First off, you seem to have confused atomic number, which is the number of protons in the atom, with atomic weight. The atomic weight of copper is 63.5 and the atomic weight of FeCl3 is 162 (molecules have no atomic number).
More important, you need to define the equation, You seem to be using this equation:
2FeCl3 + Cu ===> 2FeCl2 + CuCl2
Certainly, that is one appropriate equation. But are there others? How can you explain that CuCl2 itself can be used to etch copper? Is it possible that some of the ferrous (II) chloride could be converted to an iron oxide, thus leaving more chloride to form copper chloride? Or, perhaps, the copper chloride converts to copper (II) oxide, which also releases chloride? Also, remember that copper has two oxidation states that, so Cu2O and CuO are both possible. When doing etching, particularly if there is excess HCl, several reactions are occurring.
Simply, the chemistry involves oxidation, so oxygen in the solution can also play a part, and in the presence of excess HCl, oxygen can play a large part.
The article by Adam Seychell has been referenced here before times and is posted on the site. He explains how CuCl2 can be used as an etchant and be continuously regenerated by bubbling air through the solution with occasional addition of HCl.
Finally, when people refer to ferric chloride as an acid, remember there are different definitions for acids. When dissolved in water, ferric chloride produces and acidic solution.
As for the original question, don't worry about the ratio of ferric chloride to copper. Some people actually just sponge it on and get quite rapid etching. The advantage of sponging is that exposure to oxygen is maximized. Others spray the ferric chloride, which is also a rapid method. Another method is to put both the PCB and ferric chloride in a plastic bag and squish it around. If you use a bath to immerse the PCB for etching, remember that the concentration of ferric chloride in the bath will be about 40%. If you have 1 oz copper clad for your PCB, that is 1 oz of copper per square foot. Only some of that is removed. So, it is unlikely you could ever run into a situation of having too little ferric chloride assuming you have sufficient liquid present to cover the board.
John
----- Original Message -----
From: smilingcat90254
To: Homebrew_PCBs@yahoogroups.com
Sent: Thursday, November 22, 2012 9:51 PM
Subject: [Homebrew_PCBs] Re: Ratio of Ferric Chloride to copper?
hmmm... can't really go by volume of FeCl3 (Ferric chloride) and volume of copper. Wht you need to know is how many moles of each (chemists way of counting number of atoms or molecules) will react together. read about atomic weight, Copper weighs 29gram/mole whereas FeCl3 weighs 77g/mole.
And now you want to know how much ferric chloride can "eat" copper.
It takes two molecules of ferric chloride to "eat" one atom of copper and turn it into one molecule of Copper chloride (CuCl2). I think it was...
So you need 154g of ferric chloride to etch 29g of copper.
You have to know the concentration of your ferric chloride solution.
BTW, ferric chloride isn't an acid, its a salt like sodium chloride (table salt).
hope this answers your question.
--- In Homebrew_PCBs@yahoogroups.com, "RickS" <rgsparber@...> wrote:
>
> Does anyone know the ratio of needed Ferric Chloride by volume to the volume of copper being removed? I am assuming 110F for the acid and constant agitation.
>
> Thanks in advance,
>
> Rick Sparber
>
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