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When I use $\ce{FeCl3}$ solution for etching circuits it gives off an acidic smell. I was wondering what is the cause of the smell. I don't think it's caused by the $\ce{Cl2}$ from $\ce{2FeCl3->2FeCl2 + Cl2}$ because chlorine has a different smell.

The product used is the one in this picture.

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  • $\begingroup$ Welcome to Chemistry.SE. Could you tell us a little more about your $\ce{FeCl3}$ solution? Is it just $\ce{FeCl3}$ in water? Are there additional components? Do you make the solution or is it commercial? It if is a commercial product, identifying it would be helpful. This is information that can help us provide a better answer. I also cleaned up the chemistry in your post. We have the MathJax plugin for chemistry and maths. You can find out more on our help center page. $\endgroup$ – Ben Norris Oct 13 '13 at 11:02
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    $\begingroup$ Thanks for the tips Ben. It is a commercial product but its label isn't very informative. I think there might be impurities as it is used for etching hobby printed circuit boards. vitacom.ro/images/products/PictureVitacom/media/c/… $\endgroup$ – titus Oct 13 '13 at 15:35
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The only clue I have been able to find is a one-off sentence in the Wikipedia article for $\ce{FeCl3}$, for which no reference is given:

Iron(III) chloride undergoes hydrolysis to give an acidic solution.

If so, then the reactions are probably those along the conversion of $\ce{FeCl3}$ to $\ce{Fe2O3}$, producing hydrochloric acid $(\ce{HCl})$. The hydrogen chloride gas that escapes from the solution has a pungent acidic acrid smell.

Possible reactions include:

$$\ce{FeCl3 + H2O <=> FeOCl + 2HCl}$$ $$\ce{2FeCl3 + H2O <=> Cl2FeOFeCl2 + 2HCl}$$ $$\ce{2FeOCl + 2H2O <=> Fe2O3(s) + 2HCl}$$

Because these reactions are in equilibrium, solutions of $\ce{FeCl3}$ can be stabilized by addition of $\ce{HCl}$. Addition of $\ce{HCl}$ also promotes the formation of the tetrachloroferrate ion $\ce{FeCl4-}$, which is more resistant to hydrolysis.

$$\ce{FeCl3 + HCl -> FeCl4- + H+}$$

Ferric oxide (Iron(III) oxide $\ce{Fe2O3}$), is insoluble. Does your solution develop reddish precipitate as it ages? That would be evidence in favor of hydrolysis as the source of $\ce{HCl}$.

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  • $\begingroup$ I observed some murky-brown precipitate when I used the solution for etching copper. I thought it was $\ce{Cu2Cl2}$ which precipitates. Now I've looked it up and $\ce{Cu2Cl2}$ is white not brown. $\endgroup$ – titus Oct 14 '13 at 7:18
  • $\begingroup$ I learned something useful. Adding $\ce{HCl}$ to the solution will make it last longer by converting $\ce{Fe2O3}$ to $\ce{FeCl3}$. Thanks Ben! $\endgroup$ – titus Oct 14 '13 at 7:25
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    $\begingroup$ As you noted, the hobby-reagent grade chemicals are quite impure, so what you smell is either HCl, which you can buy separately and check, or any of the impurities, which reacts in this very dirty mixture to anything. Probably not worth research. It also causes coloration of CuCl (should become CuCl2 soon). BTW, I etch PCBs with much cheaper process: sites.google.com/site/ssavcoblog/electronics/pcb $\endgroup$ – ssavec Oct 14 '13 at 7:33

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