COPPER, SILVER, AND GOLD 483 



bromide by heating it with silver acetate, 2C 2 H 3 O 2 Ag. The insolubility 

 of the halogen compounds of silver is still more frequently taken ad- 

 vantage of in determining the amount of silver and halogen in a given 

 solution. If it is required, for instance, to determine the quantity of 

 chlorine present in the form of a metallic chloride in a given solution, 

 a solution of silver nitrate is added to it so long as it gives a pre- 

 cipitate. On shaking or stirring the liquid, the silver chloride easily 

 settles in the form of heavy flakes. It is possible in this way to 

 precipitate the whole of the chlorine from a solution, without adding 

 an excess of silver nitrate, since it can be easily seen whether the 

 addition of a fresh quantity of silver nitrate produces a precipitate in 

 the cleap liquid. In this manner it is possible to add to a solution 

 containing chlorine, as much silver as is required for its entire precipi- 

 tation, and to calculate the amount of chlorine previously in solution 

 from the amount of the solution of silver nitrate consumed, if the 

 quantity of silver nitrate in this solution has been previously deter- 

 mined. 25 bis The atomic proportions and preliminary experiments with 

 a pure salt for example, with sodium chloride will give the amount 

 of chlorine from the quantity of silver nitrate. Details of these 

 methods will be found in works on analytical chemistry. 25trl 



25 bis j n or aer to determine when the reaction is at an end, a few drops of a solution 

 of K 2 CrO4 are added to the solution of the chloride. Before all the chlorine is precipitated 

 as AgCl, the precipitate (after shaking) is white (since Ag 2 CrO 4 with 2RC1 gives 2AgCl) ; 

 but when all the chlorine is thrown down Ag 2 CrO 4 is formed, which colours tte precipi- 

 tate reddish-brown. In order to obtain accurate results the liquid should be neutral 

 to litmus. 



25 tri Silver cyanide, AgCN, is closely analogous to the haloid salts of silver. It is 

 obtained, in similar manner to silver chloride, by the addition of potassium cyanide to 

 silver nitrate. A white precipitate is then formed, which is almost insoluble in boiling 

 water. It is also, like silver chloride, insoluble in dilute acids. However, it is dissolved 

 when heated with nitric acid, and both hydriodic and hydrochloric acids act on it, con- 

 verting it into silver chloride and iodide. Alkalis, however, do not act on silver cyanide, 

 although they act on the other haloid salts of silver. Ammonia and solutions of the 

 cyanides of the alkali metals dissolve silver cyanide, as they do the chloride. In the 

 latter case double cyanides are formed for example, KAgC 2 N 2 . This salt is obtained in 

 a crystalline state on evaporating a solution of silver cyanide in potassium cyanide. It 

 is much more stable than silver cyanide itself. It has a neutral reaction, does not 

 change in the air, and does not smell of hydrocyanic acid. Many acids, in acting on a 

 solution of this double salt, precipitate the insoluble silver cyanide. Metallic silver dis- 

 solves in a solution of potassium cyanide in the presence of air, with formation of 

 the same double salt and potassium hydroxide, and when silver chloride dissolves in 

 potassium cyanide it forms potassium chloride, besides the salt KAgC 2 N 2 . This double 

 salt of silver is used in silver plating. For this purpose potassium cyanide is added to 

 its solution, as otherwise silver cyanide, and not metallic silver, is deposited by the 

 electric current. If two electrodes one positive (silver) and the other negative (copper) 

 be immersed in such a solution, silver will be deposited upon the latter, and the 

 silver of the positive electrode will be dissolved by the liquid, which will thus preserve 

 the same amount of metal in solution as it originally contained. If instead of the 

 negative electrode a copper object be taken, well cleaned from all dirt, the silver 



