Gold solution 30%W/V

[huggybear]

Hi All,

On face value this seems like a simple experiment, but I am running into some problems.

I am trying to create a gold chloride solution with the highest possible gold concentration whilst still being stable in solution. I am starting with 99.99% pure gold granules and digesting with aqua regia (adding minimum nitric) and then evaporating with repeat HCl additions to remove excess nitric. I then make up to volume. From what I have read, the highest possible gold concentration is about 30%W/V (30 grams of gold made up to 100 millilitres).

I need some guidance on what matrix conditions would be best to ensure the gold stays in solution. (previously I have had metallic gold forming out of the solution)

Should I make up to the final volume with water or add some HCl?

Is nitric required to keep it in solution? (my understanding is that nitric is only necessary during digestion)

Any other tips are greatly appreciated.

Regards,
huggy

 

[Martijn]

(previously I have had metallic gold forming out of the solution)

Then you have achieved your goal!, Maximum saturation means no more salts can be in solution, so it precipitates out as a salt or in metallic form.
this saturation level is only at a specific temperature, once it gets colder, more salts will precipitate, once it gets hotter, its no longer saturated.
If i may ask, what is the purpose of this experiment?

 

[Martijn]

Should I make up to the final volume with water or add some HCl?

i think you will have the best solubility in HCl. Read Wikipedia about solubility in water.
if you add a chemical to a solution, it will be diluted by the chemical, no longer being saturated...
So store it warmer than it was made to keep stable.

 

[huggybear]

Then you have achieved your goal!, Maximum saturation means no more salts can be in solution, so it precipitates out as a salt or in metallic form.
this saturation level is only at a specific temperature, once it gets colder, more salts will precipitate, once it gets hotter, its no longer saturated.
If i may ask, what is the purpose of this experiment?

Hi Martijn,

Thanks for the reply.

I have buyers interested in purchasing chloroauric acid for their own processes. They prefer higher concentrations since this reduces transportation costs.

I Googled solubility of gold chloride in different matrixes but couldn't really find any definitive information.

Do you know why gold sometimes comes out of solution as metal? To me this seems strange... I would expect it to present as salt crystals.

Thanks,
huggy

 

[Lou]

Most likely light.

All of the high concentration chloroauric acid I made was very sensitive to sunlight. Gold would come out with the release of chlorine.

You could also be overheating it. When I would try and make chloroauric acid crystals, I would have to dehydrate them in a vacuum over P4O10.