Test the solution in the beaker for copper as follows, remembering that it is to be used for subsequent determinations of iron and zinc:  Remove about 5 cc. and add a slight excess of ammonia.  Compare the mixture with some distilled water, holding both above a white surface.  The solution should not show any tinge of blue.  If the presence of copper is indicated, add the test portion to the main solution, evaporate the whole to a volume of about 100 cc., and again electrolyze with clean electrodes (Note 7).

After cooling the electrodes in a desiccator, weigh them and from the weight of copper on the cathode and of lead dioxide (PbO_{2}) on the anode, calculate the percentage of copper (Cu) and of lead (Pb) in the brass.

[Note 1:  It is obvious that the brass taken for analysis should be untarnished, which can be easily assured, when wire is used, by scouring with emery.  If chips or borings are used, they should be well mixed, and the sample for analysis taken from different parts of the mixture.]

[Note 2:  If a white residue remains upon treatment of the alloy with nitric acid, it indicates the presence of tin.  The material is not, therefore, a true brass.  This may be treated as follows:  Evaporate the solution to dryness, moisten the residue with 5 cc. of dilute nitric acid (sp. gr. 1.2) and add 50 cc. of hot water.  Filter off the meta-stannic acid, wash, ignite in porcelain and weigh as SnO_{2}.  This oxide is never wholly free from copper and must be purified for an exact determination.  If it does not exceed 2 per cent of the alloy, the quantity of copper which it contains may usually be neglected.]

[Note 3:  The electrodes should be freed from all greasy matter before using, and those portions upon which the metal will deposit should not be touched with the fingers after cleaning.]

[Note 4:  Of the ions in solution, the H^{+}, Cu^{++}, Zn^{++}, and Fe^{+++} ions tend to move toward the cathode.  The no_{3}^{-} ions and the lead, probably in the form of PbO_{2}^{—­} ions, move toward the anode.  At the cathode the Cu^{++} ions are discharged and plate out as metallic copper.  This alone occurs while the solution is relatively concentrated.  Later on, H^{+} ions are also discharged.  In the presence of considerable quantities of H^{+} ions, as in this acid solution, no Zn^{++} or Fe^{+++} ions are discharged because of their greater deposition potentials.  At the anode the lead is deposited as PbO_{2} and oxygen is evolved.

For the reasons stated on page 141 care must be taken that the solution does not become alkaline if the electrolysis is long continued.]

[Note 5:  Urea reacts with nitrous acid, which may be formed in the solution as a result of the reducing action of the liberated hydrogen.  Its removal promotes the complete precipitation of the copper.  The reaction is

Co(NH_{2})_{2} + 2HNO_{2} —­> Co_{2} + 2N_{2} + 3H_{2}O.]