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Author: Subject: Refreshing old Sodium Dithionite
nimgoldman
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[*] posted on 20-1-2020 at 18:14
Refreshing old Sodium Dithionite


I have about 1.5 year old sample of commercial sodium dithionite powder.

I stored it in plastic bottle and it seems to deteriorated over time. It smells sharply of SO2 and the aqueous solution turns yellow. I use it for silver precipitation and it still works.

I am thinking of recrystallizing it but it is also known to be unstable in water, especially acidic.

How would you go about cleaning the old dithionite? My first thought is to dissolve it in dilute ammonia soln. and add enough ammonia to make the solution basic/neutral, then concentrate it to the point of saturation, let the crystals of sodium dithionite precipitate and finally dry in a vacuum desiccator.

I am going to store it in a glass Wheaton bottle which seems to be more airtight than plastic...
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woelen
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[*] posted on 20-1-2020 at 23:37


Most likely your sodium dithinite has not deteriorated. The material is quite stable when dry, but a solution in water decomposes. The coloring of the solution is normal, it decomposes at low pH, but also in neutral solution. The smell of SO2 also is normal for dithionites. So, just repackage it in a glass bottle with a good airtight cap.



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DavidJR
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[*] posted on 21-1-2020 at 11:29


I agree - I don't think it's worth attempting to purify it.
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nimgoldman
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[*] posted on 21-1-2020 at 11:53


Alright, I will just repack it.

I found the 20% sodium dithionite soln. is only slightly acidic and adding a few drops of dilute ammonia easily make the soln. strongly basic, in which case only very slight yellowing occured upon standing.
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woelen
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[*] posted on 22-1-2020 at 00:32


Yes, at high pH a solution of dithionite is more stable. In the long run, however, any aqueous solution will decompose.
If you experiment with the salt, be careful with its reactivity. Under certain conditions, when it becomes somewhat damp, it can react with oxygen from air and so much heat can be produced in that reaction that a fire is started. So, repackaging should be done quickly and carefully, outside in a place where an accidental fire does not cause a disaster.




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