84 



llEPOKT — 1873. 



The Preciintation of Silver hy Copper. By Aifeed Tribe. 



It lias beeu recently shown by Dr. Gladstone and the writer that copper covered 

 with precipitated silver removes dissolved oxygen, as cuprous oxide, from a solution 

 of copper nitrate containing air, and also that the silver-copper couple moistened 

 with the same liquid removes oxygen, not only from the air, but from other gaseous 

 mixtures. 



In tlie course of the above and other experiments, it has been necessary to com- 

 pletely precipitate, at various times, large quantities of silver by copper, and it has 

 been noticed that the metal so obtained, after being thoroughly washed, always 

 contained copper. The constant presence of this metal was considered due to dis- 

 solved oxygen in the silver solutions, or to the absorption of that gas from the 

 air, by the produced copper nitrate, during or subsequent to the precipitation. 

 The experiments made with the view of ascertaining the correctness of this suppo* 

 sition are tabulated below. 



In each experiment there was employed an excess of copper, and in experiments 

 C to I about the same volume of solution. In A and B, pieces of copper foil of the 

 same dimensions were placed in open basins, and covered to about \ of an inch with 

 ordinary silver nitrate, i. c. impregnated with air. In C, D, E, bottles were filled 

 with ordinarj' solutions, and stoppered during the precipitation. In G, H, carbonic 

 anhj'dride was bubbled through the solutions prior to the immersion of the copper, 

 and the precipitation conducted as in C, D, E. In II, I, ordinary solutions were used. 



It appears from experiments A, B, D that the quantity of copper is increased 

 by exposing the couple, covered with dilute copper nitrate, to the air, and decreased 

 by precipitating in absence of air. In G, I), E the actual amounts of copper found, 

 being nearly the same, clearly indicate that its presence is not due to oxygen in 

 the copper employed. Moreover it is a result which would follow were the free 

 oxygen in the respective silver solutions the cause, since it is probable that each 

 contained about the same quantity of the gas. Experiments F and G show that the 

 effect of saturating the solutions with carbonic anhydride prior to precipitation is 

 to diminish the amount of copper 3-4 times, which, doubtless, is due to the partial 

 displacement of oxygen by the more soluble gas. 



In the experiments C and G there existed a trace of silver in solution after the 

 twenty-four hours. II and I being of short duration, there was a large excess ; and it 

 is noticeable that in every case where the silver was nearly exhausted copper was 

 found, whereas where there was an excess of silver the merest trace only existed. 



It appears from the foregoing experiments that free oxygen is intimately con- 

 nected with the presence of copper in silver precipitated by that metal ; but 

 ■whether copper exists therein as cuprous oxide, or as basic nitrate, would depend 

 upon at what stage of the operation the oxygen plays its part. If the two actions 

 {i. e. decomposition of silver nitrate by copper and of copper nitrate by oxygen) be 

 simultaneous, basic nitrate should be found. If, however, the decomposition of the 

 copper nitrate be not effected until the silver nitrate is so exhausted as to be 

 incapable of action on the produced cuprous oxide, that siibstance should be foimd. 

 One experiment made on this point, with a weak solution of silver nitrate, seemed 

 to show that basic nitrate was not formed. 



