livinthedream said:
I have no good way to determine an accurate amount of Au present, so could I still use this method?
I hope I am addressing your concern.
Using the right amount of acid requires that you have patience, and that you understand that too much HCl does no harm, and too much nitric does. I reflect on the patience because it's common knowledge that those unfamiliar with the way acid works on metals may not be something with which they are familiar. When they don't see violent action, it is often assumed that the reaction has ended, when that may not be the case. As the acid diminishes and the volume of remaining material does as well, the reaction becomes somewhat subdued. Adding more acid accelerates the reaction, so it is commonly wrongly assumed that the acid was exhausted. So then, in order for you to use the right amount, you must be patient and wait until you see NO further action, after having added some fresh HCl.
How well this system works (using the proper amount of acid) is reliant on your ability to know that all of the values have been digested. That isn't always possible. I offer you, as an example, dissolving filings from the jeweler's bench. Because the filings are typically accompanied by bits of abrasive, it isn't always clear when everything has been put in solution. Therefore, it was always my intention to use just a little too much nitric. That way I could insure that I was NOT leaving behind any undissolved values. I then added a weighed button of pure gold to the solution, which consumed the traces of unused nitric. It was important for me to process in this manner, for I did NO assaying of customer materials. They were always processed as individual batches, flagged with the name of the owner attached to the vessel in which it was processed. By weighing the button of added gold, I could then determine how much gold was added to the lot, so the owner received a proper accounting of that which was recovered.
I heartily endorse Chris' method----but it can get you in trouble if you lack experience. Play it safe and use a little excess nitric, then consume it in evaporation. The alternative is to intentionally use too little nitric, allowing the solution, which has an excess of HCl, to digest until all action ceases. You can then safely assume that the nitric acid you have added has been totally consumed, evidenced by the presence of yet undissolved values. Decant the pregnant solution, then start with a fresh batch of acid. That way, if you do happen to use too much nitric, the volume of solution that requires evaporation will be small, so evaporation will proceed quickly. You would, of course, still add a button of pure to the lot, which would consume the unused nitric acid.
Don't be concerned about breakage. As long as you use vessels that are intended to be used in the lab, or are intended to be heated, breaking is not an issue. Use a thin asbestos pad between the heat source and the vessel. Be certain you work such that should you break a vessel, the solution can't readily escape. NEVER work in a sink, not unless the fluid runs to a catch barrel instead of a drain. Keep a supply of paper towels near. You can soak up a spill and achieve a respectable recovery that way. All that is required to recover the values is to incinerate the paper and to then reprocesses the ash.
No apology is necessary. I am not the least bit concerned about your comments----I just want to insure that you have a clear understanding. Being misinformed is not in anyone's best interest.
Harold
