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How much HCN is in acid gold bath?

A discussion started in but continuing through 2020


Hello, I would like to know the solubility of HCN in acidic gold plating bath. A bad smell gas seems to release during plating. Since cyanide species in acidic bath exist mainly as HCN, can HCN release during the deposition? Any information would be greatly appreciated.

Yigen Li

"Cyanide species in acidic bath exist mainly as HCN", does not properly describe the situation here, Yigen. If one were to acidify other plating baths like copper cyanide, zinc cyanide, or cadmium cyanide, the cyanide would (as you say) exist as HCN gas, and rapidly evolve in a terribly hazardous fashion. A number of people have died in plating shops over the years due to accidentally acidifying cyanide plating baths.

However, the cyanide in acid gold baths is very tightly complexed, to the point where acid gold cyanide baths routinely operate at pH values of 3.8 to 5.0 without evolving dangerous amounts of HCN.

Just tonight I was discussing the toxicity of acid gold cyanide with a precious metal plater and a supplier of acid gold. This is 'urban legend' not hard fact, but they felt from decades of hands-on experience that the toxicity of acid gold cyanide is probably about an order of magnitude lower than sodium cyanide.

It would be foolish for me to assert from this distance, and with so few facts in hand, that you are not smelling dangerous HCN, especially since you identify the smell with the electrolytic process. I would suggest that you install an HCN detector. And if you exhibit any symptoms of cyanide poisoning, or you detect an almond-like smell, or you have any reason at all to suspect an HCN problem, the emissions should be immediately tested. But if the only reason for your concern is a misunderstanding about whether cyanide can stay complexed in an acid gold bath, rest assured that it can.

I hope another reader , especially a gold plater, can offer additional insight.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


Yes, HCN is released at the cathode during acid gold plating. I have noticed it many times, and the concentration of the gas seems to be in proportion to the amount of plating (amps). As long as there is a reasonable ventilation system in place, I doubt it is much of a concern. The gold bath itself is stable as you said, Ted, and the fumes are emitted only during plating.

The source? Electrolytic decomposition of the normally stable gold cyanide complex. The release of the "acid" HCN gas is also one of the reasons why gold bath pH rises during plating.

Bill Vins in sunny Mesa (what a place-a) Arizona. The pool is up to 79 F. Only about another week and I'll be swimming! Time to get out of the high 90's heat!

bill vins
Bill Vins
microwave & cable assemblies - Mesa (what a place-a), Arizona 

affil. link
Hydrogen Cyanide Detector


Thank you, Mr. Mooney and Mr. Vins.

In acidic gold plating bath (pH 3.5 ~5), the principle cyanide containing species are Au(CN)2- and HCN. The cathodic electrode reaction may be written as: Au(CN)2- +2H+ +e = Au + 2HCN. Before deposition, the concentration of free HCN in the bath is very low, approximately close to zero because of the high stability of the gold (I) cyanide complex, Au(CN)2-. During plating, HCN is produced on cathode and mass transfer to the solution. It's amount is proportional to the charge passed, depending on the faradaic efficiency. The equivalent amount of HCN are equal to the amount of gold deposits, if other side reactions are ignored, and the equivalent amount of HCN is also similar to the amount of hydrogen if supposed that the efficiency is about 50 %. Probably, the concentration of HCN in solution increases from zero and reaches a certain value. Initially, HCN may be not released from solution. After reaching its solubility, HCN may release. However, the solubility of HCN may be very high. Possibly, HCN exist as polymer (HCN)x. (I did not find such data, references for the solubility of HCN gas in water can be found in literature, but the condition is much different to the acidic gold plating).

Although a bad smell gas (containing H2O, H2, CO2, +?) was noticed in gold plating, is it possible that this gas contains very low concentration of HCN? The bad smell gas was also noticed by other's nose and seemed no one was killed by the suspect of HCN in acidic gold plating (pH 3.5 to 5). So, the question is at what conditions HCN may release to kill one?

Yigen Li

Let's accept your equations; then, after equilibrium is reached, two moles of HCN evolve for each mole of gold deposited. But what happens in alkaline gold baths that accounts for their 90-98 percent efficiency, compared to 30-40 percent in acid gold? How does the cathodic reaction differ?

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha



I suspect the lower cathode efficiency of acid gold is mostly due to side reactions, especially hydrogen evolution (lots of gassing!). Some small amount goes to reducing impurities such as copper, iron, silver, etc. that get dragged into the bath and the hardeners such as cobalt or nickel, if present.

bill vins
Bill Vins
microwave & cable assemblies - Mesa (what a place-a), Arizona 


The faradaic efficiency is determined by the (faradaic and also non-faraday) reactions involved in the process. Without any side reaction, efficiency should be 100%. Besides the cathodic reduction of gold, other side reactions are also involved in gold deposition. For example, hydrogen evolution reaction (HER), the oxidation of Au (I) to Au (III), the dissolution of deposited gold to Au(I) in cyanide solution, the cathodic reduction of some additives in the bath, etc. The side reaction of HER decreases largely the efficiency.
Although the dissolution of deposited gold in alkaline cyanide solution is faster than that in acidic bath (by the way, the dissolution of deposited metal in molten electrolysis is the main reason of lower efficiency, but in aqueous bath, it is not), overpotential for HER in alkaline bath is much higher than that in acidic solution. This means HER in alkaline solution is more difficult. This may be the main reason why the efficiency of gold deposition in alkaline bath is usually much higher than that in acidic solution.
In acidic bath, codepositing alloy element (Ni or Co) also facilitates the HER, leading to a lower efficiency. The cathodic reduction of some additives in acidic bath was observed ( this may be not well known yet) and it decreases the efficiency. The oxidation of Au(I) to Au(II) results in a lower efficiency. As you know, a certain reducing agent is usually added into the gold plating bath to prevent the oxidation of Au(I).

yigen Li

August 9, 2009

thumbs up sign Thanks very very for HCN gases hazards.

Atul Chumakia
laser make - Surar, Guju

Is HCN gas released from an acid gold bath

January 22, 2020

Q. Hello,

I work every day with acid gold baths. We do not have any problem with our baths but one guy asked me an interesting question and I would like to have your opinion.

We have several acid gold baths, so we have cyanide gold in the baths and the pH is acid (between 3 and 5). The question is: Is it possible that HCN gas releases in the atmosphere if we add too much acid in the bath (by "normal" acid addition to stabilise the pH or accidentally)? If it is possible, how much acid is needed to release HCN gas and be dangerous for health?

In my point of view, cyanide is strongly complexed with Gold in the bath even in acid environment. So (CN)- is released only during plating where [Au(CN)2]- + e- -> Au + 2 (CN)-. After (CN)- can be oxidized at the anode but I do not know the reactions or products, an idea please? Therefore, HCN gas released in the atmosphere cannot occur, but some people say Yes, HCN can be released in the atmosphere if we acidify the gold baths Who is right, who is wrong? What is your point of view on this subject?

Thanks you for your help.
Best Regards

Thomas Perrier
- Berne, Switzerland

February 27, 2020

A. Hi Thomas
The double salt gold potassium cyanide is very stable. Routine gravimetric analysis requires boiling nitric/sulphuric acids to destroy it.
You may be interested in an extensive dissertation on cyanides at ...

geoff smith
Geoff Smith
Hampshire, England

February 28, 2020

A. Hello Thomas,
If I may expand some on Geoff's answer a bit. In acid gold baths the ph naturally increases, as you know. To lower the ph citric acid is normally added, some platers add 5% sulfuric. I don't prefer sulfuric or recommend it however. As you mention, the cyanide in the complex is oxidized at the anode and becomes potassium cyanate, which is an organic. This is the main reason why periodic carbon polishing is key. As Geoff states, PGC salts are stable and as long as the ph doesn't drop too low, dangerous gasses will not cause any problem.

Mark Baker
Retired - Winston Salem, North Carolina USA

March 2, 2020 [ed. note: sorry, not posted 'til March 21]

Q. Hello,

Thank you for your replies Geoff and Mark! Yes KAu(CN)2 is very stable in Gold bath even in acidic bath (pH between 3 and 5). But if we add too much acid in the bath, is there a risk to release HCN gas? Mark, you said "PGC salts are stable and as long as the ph doesn't drop too low". And my question is: if the pH drop very low due to an accidental or error in the acid addition. In this case, HCN could release in the atmosphere and it will be very dangerous for people, do you agree with me?

Just another question Mark, what do you mean by "periodic carbon polishing is key"? What is carbon polishing? Treatment/cleaning with activated carbon??

Best Regards,

Thomas Perrier [returning]
- Berne, Switzerland

March 9, 2020 [ed. note: sorry, not posted 'til March 21]


Thank you very much for your replies! But...
What do you mean Mark by " carbon polishing" : a common activated carbon treatment??
You say also "PGC salts are stable and as long as the ph doesn't drop too low, dangerous gasses will not cause any problem." And my question is what is the effect if we add too much acid in these acidic baths : is it possible HCN escapes from the bath and be dangerous for health? What could be the quantity of acid is needed to release HCN for these acidic gold baths?

Thank you again,
Best Regards

Thomas Perrier
- Berne, Switzerland is possible thanks to our supporting advertisers, including this one:


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