Thank you for the comments, here goes right where I left off....
One safety note; the fumes produced from this reaction are different from the fumes created while dissolving the inquarted silver and base metals. But they are just as deadly. Make sure and do this reaction outside and away from people, pets, and metal objects (these fumes on metal will cause the metal to corrode almost instantly).
While dissolving the gold in aqua regia, newcomers (and some not so newcommers like myself until recently) add too much nitric acid in an effort to speed things up.
Adding fresh nitric acid will speed the process, but I will probably end up with much residual (excess) nitric acid that has to be removed before I can drop the gold.
By adding small amounts of nitric acid a little at a time I can better control the amount of nitric that I add to the reaction vessel.
When most of the gold has dissolved, there will be some small undissolved pieces emmiting tiny bubbles as they continue to dissolve - almost looks like the bubbles in a glass of chanpaigne.
When you see this, add the nitric acid in even smaller increments - a few drops at a time. Here is where patience really pays off.
As I said earlier - I used only about 35 to 40 ml of concentrated nitric acid to dissolve nearly 70 grams of pure gold.
Once all the gold has been dissolved, remove the container from the heat and allow it to cool for a few minutes.
I place the container with my dissolved gold inside a CLEAN corningware casserole dish or pyrex baking dish. This dish will catch your gold solution should you experience a boil over.
Placing the container inside the dish is a precaution. If you are careful and add chemicals in small amounts and wait for the reactions to stop completely before adding more, then you reduce the chance of a boil over. But they do happen from time to time especially for the beginner - better to have it and not need it, then the other way around.
I usually rinse down the insides of the reaction container with distilled water from a squirt bottle to get all the gold bearing liquid down into the main solution.
Next, I add some concentrated sulfuric acid - about 5 or 10 drops - to precipitate out any lead that may be present.
This is a good idea even if you are certain there is no lead in the solution, it does not hurt a thing (and may be helpful when evaporating later on) plus it helps ensure that no lead will make it into your final pure gold.
The resulting lead sulfate (if any) will precipitate out of the solution and get trapped in the filter during the filtering process.
The sulfuric acid can be purchased at Ace hardware. It is called Roto drain cleaner, or Liquid Fire drain cleaner.
Roto is my preference as it is nearly colorless - it comes in a white plastic bottle and is sealed inside a plastic bag.
The Liquid Fire works just as well, but it has orange dye in it. Liquid Fire comes in a red plastic bottle and is also sealed inside a plastic bag.
I believe that both of these products are 98% concentrated sulfuric acid.
Once the sulfuric acid has been added and stirred (or swirled nicely), the next thing I do is decide on what method I will use to remove excess nitric.
I have used two methods and each produces excellent results although each has advantages and disadvantages.
Method number one, and the one I use most frequently is; add some urea.
Urea is a course crystal chemical that is inexpensive and readily available. I buy mine off Ebay.
Urea will consume or "kill' residual or unused nitric acid from the aqua regia that I used to dissolve the gold.
The more excess nitric I have in my dissolved gold solution (called Auric Chloride Solution) the more urea will be required to eliminate the excess nitric acid.
I add the urea in small amounts (1/2 table spoon at a time) and stir with a glass rod to ensure that all the urea has reacted with the excess nitric in the solution. I continue to add urea until I get no more reaction or fizzing.
Adding too much urea at once, or adding more urea before the reaction from the previous addition is complete may cause a boil over. Patience and small incremental additions will greatly reduce the chance of a boil over.
If a boil over does happen, stand back and wait for everything to stop. Go get a cold soda, take a deep breath and thank yourself for putting that baking dish under the container.
Then, carefully rinse any auric chloride solution fron the outside of the container and set it aside on a clean surface. I would then pick up the dish that received the boiled over auric chloride and pour it back in with the main solution. It is critical to ensure this dish is clean before hand - you can see why.
If there is much excess nitric acid, there will be a vigoruos reaction upon adding a small amount of urea. As the nitric acid is consumed, the reaction will become less and less. I can usually tell when I have killed all the nitric acid in the auric chloride solution when there is no reaction upon addition of more urea.
The advantage of using urea; speed. There are times when I don't have time to wait for evaporation. The disadvantage; I am adding a compound that may introduce contamination to my final gold.
method two; evaporation. I place the unfiltered auric chloride on low heat and gently warm it to evaporate off most of the liquid so that my auric chloride becomes syrupy in consistency, then, I add hydrochloric acid a few ml at a time to rehydrate the syrupy liquid.
As the hydrochloric acid is added, brown fumes will be given off (if there is excess nitric acid). These brown fumes are the excess nitric acid being driven off from the auric chloride solution.
This process is repeated, evaporating down again to a syrup, then adding hydrochloric acid to drive off the excess nitric acid. When no more brown fumes are given off, then the process is complete.
If there are no brown fumes upon addition of hydrochloric acid, then congratulations. The amount of nitric added to dissolve the gold was properly meted in and there is very little if any excess nitric acid to be driven off.
Once all the nitric acid has been driven off, as indicated by no brown fumes being driven off when hydrochloric acid is added, then the solution can be rehydrate with distilled water - about three or four volumes. I usually just note the liquid level before evaporating and rehydrate to that level with distilled water after the evaporation process in complete.
The advantages of evaporation; it does seem to produce a cleaner final product and removes the nitric acid more completely than urea (in my opinion). the disadvantage; it takes much longer to do. To me, the advantage of speed is of greater value. But if your goal is ultra-pure gold, then evaporation is the way to go.
Having excess nitric acid in the auric chlorice - even a little bit - contributes greatly to allowing other, non-gold metals, dropping down with the pure gold powder when using sodium metabisulfite as your precipitant.
Also, excess nitric acid greatly increases the chance of a boil over when precipitating the gold with sodium metabisulfite. The reason for this; as the sodium metabisulfite is added to the auric chloride solution and the gold precipitates, any unused nitric acid will immediately redissolve the precipitated gold back into solution and generate lots of heat.
The bottom line is this, removing excess nitric acid is critical. You can see here how important it is not to use too much nitric acid to begin with.
More to come later - kadriver
