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Home-Made Gold Plating

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Hello there, I'm an electronic engineer, and was willing to gold-plate my soldering iron end, and some other electronic stuff. In my lab I happen to have all what's needed to make a small quantity of aqua regia, and a 40 g gold piece, and have no problems in handling dangerous/poisonous items. I have read in existing threads that gold plating is not the easiest thing to experiment with, and that I should not expect good results at first try; I've also read that gold electroplating baths are made with either cyanide or sulfur gold ions and that Cl ions (as in gold dissolved in aqua regia) are not suitable for plating. My question is WHY? Why a gold saturated aqua regia solution would not permit electroplating?

Alfonso Brevi
engineer - Milano, Italy


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Gold dissolves in aqua regia to form the auric ion, Au3+. It is generally added to cyanide based gold solutions as aurous (Au1+) potassium cyanide. As far as I know, the two oxidation states are not easily interconverted. KAu(CN)2 is made commercially by the anodic dissolution of gold metal in potassium cyanide solutions.

dave wichern Dave Wichern
- The Bronx, New York


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Thanks David, for your reply. To me, though, it's still unclear... what do you mean by "oxidation states are not easily interconverted", does this mean that my saturated Aqua Regia stuff will allow electrodeposition or not ? (in the end, I think I'll give it a try this coming weekend...)

ciao, AL.

Alfonso Brevi
engineer - Milano, Italy


First of two simultaneous responses -- +++++

I mean: if you try to dissolve the auric trichloride in a cyanide solution, the gold will be reduced to Au1+, but a goodly proportion of the cyanide will be oxidized to carbonate ion in the process. Other reducing agents are liable to give you a mixture of aurous chloride and gold metal.

If I were you, I'd win the gold metal out of the aqua regia, melt it into an anode, and make my own aurous potassium cyanide in a little divided electrolytic cell. The cathode could be encased in a semipermeable membrane (a "Naturalamb" or other natural membrane type condom would work) filled with potassium hydroxide solution. This is so you don't plate all the gold out onto the cathode as it dissolves. When enough gold is in the anolyte, there's a base for your plating solution.

Dave Wichern
- The Bronx, New York

Second of two simultaneous responses -- +++++

If you anyway are going to try with 40 g of gold, you better give me these 40 g, and I'll tell you why it doesn't work. Do we have a deal ?

Bo König
Food industry - Odense, Denmark

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Good point Bo ! ;-)
Now I kinda believe that saturating aqua regia with gold for the purpose of using the results as a plating bath is hopeless. I get your point. Still I'm curious as to WHY the baby won't act as I want... and everybody keeps telling me about cyanide.
Let's put it this way: next time you come over to Milano I buy you a good pizza.

Ciao, AL.

Alfonso Brevi
engineer - Milano, Italy


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Aqua Regia is far too acidic. All you could do with that is decompose water into hydrogen and oxygen. No gold would be deposited. For the value of 40 g of 24 K gold, you can buy a reasonable quantity of ready-to-plate gold solution which will work well.

jeffrey holmes Jeffrey Holmes, CEF
- Spartanburg, South Carolina


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Hi Al
Better be careful with promises, my wife want to go to Italy this summer
To the serious.
If you want to solder on iron you better use a barrier layer of nickel first, gold is counted as an unwanted impurity in solder, therefore you have to decide what purpose are your gold plate. Is it connectors or are you going to solder ? Connectors shall be plated with hard gold (containing 1 % nickel) on the other hand solder is pure gold but 0,08 my max, which you normally get with immersion gold in 10 minutes.
Second why can't it be done with chloride ? When you plate you need the metal a stable complex in order to control your plating. You probably know about plating copper with sulphuric acid. Without vinegar to form a good complex, copper will just plate out and not stick to you item. You can plate with chloride, but then you need another complex, and don't try with cyanide in aqua regia.

Bo König
Food Industry - Odense, Denmark

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Regarding making gold plating solution from aqua regia. Why can't you drop the fine gold out of your acidic solution {aqua regia}, with the addition of sodium bisulfite? The sulfur dioxide gas formed, reduces the gold chloride to pure gold, and the gold settles to the bottom of your beaker. This takes about 24 hours. Rinse the 24k several times and decant the rinse water [you can check for neutrality with litmus paper if you want to.] Gently, warm the gold to dryness, and you have pure gold particles that are extremely small. Only a micron or so in size. This gold can now be redissolved in a mild sodium cyanide solution. I don't know but you may have to also add an ounce or so of Hydrogen Peroxideamazoninfo to hasten the process. sodium bisulfite is cheap, I get it from a company that sells beer making supplies. Also, if you use a pure gold anode[the positive lead] the solution should last a lot longer before the solution is plated out. Solution should be diluted such that there is about 1.5 gr [1 dwt.] per liter of solution. 180 deg. temp. should be about right, that's fahrenheit. 4 to 6 Volts is good . I quite often drop gold out of aqua regia this way. I then remelt it with the addition of various alloys to get the color and karat that I need. The plating solution part is just a suggestion. I think it will work. Then again, maybe it won't.
n

JohnMcGahee
jeweler/goldsmith - Brooksville, Florida   

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