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topic 26978

Removing chloride from silver KCN plating bath


Q. I have a silver cyanide bath. I think high chloride is problem; I want to remove it. But I don't know how to remove Cl out of bath. Any help please? Thank you very much.

chemist - Bangkok, Thailand


A. Normally silver chloride is pretty insoluble and will precipitate as a white powder. I have never seen chloride added to a cyanide silver bath, so why do you think it is a problem? I am not sure whether the cyanide complex will be strong enough to prevent the precipitation of silver chloride. Perhaps you can give us more details of what you are doing and why you think chloride is a problem.

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


A. Where did the chloride come from? I may be wrong but I know of no silver cyanide bath that uses chlorides. I would bet that you cut costs and made the bath using silver chloride instead of silver cyanide. You probably dissolved some silver in nitric and then salted out the silver chloride, of which you dissolved in cyanide to make your bath. Cheap and safe way to go. Too bad it doesn't work :-(
You should have tried it in a beaker before making up a big batch. I've never seen high chlorides in a Ag cyanide bath before, so I don't know how it would affect the deposit. It may have worked at first but died as the chlorides built up from silver additions. Maybe it never worked well. I would bet it's the latter.
If I've misjudged the situation, please forgive me. I just can't figure out any other way the chlorides got in there. If I am wrong, please let me know. Now you have a big problem. You have a bunch of silver tied up in a worthless bath. I know of no way to get rid of the chlorides. If it were me, I would plate out the silver on a passivated stainless steel sheet. Then, peel the silver and sell it. Make up the bath with the right chemicals.

I would advise not making your own silver cyanide, unless you have a super fume hood and someone already experienced in doing such things. HCN, which is released, is the prison gas chamber gas.

Chris Owen
Consultant - Houston, Texas

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