kapasari wrote:If stanous Chloride does not work, how do we detect
the gold concentrate on the solution?
I try to drop them using zinc, but as I only use
small test sample, it is not very clear.
I did get some very fine brownish perticipation,
but is not much.
Stannous chloride won't test for gold in a cyanide solution because the gold has a valence of one (I). Using stannous chloride requires the gold to be valence 3 (III), which is the state of gold in a gold chloride solution
As far as I know, there is no simple method of detecting or analyzing gold directly from a cyanide solution. Exceptions would be the use of atomic absorption or ICP instrumentation.
Most all traditional analysis methods involve collecting the gold in lead and then using fire assay (there are 3 or 4 different ways to do this). An example would be to make a leak-proof box (or, boat) from lead foil and then slowly evaporate a sample (from 5ml to 50ml) of the solution in the box over gentle heat. When dry, the foil is folded up, with the dried residue inside, and then directly cupelled or fused/cupelled. A direct cupellation will "spit" some, but the losses should be minimal. I have used this method many times. If the NaOH is too high, though, it can attack the lead and the boat will leak. Instead of evaporating in the lead boat, the sample can be evaporated slowly in a small separate container (a watch glass can be used if the temperature is kept very, very low - otherwise, it will break). The residue can then be cooled, carefully and completely scraped out and collected, wrapped in lead foil, and cupelled.
For quantitative results, the zinc method you are using may work, but I would use a larger sample - maybe a liter. Add enough zinc (325 mesh zinc dust is preferred) to barely make the precipitated powder gray colored. At first, the powder will be brown (gold and/or copper). When you've added enough zinc to produce a gray color, all of the gold should be precipitated out. The zinc tends to lump up in storage, so put some through a small fine flour sifter before using. Try not to use a great excess of zinc. It should take from 1 to 2 grams of zinc per gram of gold. Filter and rinse very well to remove all traces of cyanide. Under the fume hood
, cover and leach the residue with 50/50 nitric for about 30 minutes - keep it hot. All of the zinc and copper, etc., should dissolve. A brown gold powder should remain. Filter, rinse well, dry, and weigh the powder.
If all you want to know is whether gold is present, or not, I would collect the brown powder that you say you already have (from zincing), rinse it well, leach out possible base metals with nitric, rinse, dissolve some of the residue in a few drops of aqua regia, and test this solution with stannous chloride. A white spot plate would work well for this.
There is another colorimetric way I know of to convert the sodium gold(I) cyanide to a gold(III) chloride and then use stannous chloride to check for the presence of gold, but it is quite involved .
It is also possible to treat a sample of the cyanide solution directly with very hot concentrated H2SO4 plus a few drops of nitric acid (Under a very good fume hood!!!
) for an extended period of time. This is called "wet ashing". The cyanide is destroyed and the gold comes down as a brown powder. The procedure has been covered somewhat on the forum. It is involved and can be quite dangerous if you don't know what you're doing.
If possible, I would send your solution samples to a qualified lab and let them do it.