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I've been experimenting using electroplating, and I recently made a solution by placing two copper pieces connected to a voltage source, in water. I suspect that this created copper(II) hydroxide, as it matches the color of that specific solution (green-blue) and doesn't stay dissolved in the water (it sinks to the bottom after a day).

A lot of people use copper(II) sulfate for an electrolyte during copper electrolysis, so I assumed $\ce{Cu(OH)2}$ would work the same, but if doesn't seem to be. A coin was attached to the negative wire, copper was attached to the positive wire, but attempting to copper plate a coin in this solution has failed.

Why is that? Both copper(II) hydroxide and sulfate have $\ce{Cu^2+}$. Shouldn't that be attracted to the negative wire, and append the copper to the desired item? Why does $\ce{Cu(OH)2}$ not behave like $\ce{CuSO4}$ during electrolysis?

I don't know too much about chemistry so if you explain it in simple terms, I would appreciate it!

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If you are going to do anything differently than is normally done, you should understand what you are going to do and why it should work.(*)

The essential (but often not sufficient) conditions for successful electroplating by any metal are:

  • The used metal salts/compounds are soluble, so ions can migrate within the solution in applied electrostatic gradient. That is not the case of copper in copper hydroxide nor alkalic copper carbonates, which are insoluble.
  • The metal salt has sufficient concentration, so electrolytic current reacts primarily with this salt, at least on the cathode.
  • The current density on the plated electrode is low enough ( typically $\pu{10 mA/cm^2}$).

The solution composition for successful creation of shiny metal surface can be tricky, based on intensive empirical reasearch, involving solution modifiers like sugar or soluble polymers. The goal is mostly decreasing ion mobility and/or the rate of electrode reaction to support even metal deposition.


(*) It does not fully apply in the basic research, which uses often "what if.." approach, but this is not the case.

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