stannous chloride will reduce other metals besides gold, to reduce means to get back an electron, and convert back to metal, so if your solution has other metals, if you have a lot of these metals in solution it would not take an overuse of stannous chloride to reduce some of these metals back to metal to show a dark color in your test, on the other hand if you had very little base metals dissolved with your gold in solution in the test the violet color may be seen better as gold is the predominant metal reduced in solution giving the solution the recognizable purple color, then again if you only had a little metal in solution and gold was precipitated out of solution and you did not overuse the SMB the stannous would not have much metals to reduce, and you also did not reduce some of these metals with using too much SMB. so the test may show a yellow color from just normal acids and base metals, but the use of too much SMB reducing some base metals, plus the stannous chloride reducing them no wonder you get a brown reaction of reduced metals in the false positive test.
The stannous test works great when used properly, but just like our recovery and refining process we need to be able to do them properly, the name of the game seems to be the less base metals in solution, not overusing acids and chemicals, and proper procedures help to make everything work smoother.
SnCl2 + CuCl2 --> Cu + SnCl4
I am not sure but for copper chloride, and sodium metabisulfite I suspect something like this:
CuCl2 + Na2S2O5 --> 2NaCl + 2 SO2 +SO3 +Cu
I ended up with some salts and gold on the bottom. But how do I see if there is more gold in the solution?
