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Every reasonably effective method of electroplating $\ce{Au}$ onto another metal's surface (e.g. $\ce{Cu}$,$\ce{Ni}$,$\ce{Ag}$, etc.) involves use of $\ce{KAu(CN)2}$ or in some cases $\ce{KAu(CN)4}$ in acidic, neutral, or (in almost all cases less effective) basic solution. Ideally it should be $60$ to $80~\mathrm{^\circ C}$ with a certain current density applied (c. $0.5~\mathrm{A/dm^2}$). In the past I have tried a direct route of dissolving the gold in warm $\ce{HCl / HNO3}$ as the deposition solution to little effect (a brownish/blackish tint is left on the receiving surface with very little tint of gold). The short story is that I have found the use of cyanide unavoidable.

$\ce{KAu(CN)2}$ runs $157/g from Sigma-Aldrich so I would like to synthesize it, if possible.

I have:

  • 1g solid $\ce{Au}$
  • Excess $\ce{KNO3}$, $\ce{KI}$
  • Various $\ce{C-N}$ organic compounds, e.g. urea, uric acid, etc.

I think my best route is indirect — to first synthesize $\ce{KCN}$ and then form the product:

$$\ce{4 Au + 8 KCN + O2 + 2 H2O → 4 K[Au(CN)2] + 4 KOH}$$

...and follow by vacuum filtration on the solid precipitate. Which begs the question of the best method for safe synthesis of $\ce{KCN}$.

Much of my information comes from this difficult-to-find and expensive book, "Properties of Electrodeposited Metals and Alloys: A Handbook" by Safranek. It's got quite a lot of proprietary data. Also Kohl's chapter on gold electrodeposition.

Jan
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khaverim
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  • please clarify how your workplace looks like. It is a kind of pro-lab, a DIY garage, something in-between ? – permeakra Jan 26 '16 at 14:13
  • It's a makeshift lab at home. I have a good amount of equipment but any synthesis of CN I'd be doing outside. Wearing PPE of course. I'm a computational chemist at university but do experimental work as a hobby at home – khaverim Jan 26 '16 at 14:43
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    I kindly suggest to order it by mail than. As a last resort you may find a buddy with access to a proper fume hood and ask him. I actively discourage producing potassium cianide at home. Even on open air. Hydrogen cyanide is an intermediate and you definitely do not want to deal with it outside a fume hood. This said, potassium cyanide is sometimes employed in organic synthesis, so you may find some there. – permeakra Jan 26 '16 at 15:19
  • Related: How to prepare potassium cyanide from potassium ferrocyanide –  Jan 26 '16 at 23:05

2 Answers2

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I read that you are a computational chemist at university, so I assume you went through chemical training. This is a good thing. But please remember that cyanides are potent poisons and that protonated cyanide ($\mathrm{p}K_\mathrm{a}(\ce{HCN}) \approx 8$) aka hydrogen cyanide is a poisonous gas.

If you are not a trained chemist (or if you are one but do not feel $100~\%$ safe about using cyanides), stick away from them!

With that in mind this leaves two methods of acquiring $\ce{K[Au(CN)2]}$ — either buying it directly from YourSuppliers™ or by synthesising it. My preferred route of choice would definitely be to buy it from YourSuppliers™, as that is the safest way possible — and always always ensure that it remains in an alkaline solution.

I have no idea about the experimental procedure to synthesise $\ce{K[Au(CN)2]}$ according to the reaction you propose, but it didn’t escape my notice that you are generating $\ce{4KOH}$. This means, I’ll have to assume that the reaction runs under acidic conditions. If that is the case, do not use that method unless you have access to a proper and tested fume hood! If there are other procedures out there that employ alkaline reaction conditions you may (as a trained chemist!) decide to use them at your own risk.

Assuming you did find a method that employs alkaline conditions, the question remains where to get the cyanide from. Well, SigmaAldrich gives me a price of 38 € for $100~\mathrm{g}$ which means that the chemical is very cheap. Note that acquisition of potassium cyanide may well be regulated in your jurisdiction — and rightly so!

Whatever you do, do not attempt to synthesise $\ce{KCN}$ (or any other cyanide for that matter) at home!

Remember that a certain percentage of the population can sense $\ce{HCN}$ which they perceive as an almond-like smell. Don’t attempt to test which part you belong to. If the concentration of $\ce{HCN}$ is such that you can smell it, you are in severe danger!

This bears repeating for future visitors:
Do not do this unless you are a trained chemist and have access to a proper fume hood! Never, ever try this at home! Never work with cyanide in the open air or in any other place where unsuspecting passers-by may be endangered!

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Jan
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    It seems the wise answer here is a focus on matters of practicality and safety and not chemical procedure. I think buying KCN and borrowing a hood is indeed best. Going to do further research on options – khaverim Jan 26 '16 at 21:00
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Sodium cyanide can be very easily made by fusing sodium ferrocyanide with sodium carbonate. The guy in the video is still alive, but I would recommend wearing gloves at least.

Video including Au dissolution

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