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HCN concentrations in plating baths

Gold plating baths can contain HCN at acidic pH's. As I understand it, the gold/cyanide bond will hold at lower pH's but the free cyanide has gassed off. Am I wrong in what I am thinking? In what concentrations can cyanide exist in an acid solution 4-6 range?

Kelly Smith
- Mount Vernon, Washington

Indeed there are acid gold cyanide plating baths much like you describe, and your understanding is correct. I don't know the maximum concentration of free cyanide (HCN or NaCN) that can stay in solution in this pH range, and it probably depends on a number of other factors like temperature, but it's not much--which is why acidifying cyanide solutions is so dangerous.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Ted is available for instant help
or longer-term assistance.

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