Dithionite for silver recovery from spent fixer

Raghu Kuvempunagar · Mar 13, 2020

Sodium dithionite is often used to recover silver from spent fixer. What are the chemical reactions involved in this process?

AgX · Mar 13, 2020

Dithionite:
S2O42- + 2 H2O <> 2 SO32- + 4 H+ + 2 e-

grainyvision · Mar 13, 2020

Dithionite is a very powerful fogging developer, especially in alkali solution. I assume it will reduce any soluble silver halides (including silver thiosulfate) to metallic silver which is of course insoluble and will drop out of solution. I'm not sure how to actually extract the silver after it precipitates though. In my tests the powder was too fine for filtration and if left to dry would also contain a lot of sulfur. It does work easily for making the used fixer liquid safe to dispose of down the drain and then you can just throw away the precipitate since nothing there is toxic really.. but if you want to extract the silver and melt it down etc then it requires some kind of other step

NedL · Mar 13, 2020

This reminds me that I've been meaning to find some "rust out" to try as a fogging developer in H2O2 + CA paper reversal process. I went to two different hardware stores one day that didn't have it. Thiourea toner works, but it's possible that dithionite would not work because the bleach process leaves silver citrate behind rather than removing the metalic silver completely. But I've been using sufide to precipitate silver from waste solutions with AgNO3, and it might be great for that and for treating used fixer as well. I wonder if there is a way to test if any silver thiosulfate complexes are left in solution? Any idea?

Raghu Kuvempunagar · Mar 13, 2020

AgX said:
Dithionite:
S2O42- + 2 H2O <> 2 SO32- + 4 H+ + 2 e-

Thanks! What happens next? I mean, how does this work with silver thiosulphate complexes to precipitate silver?

AgX · Mar 14, 2020

S2O42- + 2 Ag+ <> SO32- + 2 Ag

for the Thiosulfate-complex something like this:

[Ag(S2O3)2]3- + e <> 2 S2O32- + Ag+ + e <> 2 S2O32- + Ag

NedL · Mar 14, 2020

That fits what jnantz said about it stinking... sounds like this is something you'd want to do outdoors I'm not a chemist but maybe the 2 e- part means that it would do a very thorough job of removing the silver from fixer, with very little left in solution. I think I made a mistake in my earlier post.. the product in the US with dithionite is called "iron out". If it can really precipitate all the silver from spent fixer, it would be very useful for those of us on septic systems who save our fixer to take to the hazardous waste facility. I want to learn more.

Dithionite for silver recovery from spent fixer

Raghu Kuvempunagar

AgX

grainyvision

NedL

Raghu Kuvempunagar

AgX

NedL

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