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Impurities appear when precipitating gold from aqua regia with nitric acid & sulfuric acid

I have a serious problem that is "Impurity appear after I precipitate gold from aqua regia solution by using nitric acid & sulfuric acid?"

My sample came from the refinery company(about 150 ml sample), the sample contained impurities, gold and aqua regia. My main task is analysis of the gold content by gravimetric method (GA). The sample is about 2-10 ppm. I pipette [pipettes on eBay or Amazon [affil links] 20 ml sample, then add 20 ml nitric acid then 30ml sulfuric acid. Reflux it in hot condition until the sample turns clear. Gold will be precipitated out. Cool it and add water, boil it again and wash the precipitated gold by hot water. After that filter the sample and dry it on 180C oven and weigh it.

However, in the stage of reflux, impurities appear. then I can't use the basic concept of GA to determine the gold in the aqua regia.

Is there any method to eliminate the impurities before or after analysis? The impurity is dissolved in the aqua regia and besides, after I got the impurities, it can't be dissolved in HCl, HNO3 and H2SO4 also

How can I apply a simple method to eliminate the impurities!

Thanks!

Sylvian Wong
technician - Kwai Chung, NT, Hong Kong
2007

2007

Are you talking about a lightweight, red precipitate that obviously isn't gold? I commonly see this. I generally assume that it weighs little relative to the gold, and ignore it. When we analyze such samples via "fire assay" techniques, we see answers that are very slightly lower.

You should be heating the aqua regia/sulfuric acid sample prep. to the point where all of the HCl and HNO3 are fumed off, and white fumes of SO3 are observed.

Dave Wichern
Consultant - The Bronx, New York

Thanks, David, for the information.

However, there is large amount of impurity in my sample. The impurity must influence my result when I am using the gravimetric analysis. The impurity is not red color and it is grey in color.

In addition, I had tried to do this experiment by evaporated the aqua regia almost to dry, let it cool and addition of water and sulfuric acid to relux it. Nevertheless, eventually, impurity still appear after addition the hydrogen peroxide solution in the final step.

I do want to know how to eliminate the impurities that can be precipitated out in this method.

Thanks all!

Sylvian Wong
- Kwai Chung, NT, Hong Kong
2007

I guess there was some complication in the GA process. I would prefer you using HCl + Nitric to form AQ, nevertheless it still can be done; your impurities may be due to too high a mixture you couple with such a problem it should be advise to inquart it with silver to purify it. if you know the way of doing
Jimi

Jimi Lim
- Singapore
2007

I am gold refining officer.you take precautions mainly by filtering the sample,purity about your chemicals,and also with water.

Bhupesh Mulik
jewellery - Mumbai, India
2007

April 4, 2011

If you are using sulfuric acid when refining and if you have lead in the materials being refined it (sulfuric acid) will precipitate out the lead. This could be what is being reduced or dropped out.
Donnybrook Australia

Don Buckley
mistyview - Braidwood NSW, Australia

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