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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: thdbird83 on July 27, 2005, 03:50:29 AM

Title: Gold Electrolysis
Post by: thdbird83 on July 27, 2005, 03:50:29 AM

Does anyone knows the theory behind gold electrolysis? With the following setup:

Anode: 99.99% gold foil
Cathode: 99.999% gold foils
Electrolyte: Gold in Aqua Regia

From wat i heard is that Au3+ will propogates towards the Cathode side... Is this true?

Title: Re:Gold Electrolysis
Post by: Borek on July 27, 2005, 04:11:33 AM

Quote from: thdbird83 on July 27, 2005, 03:50:29 AM

Anode: 99.99% gold foil
Cathode: 99.999% gold foils
Electrolyte: Gold in Aqua Regia

From wat i heard is that Au3+ will propogates towards the Cathode side... Is this true?

In general it is possible, but I doubt it will be doable using Aqua Regia as electrolyte.

Solution of cyanides (or some other complexing agent) seems better choice for me. Plus proper selection of pH to avoid hydrogen evolution.

Title: Re:Gold Electrolysis
Post by: xiankai on July 27, 2005, 04:17:59 AM

but i heard that most of the gold is dissolved is in the form of AuCl₄^-, ?

Title: Re:Gold Electrolysis
Post by: Borek on July 27, 2005, 05:16:11 AM

Quote from: xiankai on July 27, 2005, 04:17:59 AM

but i heard that most of the gold is dissolved is in the form of AuCl₄^-, ?

It is. But there are three problems with Aqua Regia:

1. It will dissolve electrodes on its own, and that will interfere with your idea of electrolysis.

2. It has low pH and thus hydrogen evolution is very easy - that will interfere too.

3. Cl^- presence in electrolyte can end up with Cl₂ evolution.

You will have to check exact potentials and complexation constants to calculate what happens in the solution (you will also need overpotantial for hydrogen evolution on gold), but my gut feeling is that you have to go for high pH and no chlorides. Adding strong complexation agent (like CN^-) you are facilitating electrolytical Au dissolution, as the oxidized Au³⁺ is removed (in the form of complex) from the solution.

If I recall correctly CN^- complexation is strong enough to allow oxidation of gold by the atmospheric oxygen.

Title: Re:Gold Electrolysis
Post by: thdbird83 on July 27, 2005, 09:59:24 PM

BUt wouldnt charging the aqua regia electrolyte with current would sperate out the Cl ions from AuCl?
thus forming Cl2. And when theres no more Cl ions, gold dissolution would not be possible anymore (in that particular batch of electrolyte @ least).???

Title: Re:Gold Electrolysis
Post by: Borek on July 28, 2005, 04:14:59 AM

One more reason to not use Aqua Regia as electrolyte.