Methods of Analysis 235 



The electrolysis should be run at least nine hours. "When com- 

 pleted, the electrolyte should be washed out as before without 

 breaking the cui'rent, the electrode lifted from the solution, dis- 

 engaged from the supporting hook, and washed and dried by 

 dipping successively in water, alcohol and ether and placing in a 

 desiccator over sulphuric acid. After having remained in the 

 desiccator for an hour the electrode is ready for weighing. 

 Weighings should be made on an assay (button) balance adjusted 

 to maximum sensibility. After weighing, the copper is removed 

 from the electrode by dipping in concentrated nitric acid, and 

 the electrode cleaned and dried by dipping successively in dis- 

 tilled water, alcohol and ether and placing in a desiccator. It 

 is again weighed as before and the difference of the two weights 

 gives the copper obtained. 



The electrolyte (from each electrolysis) which has been 

 washed out of the dish by means of the suction flask, is evapor- 

 ated to dryness taken up with water, acidified with nitrie acid 

 and tested for copper by electrolyzing, using the point of 

 platinum wire as cathode. In this way any possible loss of 

 copper by incomplete precipitation in either of the electrolyses 

 is prevented. If any copper is found in this check test it should 

 be dissolved from the platinum wire, added to the solution ob- 

 tained by dissolving the copper from the small electrode, and 

 the electrolysis repeated in order to get the true weight. 



In case a quantity of copper too small to be weighed is 

 obtained its identity as copper may be most easily established 

 by electrolyzing it onto the point of a platinum wire as described 

 above. In these electrolyses with the platinum wire as cathode 

 the current must, of course, be kept low in order to obtain satis- 

 factory deposits. If this precaution is observed the deposit on 

 the platinum wire will be of a brilliant red color and easily dis- 

 tinguishable as copper. If the deposit is brownish or blackish 

 its identity as copper may be established by the green flash when 

 the point of the wire is held in the colorless flame of the Bunsen 

 burner, particularly if the wire has been first dipped in hydro- 

 chloric acid. Nitric acid must not be used, as nitric acid itself 

 will give a green flash in the Bunsen burner flame. 



The reagents used in the above process should all be tested 

 as to freedom from copper. The water used should be doubly 

 distilled and, at least the second time, from glass. All utensils 

 should be cleaned by boiling in nitric acid. Care must also be 

 taken to conduct the operations in rooms free from dust which 

 might possibly contain copper. 



