Gold 9999 with oxalic acid.

The Use of Oxalic Acid. This method was mentioned briefly as being especially suitable for the recovery of gold from solutions that contain relatively few impurities. Its disadvantage is that it will work only when all the solutions are hot preferably boiling which means that the precipitation must be carried out in vessels that will withstand considerable heat. It costs more than copperas or sulfur dioxide, about the same as sodium nitrite. Under most conditions, the gold recovered is of high purity. For each Troy ounce of gold, you will need at least a Troy ounce of oxalic acid crystals. If you failed to expel all the excess nitric acid, you will need more. Dissolve the oxalic acid in hot water. Add a little ammonia, say a teaspoonful or less to an ounce of oxalic acid; add slowly to avoid spattering. Do not use too much ammonia the mixture should not smell of ammonia after being stirred. If it does, add enough additional oxalic acid to kill the ammonia smell. If you have litmus paper, use it here, and let the mixture show a slight acid reaction. Have the mixture in a big beaker or jar that will withstand heat. Next, heat your aqua regia solution to boiling. Also, have the oxalic solution hot boiling if possible. Pour the gold solution slowly into the oxalic acid solution. Gold comes down at once with hot oxalic acid but takes hours or even days if it is cold. Some people pour the oxalic into the gold; we think it is better to pour the gold into the oxalic. There will be bubbles and fumes, so work slowly or the liquids may boil over when they come together. If you failed to expel all the excess nitric acid, brown fumes will appear here; if this happens you may have to add much more oxalic acid to kill all the nitric acid. Let the mixture stand for some time, keeping it hot if possible. The gold comes down as bright clean crystals, often very beautiful, which usually settle promptly. Finally pour or filter off the liquid, after testing some of it with the stannous chloride testing solution to be sure you used enough oxalic

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