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I tried a simple water electrolysis experiment at home, with $\ce{NaCl}$ as electrolyte, and a $12 \,\rm{V}$ battery.

My wires were made of copper and $\ce{H2}$ bubbles were only forming at negative terminal (cathode). On the positive side (anode), nothing was happening.

After $3$-$4$ minutes, water started to change into a greenish-blue color and I realized that it must be copper reacting with chloride ions. After $15$ minutes, my solution was light blue with exceed $\ce{NaCl}$ at the bottom, after taking the wires out the solution started to turn into a yellow color in about $10$ minutes.

What was the yellow solution and why didn't cathode produce any oxygen?

I'm also worried that the yellow solution might be chlorine because I later boiled the solution.

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The color of the solution is probably due to the presence of blue copper(II) ions ($\ce{Cu^2+}$) and greenish tetrachlorocuprate(II) ions ($\ce{[CuCl4]^2-})$.

The change in color comes from the equilibrium

$$\ce{Cu^2+ + 4 Cl- <=> [CuCl4]^2-}$$

Low concentrations of chloride favor the formation of the blue copper ions. High concentrations of chloride favor the formation of the greenish tetrachlorocuprate ions.

If a significant amount of chlorine had been created you should have observed bubbles at the anode and the typical chlorine smell.

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  • $\begingroup$ I wouldn't expect any significant amount of chlorine (or oxygen, for that matter) to appear in this setup. Dissolving the anode is so much easier. $\endgroup$ Apr 16, 2016 at 17:43
  • $\begingroup$ Well I didn't observed any bubbles at the anode but I think it was chlorine I was smelling.(smelled just like swimming pools) I've just noticed bright green color around the anode, I'm assuming that is tetrachlorocuprate(II), after few hours the solution now looks almost clear yet huge amount of blue and yellow precipitation are floating at the bottom. I'm leaving it outside waiting for the water to evaporate. If the blue precipitate is copper (II) or copper(II) chloride ions then what is the yellow precipitated? $\endgroup$ Apr 16, 2016 at 20:26
  • $\begingroup$ Copper(II) chloride is readily soluble in water, so I would not expect it to precipitate. I guess that the precipitate rather contains copper(II) hydroxide. $\endgroup$
    – aventurin
    Apr 17, 2016 at 16:54
  • $\begingroup$ Copper is not an inert electrode especially at 12 volts. Even platinum electrodes can dissolve at that voltage. If something is happening at the cathode something must be happening at the anode if current is flowing. $\endgroup$
    – jimchmst
    Dec 16, 2023 at 9:10

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