The authoritative public forum
  for Metal Finishing since 1989
-----

Details about plating's redox reactions?

2007

I have been pondering this for a while and I was wondering if someone could help with a few questions about the electro-chemical processes in electroplating.
I know that the anodic electrode undergoes oxidation and then the dissolved metal ions from the anode are reduced upon contact with the cathode. But-
1) Am I correct in thinking that the electrons flowing through the circuit are from the anode's metal atoms lost in the oxidation process?
2) What is the electron transfer mechanism when an insoluble electrode is used for the anode in say for instance a Silver Cyanide Strike?
3) Why is it necessary for the cathode and anode to be placed in a continuous solution? To clarify this problem, take the electrolysis of water for example-

anode: 2H20 -> 02 + 4H+ +4e-
cathode:2H20 + 2e- -> 2OH- + H2

All constituents of both sides of each reaction are present whether or not the electrodes are in the same solution or not.

Thanks for any assistance.

Mark Wallace
Lab Tech - Florence, South Carolina

---

2007

1. Yes in the case of your soluble anode. But for your subsequent questions it may be better to view the situation as "electrons are sucked from the anode by the power supply". If you state it that way, then you can say that the effect of this may vary depending on the solution and what the anode is made from.
2. If the anode is not dissolving, i.e., is not being reduced, i.e., then the electrons flowing to the anode side of the power supply must be coming from elsewhere. And the elsewhere is components of the solution, i.e., the water and the complex cyanides dissolved in them. When you use insoluble anodes you can't have an equilibrium situation, things get exhausted, and that's one of the disadvantages.
3. The metallic wiring path carries electrons from the anode to the cathode. For current to flow, it is neccesary that the solution either conveys those electrons from the cathode back to the anode, or that there is a transfer of positively charged atoms from the anode to the cathode.

But what are you trying to do and why?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

---

2007

Thanks for your response. For now I am just trying to learn a little more detail about what exactly is going on inside the bath. I want to be able to trace the components of the bath and see how they interact together in a system.

In the case of the silver strike bath because we are using insoluble anodes I know that the silver ion supply must be replenished but if I understand correctly one of three things has to happen to allow the flow of electricity.
Positively charged metal ions previously oxidized at the anode move in the direction of the cathode. (which can't be the case here because we are using insoluble anodes)
CN- ions move toward the anode and become oxidized. (I don't know but this doesn't sound right ?)
Water begins to split at the anode releasing electrons through the circuit and H+ ions into the bath. (I don't think this could be it either because I am not dead yet)
What am I missing?

In the case of the electrolysis, if we are splitting water at both electrodes and hence have an electron source and an electron 'sink' why is there any need for any medium between the electrodes outside the circuit.

Thanks,

Mark Wallace
Finishing - Florence, South Carolina

---

2007

I agree with the majority of what Ted has said. The main thing I disagree with is that when the metal in the anode is dissolved, it is oxidised, not reduced. This process involves the loss of one or more electrons to create an ion that is depleted of a negative charge and is hence becomes a positive ion. This is then attracted to the electron-rich cathode, where the positive ion regains its lost electron(s) and becomes a solid metal again.
As far as circuits are concerned, it is essential that the electric circuit is complete, like a simple circular model electric train set, so that the electrons can keep on going round and round (just like the electric train!). If the circuit is broken, the electrons cannot flow and the process stops.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

---

Yes, I misworded that, Trevor; thanks for the correction.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007