When copper is first obtained from its ore, it is about 99% pure. The impurities - mostly silver, gold, platinum, iron, and zinc-decrease the electrical conductivity of the copper enough that even 99% pure copper must be further refined before it can be used in electrical wire.
The impure copper is used as the anode in an electrolysis cell that contains a solution of copper sulfate and sulfuric acid as electrolyte (see Figure 20.24). The cathode is a thin sheet of very pure copper. When the cell is operated at the correct voltage, only copper and impurities more easily oxidized than copper (iron and zinc) dissolve at the anode. The less active metals simply fall off the electrode and settle to the bottom of the container. At the cathode, copper ions are reduced, but the zinc ions and iron ions remain in solution because they are more difficult to reduce than copper. Gradually, the impure copper anode dissolves and the copper cathode, about 99.96% pure, grows larger. The accumulating sludge-called anode mud-is removed periodically, and the value of the silver, gold, and platinum recovered from it virtually pays for the entire refining operation.
When copper is first obtained from its ore, it is about 99% pure. The impurities - mostly silver, gold, platinum, iron, and zinc-decrease the electrical conductivity of the copper enough that even 99% pure copper must be further refined before it can be used in electrical wire. The impure copper is used as the anode in an electrolysis cell that contains a solution of copper sulfate and sulfuric acid as electrolyte (see Figure 20.24). The cathode is a thin sheet of very pure copper. When the cell is operated at the correct voltage, only copper and impurities more easily oxidized than copper (iron and zinc) dissolve at the anode. The less active metals simply fall off the electrode and settle to the bottom of the container. At the cathode, copper ions are reduced, but the zinc ions and iron ions remain in solution because they are more difficult to reduce than copper. Gradually, the impure copper anode dissolves and the copper cathode, about 99.96% pure, grows larger. The accumulating sludge-called anode mud-is removed periodically, and the value of the silver, gold, and platinum recovered from it virtually pays for the entire refining operation.
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