I normally try and keep any tin or stannous chloride out of a solution where I may have gold dissolved, even in the stock pot, where we may be trying to cement missed gold out of solution with a metal (our reducing agent in this case).
some discussion to help in understanding some of the terms used:
oxidation=take away electrons from a metals atom making ions of that metal dissolved in solution, as a metal salt of the metal and the reduced acid used.
Reduction= give back electrons to the ion of the metal making again elemental atoms of the metal.
If something is oxidized something has to be reduced, if we dissolve gold in acids like aqua regia the aqua regia is the oxidizing agent, it oxidizes the gold, takes electrons from the golds atoms making gold ions (Au+ ions {cations}), the aqua regia is reduced (gaining electrons from the gold)(hydrochloric acid here reduced to chloride ions (Cl- ions {Anions}), these ions in solution are our dissolved salt of gold we call gold chloride AuCl3.
If we use a reducing agent like copperas (SMB, copper metal..) to precipitate (or cement) our gold (or give gold ions back electrons so that it again forms elemental gold atoms in clusters of these atoms large enough to sink to the bottom as brown powder precipitate of gold, ready to wash and melt back to a bar of gold
The reducing agent is oxidized in solution (to a higher oxidation state),as it gives electrons to the gold ions to form the gold atoms.
Or in the case of the metal used like a copper bus bar, the copper is oxidized to copper ions in solution, as it reduces or cements the gold from solution (copper metal giving the gold ions electrons so it cements as a powdered gold metal), the copper acts as the reducing agent for gold in solution, the copper being oxidized to copper ions as the gold ions gaining electrons are reduced back to metal atoms (powders of metallic gold).
We can oxidize some things to a higher oxidation states, or reduce some things to lower oxidation states...
But gold metals atoms having a full shell of electrons cannot be reduced further (it already has all of its electrons), (it cannot be given excess electrons), or cannot be reduced again it does not need any electrons...
The reasoning in my mind for keeping the stannous solutions seperate from my stock pot or anywhere I suspect (or know where) gold is dissolved in solution (as gold ions), the stannous can reduce the gold to insoluble colloidal gold, this (reduced) colloidal gold cannot be (reduced) again (the gold already being reduced by the stannous) has already gained electrons from the tin chloride.
These elemental gold atoms or colloids of gold atoms, being reduced are elemental gold metal atoms (no longer gold ions) the colloids in such a fine state, they develop a polar charge causing them to oppose each other like small magnets of the same polarity, pushing each other away, so they no longer come into contact or join to grow to be seen or grow to large enough particles of gold atom clusters to over come gravity or sink, or stop moving in solution to precipitate
The gold colloid will virtually never settle (unless the colloids same polar charge is broken of it same charge, since the gold ion has already been reduced to elemental gold atoms it cannot be reduced again by a reducing agent (SMB, Copperas...) or cemented out of solution with the coppers metals electrons.
Once the gold has formed a colloid (already being reduced to metal atoms) (we cannot reduce the already reduced gold we cannot test for it in solution with stannous chloride or copperas (our iron sulfate crystal), because these tests rely on our test to reduce by the reducing agent we are using to perform the test SnCl2 (where the gold forms a purple color) or FeSO4 (where the gold reduces to show a brown color of reduced gold around our copperas crystal).
Normally the stannous waste is small compared to our other waste, I try to keep it out of my stock pot (if I suspect it holds gold colloids in solution I treat it to try and recover that gold by breaking the colloidal bonds), I treat the waste separately or use it to change the pH of another caustic waste I may be in the process of treating...
Similar to keeping ammonium solutions from my other waste and treating them separately from my other waste solutions, for safety reasons where I could possibly make a dangerous situation in the lab, by mixing these.
