234 Bulletin 80. 



volts and a current not greater than one ampere. The voltage 

 may be higher than 21/0 volts if necessary but should not be high 

 enough to raise the ciirrent beyond the limit given. The elec- 

 trolysis should be continued at least three hours and preferably 

 nine to twelve hours. The dish is, of course, the cathode. Wlien 

 the electrolysis is complete the electrolyte is washed out of the 

 dish by means of the sucking-bottle and the dish is thoroughly 

 washed with distilled water. In case the deposit of copper on 

 the dish is spongy and loosely adherent it is not safe to wash 

 out the electrolyte. In this case the copper should be redissolved 

 and the electrolysis repeated, using a little more sulphuric acid. 

 If the copper still refuses to come down in adherent form the 

 addition of 2 to 5 c.c. of a one per cent solution of gelatine will 

 often assist the precipitation. In case of a stubborn refusal of 

 the copper to give an adherent deposit it is necessary to dissolve 

 it, evaporate to dryness with sulphuric acid, and reprecipitate 

 with hydrogen sulphide, continuing the process from this point 

 as before. 



If the copper refuses to come down at all the trouble is 

 probably an excess of acid in the solution. This may be corrected 

 by the addition of a few drops of ammonia. The concentration 

 of acid in the solution nuist lie between one and five per cent. At 

 least a small part of this should be sulphuric acid as nitric acid 

 will be destroyed in the course of the electrolysis if it alone is 

 present, and the solution may become alkaline (from NH^OH), 

 which will prevent proper precipitation. Chlorides and organic 

 salts, such as acetates and tartrates, should be carefully avoided. 



The resulting deposit of copper will probably contain traces 

 of carbon and possibly of platinum. In order to eliminate these 

 and at the same time precipitate copper upon an electrode more 

 suitable for accurate weighing, a second electrolysis is made, 

 using this time the dish as anode and using as cathode a small 

 spiral of platinum wire suspended from a hook of silver (or 

 platinum) wire which in turn is connected to the battery. The 

 electrolysis should also be conducted in nitric and sulphuric 

 acid solution and what is said above as to obtaining satisfactory 

 deposits applies with equal force here. In this case, however, 

 owing to the small surface area of the cathode, it is necessary to 

 work with very much smarller currents than were used in tlie 

 first electrolysis. The maximum current to be used must be so 

 adjusted by trial as to give bright and adherent deposits. 1-lOOth 

 ampere and 1.8 volts is a good current for the purpose. It is well 

 to use as the source of current for this electrolysis four Edison- 

 Lalande cells and to have in the circuit a resistance of from 30 

 to 80 ohms. This gives an electromotive force at the dish of about 

 1.8 volts. Two determinations may be run in parallel. In this 

 case it is not permissible to use a gelatine solution in order to 

 secure satisfactory deposits, as the copper will be slightly contam- 

 inated with gelatine and the obtained weight will be too high. 



