Suppose we have an electrolytic cell with copper(II) sulphate solution as the electrolyte, and lead metal as the electrodes.

Will lead ions be discharged from the anode into the solution? What will be left in the electrolyte after we allow this experiment to run for a while?

My question stems from the fact that during electroplating, either carbon (which is always taken to be inert) or copper is always used but no one ever discusses what happens if the electrode is made of a different metal from the cation. Will the electrodes be active in such a scenario?

  • $\begingroup$ For given cathode material, surrounding electrolyte and forced potential, what happens on cathode does not depend on processes on anode and vice versa. // Pb anode would get oxidized to insoluble PbSO4(s), like in an acid lead battery. $\endgroup$
    – Poutnik
    Nov 13 '21 at 17:11
  • $\begingroup$ Also, consider that copper ions can oxidize lead metal even without powering the electrolysis cell. We normally do not want that parasitic issue. $\endgroup$
    – Ed V
    Nov 13 '21 at 17:17
  • $\begingroup$ @Poutnik Will this PbSO4(s) form a coating around the existing Pb anode or will it sink to the bottom of the solution? Also, just to make sure I understand you correctly, it doesn't matter what metal the anode is made of, it will get oxidized so long as it is not carbon or platinum. Is this correct? $\endgroup$
    – Tham
    Nov 13 '21 at 17:36
  • 1
    $\begingroup$ Remember oxidation of water is a thing. With silver, water gets oxidized. Likewise with gold. $\endgroup$
    – Ed V
    Nov 13 '21 at 17:39
  • 1
    $\begingroup$ @Tham It is the topic for an independent question, or rather for studying of electrochemistry and chemistry of elements. $\endgroup$
    – Poutnik
    Nov 13 '21 at 18:11

As with so many questions in chemistry, the answer to "What will happen if we do x?" is going to be "It depends." unless x is very precisely defined. While an electroplating setup that contains two electrodes and a solution may seem conceptually simple, there are several intertwined things to consider:

  • The composition of the electrodes
  • The composition of the solution (and solvent)
  • What voltage is applied to the cell

A good starting place to see what might happen is to examine a table of standard electrode potentials. This is essentially a table of how thermodynamically favourable half-reactions are under standard conditions. You will quickly see that there are usually many possible half-reactions for any given element (and you must consider also reactions that could happen with the solvent, e.g., the oxidation or reduction of water in an aqueous solution). Once you have a list of potential half-reactions, the Nernst equation can then be used to adjust the potentials for the actual concentrations present in the cell and you will be left with a set of possible half-reactions and their relative thermodynamics. The voltage being applied serves to tilt this thermodynamic balance to one side or another.

This approach alone, however is unlikely to give a good impression of what will actually happen in a given cell arrangement because it neglects the other important component of a chemical process: kinetics. While a given reaction may be thermodynamically favourable, depending on the circumstances, it may proceed very slowly, making a less favourable reaction more significant. This may depend on many factors, such as catalytic effects by the electrode (e.g., reduction of aqueous hydrogen ion to hydrogen gas occurs at a much higher rate on platinum electrodes versus mercury electrodes), and mass transport (a reaction can only occur if the reagents are present at the electrode surface, so reactions may proceed slower if it is more difficult for the reagent to reach the surface, e.g. an electrode recessed into a tube).

So it should be clear that electrochemical cells can be very complicated, but in the case of electroplating, there is mostly only a single thing we care about: taking the cation in solution and reducing it onto the workpiece without interference, and all the parameters are selected to achieve this condition.

Now, to touch more directly on your question: why is electroplating normally done with a cation in solution and a matching (or inert) counter electrode and what happens if this is not true. While electroplating, a metal cation in solution is being reduced on the workpiece, and one or more oxidation reactions are occurring at the counter electrode to maintain electroneutrality in the cell. These oxidation reactions are necessary, but not the intent of the process, so it is desirable to ensure that the reactions that occur do not affect the plating process.

Fundamentally, when you have a counter electrode that matches the metal you are trying to plate, as the counter electrode dissolves, it simply replaces the metal cations in the solution that are being plated on the workpiece. An "inert" counter electrode is an electrode that will not produce products that interfere with the plating reaction; on something like a carbon electrode this may simply be the evolution of oxygen, which leaves the cell as a gas and (assuming the counter electrode is not so close to the workpiece that the bubbles cling to it) does not interfere with plating. If you chose a counter electrode that will oxidize and dissolve into the solution (a non-inert electrode), the cations may eventually diffuse towards the workpiece and be plated along with the desired metal, creating an impure coating (and generally adding to the list of possible half-reactions assembled above). The electrode may also interfere in other ways, e.g., if your counter electrode forms an electrically insulating oxide layer, it may quickly cover the electrode and stop any further plating.

There are certainly materials other than carbon that may be inert in a typical electroplating cell, though carbon electrodes are inexpensive compared to something like platinum, and there is a way to separate the two halves of the electrochemical cell so that the products of the counter electrode do not contaminate the plating solution using a salt bridge, though this adds unnecessary complication and may impose some limits to how quickly the plating reaction can occur. There may also be an advantage to using a matching metal counter electrode rather than an inert counter electrode in that as the counter electrode dissolves, it is replenishing the supply of metal cations in the plating solution, and there is a mass transport advantage of not needing an aqueous reactant e.g., $\ce{OH-}$ to diffuse to the electrode surface, since the electrode itself is the reactant.

So in essence, the reason to use an inert or matching counter electrode are to avoid adding things into the plating solution that will interfere with the coating process, and while there is no general reason to not use a counter electrode made from a different metal, so long as it is inert, graphite is often chosen because of its cost, especially at large scales.

  • $\begingroup$ Such a comprehensive answer; thank you so much! $\endgroup$
    – Tham
    Nov 25 '21 at 12:42
  • $\begingroup$ No problem. I got a bit carried away, but I hope it was helpful. :P $\endgroup$ Nov 26 '21 at 4:45

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