SILVER MERCURY. 335 



5. Solution of alkali hydroxides precipitates dark-brown silver 

 oxide, soluble in ammonia water, forming a complex hydroxide 

 Ag(NH 3 ) 2 .OH. 



6. Solution of potassium iodide or bromide gives a pale-yellow 

 precipitate. 



7. Metallic copper or zinc precipitates metallic silver from solu- 

 tions of silver in any form of combination. (See page 319.) 



Test 2 is sufficient to identify silver in ordinary solutions, for, 

 although mercurous and lead chloride are insoluble in water and 

 dilute acids, silver chloride alone of these three insoluble chlorides 

 is soluble in ammonia water. Silver forms a number of complex 

 combinations with organic compounds from which it is not easily pre- 

 cipitated, but by reduction of the silver to the metallic state, and 

 solution of the metal in nitric acid, the tests may be applied. The 

 same procedure applies also to inorganic insoluble silver compounds, 

 as AgCl, AgBr, Agl, Ag 2 S. 



All silver salts are soluble in ammonia water, except the iodide and sul- 

 phide ; hence the latter alone can be precipitated from an ammoniacal solution. 

 In these solutions complex combinations, as Ag(NH 3 ) 2 .NO 3 , Ag(NH,) t Cl, are 

 formed, which are salts of the hydroxide, Ag(NH 3 ) 2 .OH. In this respect silver 

 acts very much like copper. These compounds dissociate thus: 



Ag(NH 3 ) 2 N0 3 5 Ag(NH 3 ) 2 - + NO/. 



The Ag(NH 3 )./ ions yield a slight amount of Ag* ions, but not enough to be 

 precipitated by any of the reagents except iodide and sulphide. The concen- 

 tration of Ag' ions given by the latter precipitates is less than that given by 

 the Ag(NH 3 ) 2 * ions, hence precipitation takes place. (See page 193.) 



Solutions of alkali cyanides and thiosulphates dissolve all silver compounds, 

 giving complex substances of the form KAg(CN) 2 and Na 3 Ag(S 2 O 3 ) 2 . The ions 

 of these are K* + Ag(CN)/, and 3Na* -f Ag(S 2 O 3 )/ // . Such a slight amount 

 of Ag* ions are formed from these compounds that no reagent will give a pre- 

 cipitate. That there are some Ag* ions, however, is shown by the fact that 

 active metals, like zinc, separates the silver from the solutions. Also that in 

 electroplating silver is deposited from cyanide solution. 



Soluble silver salts are not hydrolyzed, and are, therefore, neutral. 



Mercury, Hydrargyrum, Hg = 198.5 (Quicksilver). Mercury is 

 found sometimes in small globules in the metallic state, but generally 

 as mercuric sulphide or cinnabar, a dark-red mineral. The chief 

 supply was formerly obtained from Spain and Austria ; now, how- 

 ever, large quantities are obtained from California ; it is also 

 imported from Peru and Japan. 



Mercury is obtained from cinnabar either by roasting it, whereby 

 the sulphur is converted into sulphur dioxide, or by heating it with 

 lime, which combines with the sulphur, while the metal volatilizes, 

 and is condensed by passing the vapors through suitable coolers. 



