Does cyanide gold plating bath release hydrogen cyanide?

A. "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.

A. 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!

A. 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?

A. Ted,

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.

A. 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).

Multiple threads merged: please forgive chronology errors :-)

I have an immersion gold bath plating. The pH of the solution is between 5 to 6. In the worse case (dropping 10 liters of sulfuric acid in the solution) is it possible to than a high amount of HCN will be released from the solution.

Ya, this would not be something you would want to do, say standing there wearing no protection. I am also a custom painter, and whenever I'm "messing" around with cyanide and acids like that I always wear my painting positive pressure suit. Remember this is a dangerous fume, if you are leaning a few too many inches over the concentrated gas when it is released it could cause your life, do you want to take that risk? Wear protective gear!

 Thanks very very for HCN gases hazards.

A. Hi Thomas
The double salt gold potassium cyanide is very stable. Routine gravimetric analysis requires boiling nitric/sulfuric acids to destroy it.
You may be interested in an extensive dissertation on cyanides at ...
https://www.academia.edu/10635761/Cyanides_in_Metal_Finishing_Risks_and_Alternatives

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 ⇦this on eBay or Amazon [affil links] 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.

A. Hi Thomas
Firstly commercial acid gold bathe are heavily buffered and it takes a very large dose of acid to drop the pH below about 2 where GPC is still stable.
In gravimetric analysis it is common to add a significant excess of concentrated Nitric/sulfuric acid and boil which will give off a small quantity of HCN. But even this has to be done correctly or the GPC breaks down to gold cyanide, a yellow precipitate that is not affected by the boiling acids. Very annoying and ruining the analysis!
In many years of operating and servicing acid gold baths, I have never experienced any detectable cyanide gas being given off.
https://www.academia.edu/10635761/Cyanides_in_Metal_Finishing_Risks_and_Alternatives

The only significant problem I found was over-enthusiastic safety officers with little knowledge of the minimal realistic level of risk and a mission to eliminate cyanides and replace known risk with a totally unknown one.

A. The rate at which cyanide is destroyed at the anodes is much slower than the rate at which gold is plated out. This can result in a build up of a low concentration of free cyanide or higher order gold cyanide complexes like Au(CN)3 in the bath, but is unlikely to result in significant release of cyanide to the atmosphere at plating bath pH values.

A. Hi Thomas. I suspect that perhaps nobody has ever investigated whether there is any possible way whatsoever whereby a dangerous quantity of hydrogen cyanide can be released from an acid gold bath because they've seen no evidence that a serious poisoning has ever occurred, and it can be almost impossible to examine every arrangement of variables and every possible scenario? I've certainly heard of fatalities from cyanide in plating shops, but never from acidifying an acid gold bath; and reading between the lines, I'd say that neither Geoff (who has studied CN extensively), nor Mark, nor Lyle has either. Although cyanide is a very dangerous poison, it's a natural one, present in low dosage in fruits and vegetables and in building materials. What if you're 10 times more likely to die from exposure to CN fumes from a building fire in your shop than from acidifying your acid gold solution, and you focused on this instead of fire prevention and building material selection? My point is that maybe efforts might be better spent researching whether such an accident has ever actually happened in this art which has been practiced since the early 1800s than trying to determine what arrangement of variables could theoretically cause a serious release? Sometimes history is the most promising path. Best of luck.

Luck & Regards,