Chemistry Forum

I had some scrap gold I tried to purify. I melted the gold down into tiny beads to make for greater surface area for chemistry. Typically what I then do is add the beads to [ HCl ] to dissolve any impurities and leave just the 'pure' gold. Then I would pour the dissolved impurities off for recovery later.

Instead, this time I unwittingly added the beads to a mix of HCl and HNO₃ (aqua regia). Thus the impurities dissolved ALONG with the gold. Not yet realizing my blunder, I then added sodium metabisulfite to precipitate the gold. No gold precipitated ... obviously. I just got a clump of impurities and gold mixed. Can I just put the clump in HCl and start the reaction over again?

I am not sure how to proceed. Any genius chemists out there to help? Can someone help me, please?

Thanks in advance,
Rookie Mistake Maker

First are you sure you had gold? The precipitation should take place at neutral or slight alcaline pH. In aqua Regina you have very acidic and also oxidising condition. So the best is to add again aqua Regina to dissolve everything. Then boil it to very less volume, then neutralise it carefully with sodium bi carbonate to pH 7-8 .Then do reduction with bisulfite.

Hi ChenBeier,

Thanks for the super fast response. I am sure that I had gold. Gold measured around 48% (~12k). Other impurities included Ag, Cu, Zn, Sn... I digress.

If I start the reaction over again in aqua regia, then boil, neutralize to Ph 7-8, and hit that mix with the Sodium Metabisulfite (SMB), won't I be in the same boat that got me here in the first place? Won't I still have the impurities in the solution that the SMB is going to re-precipitate? Are you thinking the neutralization makes the SMB 'work' only on the Au? I'm just trying to learn.

Normally only the gold will be reduced.
Instead of the SMB also zinc powder can be used.