The cyclic precipitate/dissolve reaction with precipitants
Other wise known as a redox reaction
So - to help the original poster that started this thread understand the reason he is having trouble precipitating his gold
Redox is a reaction when both reduction & oxidation take place at the same time
When we dissolve gold with AR we use nitric along with the HCl because the nitric is the "oxidizer" that makes the gold dissolve - so it is an oxidation reaction
Then - to recover the dissolved gold back out of the AR we use a reducing agent (such as SMB in this case) so we are looking for a reduction reaction when we add the SMB
However - if there is an excess of "free nitric" in the AR because you used more nitric then needed to dissolve the gold --- then when you add the SMB to reduce the gold what happens is the SMB "tries" to precipitate (reduce) the gold - BUT - at the same time - the excess "free" nitric goes to work re-dissolving (or oxidizing) the gold that is trying to precipitate (reduce)
In other words - you have a redox reaction taking place wherein the SMB is trying to reduce the gold - but -
at the same time - the "free" nitric is oxidizing (re-dissolving) the gold that is trying to precipitate (reduce)
So - in order to stop that redox reaction from happening - you need to get rid of the "free" nitric in the AR that is causing the gold to re-dissolve when you try to precipitate the gold --- this is known as de-Noxing the AR
There are a number of different ways to de-Nox the AR & what works best for de-Noxing your AR depends on how much "free" nitric is in the AR
1) if there is not a lot of free nitric then you can simply leave it on the hot plate until you no longer see brown fumes coming off the AR - as Jon has pointed out
On the other hand - if there is A LOT of free nitric in the AR you will have to evaporate the AR down to a (thicker) syrup & then add HCl to the syrup to drive off the free nitric (this produces brown fumes when the HCl is added to the syrup) & this may (or not) need to be done more then one time - as Yaggdrasil pointed out --- this is the most difficult way to try to de-Nox your AR
2) is to simply let the free nitric get used up dissolving more metal & there are a number of ways to do that
a) - if there is not a lot of free nitric you can precipitate the gold with SMB & then let it sit on the hot plate so that the free nitric re-dissolves the gold (or at least "some" gold) & then re-precipitate with more SMB - this only really works if there is not a lot of free nitric in the first place - if there is A LOT of free nitric this does not work well
b) add more gold to the AR so that the added gold uses up the free nitric - as 4metals pointed out
c) put copper in the AR - the copper (like adding gold) will use up the free nitric & then when the copper has used up the free nitric it will also precipitate the gold --- this is called "cementing" the gold out of the AR --- I have posted "a lot" about this - so you can do a search about this using "cementing" by kurtak in the search
3) or you can do it chemically - some people use urea to de-Nox their AR
but we do not recommend using urea - the other chemical method to de-Nox AR is to use sulfamic acid (not sulfuric acid) & this is the chemical we recommend
I have used all of the above methods & have personally stopped using all of them many years ago - with the exception of ether method (2) (b) or (2) (c)
In other words the ONLY methods I ever use to get rid of the free nitric in my AR is to ether add more gold until the AR will not dissolve any more gold & then precipitate the gold with SMB - or - cement the gold with copper
Kurt