A solution of FeCl3 dissolves copper metal fairly quickly. It is used as PCB etchant in electronics and the hydrated form, FeCl3.6H2O can be purchased at many electronics parts and hobby shops.
FeCl3 can act as an etchant, because of two reasons:
Fe(3+) is a fairly strong oxidizer.
Cl(-) is a strong complexing agent for copper (II) ions.
The reaction in solution often is written as
2Fe(3+) + Cu -> 2Fe(2+) + Cu(2+)
This, however, is a too strong simplification. If you put copper in a solution of iron (III) sulfate then it does not dissolve. The chloride plays an essential role:
2Fe(3+) + Cu + 4Cl(-) -> 2Fe(2+) + CuCl4(2-)
In reality even this is simplified, because iron also is present as complex with chloride. So, a slightly better description of the reaction may be
2FeCl4(-) + Cu -> 2Fe(2+) + CuCl4(2-) + 4Cl(-)
When all iron (III) is converted to iron ()II), then the CuCl4(2-) ion in turn also oxidizes copper metal quite well, where copper (I) species are formed:
CuCl4(2-) + Cu -> 2CuCl2(-)
These copper (I) species form very dark-colored mixed oxidation-state species with copper (II) species, when all iron (III) is used up. This causes the PCB-etchant liquid to turn very dark, when it is near exhausted.
You cannot simply say that copper is more noble than iron and so the reaction cannot proceed. This only is true for Fe(2+) and Cu. Fe(3+) is a more oxidizing ionic form of iron. With chloride ions present, the nobleness of copper is reduced, due to complex formation, as described above, and that effect makes Fe(3+) a sufficiently strong oxidizer to dissolve copper.
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Try the following to make your FeCl3 work:
Use a very concentrated solution, or add a lot of NaCl also. The chloride concentration needs to be really high. The complex formation is one of the main driving forces behind the reaction and that complex only is formed at very high concentration of Cl(-).
If your solution is very turbid, then you probably have impure FeCl3 with a lot of basic Fe(3+) compounds, e.g. FeCl2(OH), FeCl(OH)2 and Fe(OH)3 and all kinds of hydrated forms of that. If this is the case, then add a few drops of HCl, until the solution is clear.