plating, anodizing, & finishing Q&As since 1989
Treatment of Acid Gold Cyanide Baths
Hi. I'm trying to gather information on treating acid gold cyanide baths. I work in a reel to reel shop. We use cobalt hardened acid gold cyanide baths for gold plating. These baths initially are a purple color attributable to the cobalt brightener (I believe Cobalt (+2) EDTA). Over time the baths develop a straw brown color. My best guess (from talking to others on this subject) is that the cobalt is gradually converted to Cobalt (+3) Cyanide. There is a treatment process for gold baths which I have learned about. Although the exact procedure and interpretation of the purpose varies significantly depending on who you're talking to. The process involves heating the bath, adding H2O2, adding EDTA, carbon treating, adding free cyanide. I'm hoping some of you can supply me with some information on this procedure.JD Kronicz
- Hanover, Pennsylvania
Why do you want to treat the bath? If there is a quality issue, it makes sense (e.g. deposit hardness out of limits, brightness gone, etc.). If you just like the color of a new bath forget it! On the other hand, if the bath performance has deteriorated, check with the supplier. They are the ones who will know what to do. If you screw up a gold bath it's a pretty expensive mistake. I would certainly want the vendor on my side!
microwave & cable assemblies - Mesa (what a place-a), Arizona
I am looking for information on treating a acid gold cyanide bath (cobalt hardened). We have an in house procedure but I'm not sure it is effective or exactly what is going on chemically. It involves heating to 150F, adding H2O2, EDTA and carbon treatment. I'm hoping someone can give me some insight into treating these types of baths. It is thought that periodic treatment by this method is beneficial. I do know that the baths go from their makeup color of purple to a straw brown after a few months use. I have been told that the color change is largely due to the conversion of cobalt from the +2 valence to the +3 valence. Whether the treatment has been done historically for cobalt reconversion or organics removal or a combination of the two I am not sure.
A related issue is in regards to the cobalt. We measure cobalt by ICP and therefore measure total cobalt. We make adds to the bath based on ICP data. Over time I suspect the active form is much lower than the total and so we probably have much less active cobalt in the bath than we should. Any thoughts or experience with this issue?
Any help is greatly appreciated.
- Hanover, Pennsylvania
First of all, I am not too familiar with cobalt acid gold baths, ( I work with nickel acid golds ) but here is what I think is happening:
1.- For each gold atom plated, two cyanide ions are released into the bath. Most of the cyanide ions leaves the bath as HCN gas at acid pH.
2.-Some cyanide ions complexes the cobalt, forming a complex stable at acid pH, thus the change in color.
3.-Hot peroxide treatment destroys the cyanide complex, ( destroying the cyanide ) ,the organic reaction products being absorbed in the activated carbon ( some other organics are absorbed as well )
4.-EDTA is added to complex the cobalt and restore its brightening capabilities.
Only the EDTA complexed cobalt will function as a brightening agent in acid gold, the cyanocobalt complex being to strongly complexed to co-deposit with the gold.
Hope this will help you.Joal Paiva
Can somebody please tell what precautions to take while working on acidic gold cyanide baths specially against HCN gas liberated during plating. Also please tell how to detect that HCN gas in surroundings is exceeding the maximum safety amounts.
- MUMBAI, Maharashtra, INDIA
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