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Ferrous sulfate, copperas or green vitriol (FeSO4.7H2O) is used for precipitating of gold from acidic solutions and also in testing. It is a pale green crystal substance or liquid.
Ferrous sulfate bought as copperas and used as plant supplement is often oxidized into brown or white ferric sulfate and useless for precipitating gold.
There are ways to remove most of the water in ferrous sulfate, but in refining that isn't useful. The dehydrated ferrous sulfate is easily oxidized to useless ferric sulfate.
Usage in testing
When testing solutions with gold in it with stannous chloride the presence of PGM:s can be masked by the gold reaction. To remove gold from solution a drop of solution is put into a spot plate and a crystal of copperas is added, this drops the gold and the remaining drop of solution can be tested with stannous without the gold interfering.
The copperas crystal in the spot plate also develops a dark cloud of metallic gold that is easily spotted. So it can be used for detection of gold in solution too.
Making ferrous sulfate
Ferrous sulfate is easily done from soft iron (for example transformer cores) dissolved in sulfuric acid. The resulting crystals can be stored if protected from oxygen.