The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Anion supporting salt common-ion effect

I have a question about plating baths. Why does the supporting salt always have the same anion as the metal salt? Doesn't this reduce the solubility of the metal salt (common-ion effect)?

Shane Beattie
Research scientist - Halifax, Nova Scotia, Canada
2004

---

You are right, there will be a common ion effect, but the concentrations of metal salts in a plating bath are usually well below the solubility limit of that salt. There are numerous reasons for choosing particular anions, such as their inertness as far as the plating process goes, their benefits on the dissolution of the anode, or the conductivity of the electrolyte. It is not advisable to have a wide range of anions in a bath because they can cause unwanted characteristics in the final metal deposit. It is also sometimes the case that the cations are tightly complexed with other anions that prevent a slightly insoluble salt being crystallised out by the common ion effect.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004