Harold_V said:
The only other viable option is to add nitric sparingly, until all of the gold has been dissolved. That is the method that is advised by GSP. The problem with that system is that if you are dealing with fairly large volumes, as the amount of un-dissolved gold diminishes, the solution reacts slower and slower with the remaining gold, due to the increased volume of solution diluting the small amount of nitric that may remain, and the decreased amount of surface area of gold remaining. It would be very easy to add too much nitric and not realize you have done so. I would always recommend the addition of a solid piece of gold in evaporation, even if his method is incorporated. It insures you do not leave nitric behind.
There's a little art in this. After a few times, you know when it's nearly finished and when it's finished. After a few times, the method feels normal. It fools you very rarely, but usually because you screw up your calculations or measurements. Just remember, it takes both HCl and HNO3 to dissolve gold. They are used up at individual rates as they dissolve the gold. When either is spent, the reaction stops.
The trick is to start with the scrap covered an inch or so with straight HCl. This is heated and a small amount of nitric is added. The reaction will fizz and foam a bit. When this dies down, give it a stir and add a little more nitric. As time goes by, make your nitric additions smaller. At some point, an addition of nitric will produce no reaction. At this point, you could be out of nitric or, you could be out of hydrochloric (muriatic). You don't know which.
You add a splash of HCl, heat, and observe. If it reacts, there's nitric left. When it slows, add a little nitric. Repeat all this until neither HNO3 or HCl produce a reaction. You are finished.
The back and forth thing can be avoided by using an excess of HCl to start with. That way, when an addition of nitric produces no reaction, you know the gold is dissolved.
At first, I always calculated the gold content and measured out the nitric before starting. It never was 100% right on, but it was close. The HCl was just estimated. The guy that taught me this just poured the additions from a plastic jug. He'd been doing it for 20 years and he never overshot.
I also poured off the acid, when it got saturated, a couple of times during the process and put on fresh HCl- then HNO3. The amount of HCl you use depends on how much gold you have left. I never let the acid level be more than an inch or two above whatever I was dissolving.
Keep it hot but don't boil it. The boiling action masks the dissolving reaction and makes it very hard to see. After any addition, give it a minute, or so, to heat up again before you observe the reaction. Only when the acid is hot can you trust the absence of a reaction.
When dropping the gold from AR solutions using this method (or, any other method), you can stand a little free nitric. However, there is a limit to this, since a little HNO3 kills a lot of sodium sulfite or SMB. I used sodium sulfite to drop my gold. It first reacted with any excess nitric - didn't affect the purity at all. When the nitric was gone, the gold dropped. I assume it will work the same SMB.
Mainly, I'm talking about karat gold. However, I use the method for most all forms of gold. Many times, you don't know how much gold you have and have to play the additions by ear. To keep nitric additions small, I may use a 10ml beaker or an eye dropper, at the end of the dissolution. The bigger the nitric container, the easier it is to add too much. When dissolving powder, start with very small nitric additions until you get the feel for how much it will take at a time.
The way I've described this, it sounds slow. It reality, it goes fast if you keep after it. Ideally, keep it going as fast as possible without adding too much total nitric or producing a foam-over. Keep the heat right under simmering.
. But current I only have 1 gram of newbie-gold, which rather will precipitate gold instead of
