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I've just got hold of a nice little PCB etching kit, which uses ferric chloride as the etchant. I know it's re-usable for at least a few etches (though the kit doesn't actually specify an estimated re-use count) but after that I'll need to dispose of the used chemical.

I've read the accompanying safety sheet thoroughly, which states that I should use add sodium carbonate to form ferric oxide, which can then be disposed of. It's not very clear about the details though.

It seems to indicate that a 1:1 mixture of Na2CO3 and FeCl3 should be achieved, but (from what I can tell) the Na2CO3 is usually sold as a powder. How do I measure a 1:1 mixture between the two?

As a side question, I see a lot of people here using boiling water with their ferric chloride etchant, but the instructions for this kit state 21°C - 24°C is the optimal temperature. Are they just trying to cover their asses in case someone gets boiling hot corrosive liquid on themselves, or am I missing something?

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    \$\begingroup\$ What the heck!! The bottle of Ferric Chloride I got from Radio Shack says to pour it down the drain! There used to be articles published about using ferric chloride for certain water purification processes. All the answers here are treating FeCl3 like it was Hexavalant Chromium! \$\endgroup\$ – londo Mollari Mar 29 '18 at 22:25
  • \$\begingroup\$ @londoMollari The problem of disposal isn't the FeCl3. That's pretty innocuous. The problem is the copper. After etching the FeCl3 becomes a mixture of FeCl3 (unused etchant), FeO (rust), and CuCl2 (Copper(II) Chloride). It's that Copper(II) Chloride that's the problem - you can't just tip it down the drain. You gotta get rid of that copper from solution before you can dispose of the rest of it. Unspent FeCl3 can be poured down the drain I guess. But not the CuCl2. \$\endgroup\$ – Majenko Apr 30 at 12:07
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Washing soda is the harmless of the two reagents, so it doesn't matter if your mixture contains some excess, unreacted soda.

Soda is cheap enough that you don't have to worry about optimizing your use to the absolute stoichiometric ratio.

You can use baking soda also (bicarbonate).

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  • \$\begingroup\$ Isn't baking soda sodium bicarbonate? Pretty sure NaHCO3 and Na2CO3 aren't the same thing. I don't know the chemical process that well, but I really don't like the idea of mixing hydrogen and chlorine atoms as a safe disposal method. \$\endgroup\$ – Polynomial May 9 '12 at 23:13
  • \$\begingroup\$ Yes, sorry. Commonly speaking, one is "washing soda" and one is "baking soda". However, either one can be used to neutralize ferric chloride. I will fix my answer to say washing soda. \$\endgroup\$ – Kaz May 9 '12 at 23:24
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    \$\begingroup\$ @Polynomial better avoid salt water then. Tons of chlorine and hydrogen atoms all up in there. Actually you could bleed yourself to remove some of those harmful chlorine atoms from your body. \$\endgroup\$ – Nick T Dec 25 '14 at 8:22
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Put Sodium carbonate little by little to minimize frothing that leads to messy spills.

Keep adding it until no additional fizz. It might take hours... or even days, depending on the amount you have.

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Heat really accelerates the reaction but needs to be temperature controlled and pumped and evenly spread for consistent etching. Room temp soak is just slow and simple, I think.

Disposal info

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  • \$\begingroup\$ In which case, I'll take the middle ground and try warm water (e.g. ~40C) to start off with. \$\endgroup\$ – Polynomial May 9 '12 at 23:20
  • \$\begingroup\$ Useful hint in that link about the indicator paper. Nice one. \$\endgroup\$ – Polynomial May 9 '12 at 23:33
  • \$\begingroup\$ 404 Page not found \$\endgroup\$ – CL22 Feb 6 at 23:47
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Your disposal procedure will depend entirely on your municipality. You need to call city hall, or the relevant department thereof.

That said, if you want to avoid this headache (and why wouldn't you?) look into photoemulsion.

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  • \$\begingroup\$ Can you explain how photoemulsion removes copper from your clad to reveal the circuit? \$\endgroup\$ – Kaz May 9 '12 at 23:10
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    \$\begingroup\$ I forget the exact process, but I believe the boards are specially treated. It's been awhile since I reviewed the process. Protip: simply because you tell city hall that you have safely disposed of a dangerous chemical does not mean they will believe you. \$\endgroup\$ – Joe Stavitsky May 9 '12 at 23:11
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    \$\begingroup\$ I live in the UK, so we don't really have "city hall". \$\endgroup\$ – Polynomial May 9 '12 at 23:15
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    \$\begingroup\$ I think that if you have a choice between talking to authorities and not talking to authorities, the most rational choice is not to talk to authorities. \$\endgroup\$ – Kaz May 9 '12 at 23:21
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    \$\begingroup\$ If you're concerned about disposal of chemicals, consider CuCl etching (you know, that process involving a tank with air bubbles, hydrochloric acid and copper.) This is reused "indefinitely". \$\endgroup\$ – Kaz May 9 '12 at 23:31
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This thread is old now and I am sure the question has been answered but for future users who are worried about the stoichiometric ratio. I personally recorded the initial weight of my storage container on a post it and stuck it on my cork board in my work space. I just subtract that from the total current weight and use that to setup a proper conversion to find the amount of sodium bicarbonate to add.

-happy etching

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