Iron (III) Chloride to dissolve copper?

[jowrose]

Ok, I have some FeCl3 that I have created, and somewhere I heard that this is used to dissolve copper. I took some of my (hopefully) FeCl3, put it in a test tube, and placed some copper inside, and there was no reaction. Does this reaction take a long time, or is what I think is FeCl3 something else (perhaps with lots o' impurities)?

 

While I'm at it, I might as well ask why this reaction works, seeing as copper is a very unreactive metal.

[woelen]

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.

[YT2095]

another thing worth mentioning is that although copper will dissolve quite well in ferric chloride, there`s no violence to this reaction that gives a clue that it`s working, and as with PCB etching the reaction takes place better if the liquid is kept moving we use tray agitators that rocked the soln from side to side.

it takes about 20 mins to etch the thickness of the copper on a pcb and that becomes increasuing longer as the soln gets worn/used.

you don`t mention what copper you put in, but something like a penny could well take half a day to vanish entirely!

[jowrose]

I put in some copper wire a few mm thick. It seems that it has gotten thinner, and the solution has turned a very dark brown. Only half of the wire was submerged, and the other half has grown some brown compounds on the surface.

 

Thanks woelen and YT for the help. I'll wait some more, and if it isn't working I'll add some NaCl and some HCl to add chlorine and purify the FeCl3.

 

Every day, I learn how incredibly oversimplified my high school chemistry class is...

[woelen]

Yes, indeed, the highschool chemistry is oversimplified. That is not a problem (otherwise we would not be able to learn the subject), but what I think is a problem is that no one tells you that things are oversimplified. Many people think that the things they learn at school are an accurate description of reality. So, pupils should be made more aware of the implifications.

 

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The very dark brown stuff is a mixed oxidation state complex of copper ions and chloride ions. It contains Cu(+), Cu(2+) and Cl(-) ions, in such a way, that the copper has an oxidation state somewhere between +1 and +2. Yes, that also is possible! Atoms can have non-integer oxidation states.

 

Probably, the brown stuff can be described as Cl-Cu-Cl-Cu-Cl, with the copper ions having (average) oxidation state +1.5 and chlorine having oxidation state -1.

 

If you add some oxidizer (more FeCl3, but dilute H2O2 also works), then all copper is oxidized to oxidation state +2 and the deep brown/black color disappears again.

[ajkoer]

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Quote from Woelen:

"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."

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My comment is that having a very concentrated solution of FeCl3 with added NaCl, means to me that in the hydrolysis of FeCl3:

 

2 FeCl3 + 6 H2O <---> 6 HCl + 2 Fe(OH)3

 

one has changed the 'activity level' of the normally dilute (and weak) HCl to a much stronger acting acid. Hence the witnessed increase in etching behavior.

Edited April 23, 2013 by ajkoer

[mrzamani]

Hi everyone,

 

Do you know if the Cu FeCl3 etches silicon or not?

I have ~40nm copper on a patterned silicon wafer. I am hoping to remove the copper without damaging my patterns. Any suggestions are welcome.

 

Thanks.