Electroplating rhodium onto aluminum using rhodium III chloride hydrate

A. Hi Denis. You can only plate an ion out of solution and onto the cathode if it is in solution. If a salt is virtually insoluble in water, then there can be virtually no metal ions in solution, so you can't plate them out. If the salt is highly water soluble, you can reduce the metal onto the cathode with electricity. I don't know much about Rhodium III chloride, but I understand from googling that RhCl3(H2O)3 is a reddish hydrated salt that is soluble in water whereas the dense brown powder RhCl3 is not. sulfuric acid may aid the conductivity of the solution, and it may prevent the solution from going alkaline when some of the water is hydrolyzed into H2 gas and OH- ions.

Simply plating a metal out of solution, though, doesn't guarantee it will necessarily generate a sound, low-stress, robust, worthwhile, plated layer. In fact, it probably won't. My understanding is that almost all rhodium plating is done from rhodium sulphate rather than rhodium chloride. Why? Well, all I can say is that much of our knowledge of how to plate robustly (as opposed to simply getting some metal to deposit) is empirical knowledge rather than being based on first principles.

Aluminum oxidizes instantly and you can't electroplate successfully onto oxidized metal. The most common way to electroplate onto aluminum is to first "zincate" it (immerse it in a strong alkaline solution of zinc so there is a replacement reaction or immersion plating reaction, which leaves a surface of zinc. Then you can either cyanide copper plate it or alkaline electroless nickel plate it. Do not consider the use of cyanides without personal supervision from someone familiar with them. Good luck.

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by Reid & Goldie
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A. Dear Steven,

Without knowing what your aiming at, it's difficult to answer.
If you use the cyanide electrolyte for the Gold, you should keep in mind that the cyanide is unstable by nature and is oxidised to all type of by-products like carbonate and longer chained organics, influencing your deposition potential. At the same time, the Au is partly oxidised to the 3-valent form and without a suited electrolyte matrix you can even have the formation of colloidal Gold, influencing your project.
For the "simple" chloride salt, you will have stability problems, the Gold will relatively quickly be reduced to metallic colloidal gold and if you're "lucky" you will have a beautiful gold film on your glassware.
Maybe you could extend your question a bit more to what you want to investigate exactly. I think the big suppliers like Enthone, Technic, Rohm & Haas, Blasberg, Hereaus, Omnicore and the ones I forgot to mention, will probably help you with the delivery of a proper, stable electrolyte and advise on your project as well. Use the website of the World Gold Council (www.gold.org) for articles on Gold, Spend a few days to visit a good library to read books like "Gold Plating Technology" .

or other books on precious metal plating, and ask more on this forum if you like. I cannot believe that the answer Au+ + 1e- -> Au, or Rh3+ + 3e- -> Rh answers your question, so feel free to ask in more detail here too.

Best regards,