kwxj61b said:
First I filter out that solution. Then w/ that gold sample and white precipitation/residue, I put it into a beaker with 100ml of nitric acid. I add heat for 15 minutes. But I notice that white stuff did not dissolve...so no silver?
That will not work. You already have a crust of silver chloride on the undissolved metal. Silver chloride will not dissolve in nitric acid.
The reason we inquart is to prevent the situation you have run into.
If you put a piece of karat gold in nitric acid, only a bit of the alloying metals like copper and silver will dissolve near the surface, but fairly quickly the reaction will come to a stop because the gold shields the alloying metals deeper inside the chunk of metal from being reached by the acid. The result will be a piece of metal that is fairly pure gold at the surface of the piece, but pretty much the original alloy deeper inside.
If you put the same piece into AR, the AR will dissolve the gold at the surface, but as it does so, it forms the crust of silver chloride you now have, which shields the gold from being contacted by the AR.
When you inquart, you add enough silver to the original piece to bring the gold content down to about one quarter of the total metal weight. You can put this inquarted metal into nitric acid, and the nitric will dissolve the silver. Because the gold content is only about one quarter of the total, it does not shield the inner parts from the acid, so the nitric acid can dissolve virtually all of the silver. This will leave you with relatively pure gold that can then be dissolved in AR.
There is a solution to your problem. First, you can put the remaining piece of undissolved metal in some ammonia. The ammonia will dissolve the silver chloride. Once it has done so, remove the remaining, cleaned piece of metal from the ammonia.
Promptly add some HCl to the ammonia solution to acidify it.
If you don't, the ammonia and silver can form explosive compounds.
Next, take the remaining piece of metal and hammer it as thin as you can. I don't mean a whack or two. I mean beat it till your arms are sore. You need to get it very, very thin. If you get it thin enough, you'll be able to put it back into AR to dissolve the gold. By beating it very thin, you expose a lot more surface area to the acid, and while the silver chloride will again form on the surface, if you've beat it thin enough, the acid will be able to penetrate to the center and dissolve all the gold. Before you put it in the acid, twist the thin metal so it doesn't lie flat on the bottom of your beaker so the acid can get to it from all sides. If the same problem happens again, you can repeat putting it into ammonia to dissolve the silver chloride crust (remember to again acidify the ammonia), then returning it to AR till it has all dissolved.
Stop using so much acid! It's a huge waste. You started with 5 grams of 18k alloy. If it was thin enough, it could have been dissolved in about 20 to 25 ml. of AR. If you had inquarted it by adding around 13 to 14 grams of silver, the silver could have been dissolved with about 15 to 20 ml. of nitric acid and an equal volume of distilled water.
Please understand that what I've given you here is a
very brief description. You need to do more studying. All of these processed are described more thoroughly in Hoke's book and on the forum. My goal was to provide you enough information so that you can search out more complete descriptions of the processes.
Best of luck with your studies and your gold.
Dave
