# Hcl H2O2 as a means of Au digestion



## jonn (May 14, 2015)

Can we discuss the pros and cons of using H2O2 as opposed to nitric?


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## Lou (May 14, 2015)

Sure.

Let's begin:

Pros
1. Solutions produced from peroxide do not require the removal of excess oxidant--it decomposes on its own to water and oxygen gas.
2. Works well for in situ chlorine generation for digesting gold foils and gold powders. On bulk metallic gold, it's worthless if you don't have all month.
3. If added correctly (slowly), then the chlorine generated is almost entirely consumed in dissolving the values and it is less polluting than with nitric.

Cons
1. Brings molybdates and tungstates into solution which will then re-precipitate as colloidal gels of ill-defined composition, thus making filtering difficult
2. Requires the UTMOST in care in storage--hydrogen peroxide solutions, especially above 30% must be kept in scrupulously clean containers--any PGMs, silver, or iodides/bromides will dramatically destabilize it. Shelf life is limited. 
3. Doesn't work too well on finely divided PGMs or any material containing silver which catalytically decomposes it. For instance, a platinum black slurry is better dissolved with sodium chlorate and 6M HCl at 60*C, than peroxide.
4. Dilutes the solution with its main constituent, water.
5. May have an induction period where nothing happens, then it goes berserk. 


To be frank, hydrogen peroxide is best used for etchant solutions, finely divided gold powders/foils, or, preferably, to help with nitric digestions by recycling NOx back to HNO3. 

Lou


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## Pantherlikher (May 14, 2015)

Nice Lou...
Well put in terms anyone can understand.
1 thing left out though would be a better alternative such as HCL+ Chlorine(Bleach).
This works well on thin gold or other wise finely divided so it can work quickly as the Chlorine turns to gas to fast.

B.S.


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## jonn (May 14, 2015)

Great post Lou, very informative. 
Let's say for instance tungsten or molybdenum gets digested, would it pose a problem in precipitating the gold from solution? Would the tungsten or moly pull down the gold? I recently ordered some books after browsing through GSP'S library, thank you Chris! Im looking forward to some heavy reading. 

It does sound like a great method for foils and a second refine on gold powder.


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## Lou (May 15, 2015)

It's more of an issue for the tungsten than the molybdenum--tungsten in the form dissolved by peroxide.

Say you dissolved some tungsten up in peroxide, then added sulfuric, guess what precipitates? Tungstic oxides.


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## jonn (May 15, 2015)

That's genius! Thanks Lou.


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