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I found this question that describes PCB etching with citric acid. The answer to that question explains how hydrogen peroxide, salt, and citric acid can be used to ionize copper metal in water, which provides:

$$\ce{Cu + H2O2 + 2 H+ -> Cu^{2+} + 2 H2O}$$

The equation above excludes the citrate ion, so maybe it isn't party to the reaction. This observation prompted the following:

Question:

  • Assuming the citric acid ion is $\ce{C6H7O7^-}$, what is the equation for the neutralization reaction (I kept $\ce{H^+}$ in this, but it's been a while since highschool/college chem, this may not be represented right):

$$\ce{Cu^{2+} + C6H7O7^- + H^+ + Na2CO3 -> } ??? $$

  • What remains ionized in solution after neutralizing with $\ce{Na2CO3}$?
  • Will there be a precipitate? If so, what?

(This will inform our choices for responsible disposal. I'm not sure what other details to provide, as this seems like a pretty simple question. Let me know if you need more information.)

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  • $\begingroup$ You don’t dispose of any chemicals down the drain. Check your municipal hazardous waste disposal folks. $\endgroup$
    – Jon Custer
    Commented Mar 21, 2023 at 0:44
  • $\begingroup$ You would prepare a variant of Benedict reagent, copper kept in stable citrate complex. If you added glucose and boil, the blue solution would start to form yellow to brick read insoluble cuprous oxide. $\endgroup$
    – Poutnik
    Commented Mar 22, 2023 at 5:13

1 Answer 1

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Copper ion forms with citrate stable chalate, that is not disturbed even by addition of sodium carbonate, what is used in Benedict's reagent(*) for detection of reducing sacharides.

Copper etching:

$$\ce{Cu(s) + H2O2(aq) + H4Citr(aq) -> 2 H2O(l) + CuH2Citr(aq)}$$

Solution neutralization:

$$\ce{2 CuH2Citr(aq) + CO3^2-(aq) -> H2O(l) + CO2(g) + 2 [HCuCitr]-(aq)}$$ $$\ce{[CuHCitr]-(aq) + CO3^2-(aq) -> HCO3-(aq) + [CuCitr]^2-(aq)}$$

Copper can be eliminated by reducting agents, like glucose, or perhaps by sulphite/bisulphite, forming insoluble copper(I) oxide.

(*) Benedict's reagent is a deep-blue aqueous solution. Each litre contains:

$\pu{17.3 g}$ copper sulfate
$\pu{173 g}$ sodium citrate
$\pu{100 g}$ anhydrous sodium carbonate or, equivalently, 270g sodium carbonate decahydrate

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  • $\begingroup$ Does no positive ion exist in solution after neutralization? I ask because (perhaps this is where my chem knowledge breaks down) I had always thought you needed both positive and negative ions in the solution. You wrote $\ce{-> HCO3-(aq) + [CuCitr]^2-(aq)}$ and both ions are negative. So is the result really a solution of only negative ions? $\endgroup$
    – KJ7LNW
    Commented Mar 25, 2023 at 0:38
  • $\begingroup$ There are no solutions with only negative ions. Positive ions like Na+(aq) are called here bystanding/spectator ions, not taking part in reactions, just maintaining electroneutrality. $\endgroup$
    – Poutnik
    Commented Mar 25, 2023 at 4:24
  • $\begingroup$ So the NaCl that was added to the original etching solution remains Na+ and Cl- through the entire process, from etching to neutralization? $\endgroup$
    – KJ7LNW
    Commented Mar 25, 2023 at 4:30
  • $\begingroup$ True for Na+. Cl- takes part in parallel reactions of oxidation to Cl2 by H2O2, reaction of Cl2 with Cu and forming weak complex with Cu^2+. $\endgroup$
    – Poutnik
    Commented Mar 25, 2023 at 6:07
  • $\begingroup$ So does Cl2 remain in solution, or can it come out as Cl2(g)? $\endgroup$
    – KJ7LNW
    Commented Mar 26, 2023 at 0:24

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