foto_fool
Well-known
The steel wool thing works by an inorganic oxidation-reduction reaction. Because of their different electrochemical potentials, elemental iron in the steel wool oxidizes to give up electrons to the silver thiosulfate complex. This reduces the silver(II) to silver(0), the metallic state, which falls out of solution - this is the black sludge.
I have read varying reports of the efficiency of this process. In theory it should be close to 100% - I believe the report of "60%-70%" is actually the purity of the precipitate. Note that commercial labs use electrodeposition to recover the majority of silver from their spent solutions, but then add a downstream iron canister to catch that last percent - suggesting this iron replacement process is very efficient.
The dithionite and borohydryde reactions work the same way - by reducing the silver in solution to the metallic state, which precipitates. As with iron, these reactions are also nearly quantitative, and are faster and yield a more pure silver in the precipitate. Note though that these powdered chemicals are pretty seriously toxic - use gloves, a dust mask and eye protection.
As has been noted, a byproduct of the oxidation-reduction reactions is a small amount of hydrogen sulfide gas (from reduction of the thiosulfate) - the "rotten egg" smell. Don't do this in a closed space, as H2S can be toxic in concentrations over 150ppm in air. Fortunately, most humans will retch and run if the concentration reaches 10-20ppm.
I have read varying reports of the efficiency of this process. In theory it should be close to 100% - I believe the report of "60%-70%" is actually the purity of the precipitate. Note that commercial labs use electrodeposition to recover the majority of silver from their spent solutions, but then add a downstream iron canister to catch that last percent - suggesting this iron replacement process is very efficient.
The dithionite and borohydryde reactions work the same way - by reducing the silver in solution to the metallic state, which precipitates. As with iron, these reactions are also nearly quantitative, and are faster and yield a more pure silver in the precipitate. Note though that these powdered chemicals are pretty seriously toxic - use gloves, a dust mask and eye protection.
As has been noted, a byproduct of the oxidation-reduction reactions is a small amount of hydrogen sulfide gas (from reduction of the thiosulfate) - the "rotten egg" smell. Don't do this in a closed space, as H2S can be toxic in concentrations over 150ppm in air. Fortunately, most humans will retch and run if the concentration reaches 10-20ppm.
