94 COATING OP IRON WITH COPPER. 



exercised, as it is very deleterious. It will be found that "the 

 copper is not ail precipitated by the cyanide of potassium, for 

 according to this mode, when a precipitate ceases to be formed, 

 the solution remains greenish blue, probably owing to the decom- 

 position of the cyanate of potash, and the formation of ammonia 

 which holds copper in solution, and forms also some complicated 

 compounds with the cyanides of copper. If cyanide of potassium is 

 added until the blue solution disappears, still copper is held in the 

 solution, and may be detected by taking out a little, and adding to 

 it a few drops of sulphuric acid, which will give a white precipitate 

 of subcyanide of copper. The loss of copper sustained is the only 

 objection to this mode of preparing a copper solution. The cyanide 

 of potassium is added until a precipitate is no longer formed ; it is 

 then allowed to settle, the clear liquid is poured off, and the vessel is 

 to be filled with water : when the precipitate has again settled, the 

 liquor is poured off, and this washing is repeated four or five times, 

 in order to wash out the sulphate of potash which is formed during 

 the precipitation. After being thus washed, a solution of cyanide of 

 potassium is added to the precipitate until it dissolves. The copper- 

 ing solution is now complete: it is of a light yellow colour, and is, 

 well adapted for ordinary purposes. The loss of copper is, however, 

 considerable, being about one-fifth of the whole. 



Second Method. A coppering solution may also be prepared by 

 adding cyanide of potassium to oxide of copper, or to carbonate of 

 copper, until it is dissolved. But these solutions are objectionable, 

 the latter especially so, as it contains a great quantity of carbonate 

 of potash, formed from the mutual decomposition of the carbonate 

 of copper and cyanide of potassium, and the carbonate of potash 

 deteriorates the solution ; the former leaves potash in the solution, 

 but this is not so bad as the carbonate of potash. 



Third Method. The method we have adopted in manufacturing 

 purposes is as follows: To a solution of sulphate of copper, we add 

 a solution of ferrocyanide of potassium, so long as a precipitate con- 

 tinues to be formed : this is allowed to settle, and the clear liquor 

 being decanted, the vessel is filled with water, and when the pre- 

 cipitate settles, the liquor is again decanted, and we continue to 

 repeat these washings until the sulphate of potash is washed quite 

 out. This is known by adding a little chloride of barium to a small 

 quantity of the washings, and when there is no white precipitate 

 formed by this test, the precipitate is sufficiently washed. A solu- 

 tion of cyanide of potassium is now added to this precipitate until it 

 is dissolved, during which process the solution becomes warm by 

 the chemical reaction that takes place. The solution is filtered, 

 and allowed to repose all night. If the solution of eyanide of 

 potassium that is used is strong, the greater portion of the ferro- 



