[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.] [Note 6: The electrodes must be washed nearly or quite free from the nitric acid solution before the circuit is broken to prevent re-solution of the copper. If several solutions are connected in the same circuit it is obvious that some device must be used to close the circuit as soon as the beaker is removed.] [Note 7: The electrodes upon which the copper has been deposited may be cleaned by immersion in warm nitric acid. To remove the lead dioxide, add a few crystals of oxalic acid to the nitric acid.] DETERMINATION OF IRON Most brasses contain small percentages of iron (usually not over 0.1 per cent) which, unless removed, is precipitated as phosphate and weighed with the zinc. PROCEDURE.--To the solution from the precipitation of copper and lead by electrolysis, add dilute ammonia (sp. gr. 0.96) until the precipitate of zinc hydroxide which first forms re-dissolves, leaving only a slight red precipitate of ferric hydroxide. Filter off the iron precipitate, using a washed filter, and wash five times with hot water. Test a portion of the last washing with a dilute solution of ammonium sulphide to assure complete removal of the zinc. The precipitate may then be ignited and weighed as ferric oxide, as described on page 110. Calculate the percentage of iron (Fe) in the brass. DETERMINATION OF ZINC