RESET said:
Start with the calculated amount of HCl for your amount of gold then add the calculated amount of HNO3 in small increments waiting for the reaction to stop each time. If you add the last of the HNO3 and you still have gold left, add more HCL and then add increments of HNO3 again until dissolved. If you aren't dissolving 24K gold then you are going to use more acid. If you premix your acid, you may end up with Free nitric that you will have to deal with later. If you stop just shy of dissolving all the material and filter your solution then you are almost certain to NOT have free Nitric to deal with.
Well said, RESET, and I would certainly agree. About 5 years ago, when I first introduced this 2 step AR method to the forum (cover with excess HCl - heat - nitric in small increments only until the gold is dissolved - then, stop adding nitric), no one except Lazersteve listened. Everyone wanted to pre-mix the AR and, of course, they then had to eliminate the excess nitric before they could drop the gold. Now, it seems that many people use my 2 step method (for want of a better name). I haven't pre-mixed AR in 35 years, except in a couple of unique situations.
The only thing I would change in your method, RESET, is to use an excess of HCl to start with. That way, you don't have to add any more HCl later on and then go back and forth with the 2 acids, which can get confusing. An excess of HCl creates no problems. When I dissolve gold, I start with about 140ml of HCl (instead of 120-125ml) per 31.1g. With lower strength muriatic, I start with proportionally more.
What I'm saying is that everyone should learn what I am calling the 2 step AR method. When used properly, there is no excess nitric to deal with and this eliminates the need for evaporation or urea. It also cuts down on nitric usage and it allows a much greater control of the emission of NOx fumes. Everyone should realize that
any combination of HCl and HNO3 will dissolve gold. However, it requires both. Both acids are consumed in the dissolving reaction and when either has been used up, the dissolving will stop.
When the AR is pre-mixed, you lose control and most always end up with excess nitric. By far, the best way is to use the 2 acids separately. Cover the material with an excess of HCl and heat it somewhat. If you're dissolving gold powder or high copper items, you need only a little heat, say 120F, or maybe none at all. If you're dissolving more difficult things, such as karat gold or the braze under the chip on ICs, you need to get it hotter (don't boil, though). Add a small amount of nitric and allow it to work until the reaction ceases. Give it a gentle stir and add a little more nitric. Repeat until a small addition of nitric produces no reaction.
Sorry for all the repetition but a lot of people just don't understand the importance of this. As a result, we still get posts like this one, asking about the best ratio for pre-mixed AR.
