434 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



TABLE XI. 

 Loss at Anode compared with Gain at Kathode. 



In the porous cup voltameter all the anode reactions which constitute 

 the most serious causes of inaccuracy are safely eliminated by keeping 

 the contaminated liquid within the porous cup. It is ohvious that this 

 device, or some other accomplishing the same end, should always be used 

 when accuracy is desired. 



III. The Purity of the Silver Deposit. 



An important question remains to be answered, namely, is the deposit 

 thus obtained perfectly pure silver, or does it contain traces of included 

 mother liquor? 



That impurities in the solution, such as copper, or any of the common 

 metals occurring with silver, do not affect silver deposit to any great 

 extent has been shown by Lord Rajleigh. Even if the solution actually 

 turns green from the copper dissolved at the anode, not a trace of copper 

 can be detected in the deposit. We used on one occasion commercial 

 sihier nitrate with an anode of sterling silver wrapped in paper, and 

 found that the difference between this and the standard was about .024 

 per cent, or only about .02 per cent smaller than a similar deposit with 

 the purest silver. Metals of greater solution tension than silver have 

 therefore no important effect on the weight of silver, although they may 

 change the structure of the silver deposit. Of course they had always 

 been excluded in this work. 



