu(AsO 2 ) 2 .Cu(C 2 H 3 O 2 ) 2 , is obtained by adding 

 solution of copper acetate to a boiling solution of an arsenite. This 

 and Scheele's green are often called Paris green.) 



When copper sulphate solution is added to the sodium arsenate solu- 

 tion, a greenish-blue precipitate of copper arsenate, CuHAsO 4 , is 



