noxx - 20-6-2008 at 18:08
Hello guys,
I'm trying to find a way to calculate the amount of aqua regia needed to dissolve, let say, 10 grams of gold but I can't seem to be able to find or
equilibrate the equation.
I know aqua regia is made from Nitric Acid and HCl (1:3) but what's the reaction involved ?
Thanks in advance.
497 - 20-6-2008 at 20:16
From wikipedia:
Au (s) + 3 NO3 - (aq) + 6 H + (aq) ---> Au 3+ (aq) + 3 NO2 (g) + 3 H2O (l)
and then
Au 3+ (aq) + 4 Cl - (aq) ---> AuCl4 - (aq)
Net:
Au + 3HNO3 + 4HCl ---> HAuCl4 + 3NO2 + 3H2O
So the way I see it is for every mol (198g) of gold you need a minimum of 3 mols (189g) HNO3 and 4 mols (145g) HCl. Of course you need more of both
acids to keep the solution from being depleted near the end. How much extra acid is something I couldn't figure out so easy. I have seen the number of
milliliter per gram before but I can't remember where. Do some more searching and you'll find it. I find google books and google patents very helpful
although time consuming.
So your 10g of gold should consume about 10g HNO3 and about 7g HCl. HNO3 is limiting because the mass ratio of AR is 1 : 1.1 HNO3: HCl (molar ratio of
1 : 1.87) so I would hazard a wild guess that maybe 88 ml of AR would be sufficient, being at least twice as much HNO3 as needed. Its a pretty crude
calculation so don't quote me on it though. Another thing to consider: some of the NO2 produced will dissolve and make more HNO3, I don't know how
much though.
[Edited on 20-6-2008 by 497]
Klute - 21-6-2008 at 05:06
I don't think the redissolving NO2 is aquaintable here, it will be very minimal. I think 2x excess is bare minimum, as there a side reaction :
production of chlorine gas. Aqua regia bubbles pretty heavily whne freshly made.
I would start using 2-3x the amount needed, and adding more (fresh) if the metal takes too long to dissolve.
Maybe someone with a metal refining book can help?
497 - 21-6-2008 at 16:27
I just found a reference that looks fairly reliable that says 4-5 ml of AR is required per gram of scrap. Some other good info too.
http://www.goldmineworld.net/Aquaregia2.html