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Author: Subject: Purify Sodium Metabisulfite
APO
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[*] posted on 5-2-2013 at 18:59
Purify Sodium Metabisulfite


I got some Bonide Stump Out, and I have found that it is mostly composed of Sodium Metabisulfite But I want to Purify it, It releases SO2 gas and turns to Sodium Bisulfite in contact with water, so that is not a sutable solvent for fractional re-crystallization, I just got some toluene so that may be a possible solvent, or I could use something as simple as isopropanol or methanol. Any advice or help would be of great thanks.
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UnintentionalChaos
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[*] posted on 5-2-2013 at 19:05


Titrate it for SO2 content and make a note of that. Sodium metabisulfite is rarely pure and exists with varying proportions of bisulfite present.



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[*] posted on 6-2-2013 at 08:37


How would I do that, and how would it help?
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[*] posted on 6-2-2013 at 09:01


Quote: Originally posted by APO  
How would I do that, and how would it help?


I'm saying that there is no need whatsoever for very pure metabisulfite and there is probably no good way to recrystallize it. 100% of the uses for it involve it being an acidic source of SO2 gas. Unless there is sand or other junk in it, just leave it as is. This provides a method to titrate it, by treating with a known excess of I2 in the presence of a small amount of HCl, followed by titrating the excess with thiosulfate/starch to the endpoint. http://irmm.jrc.ec.europa.eu/SiteCollectionDocuments/FinRep-...

From that you can work out the weight% of "SO2" in the solid. You could also titrate with standardized NaOH to find out how much alkali (As Na2O) was present (would be an indirect measure of sulfite content vs (meta)bisulfite. The residiual weight would correspond to water content (presence of bisulfite).

[Edited on 2-6-13 by UnintentionalChaos]




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[*] posted on 6-2-2013 at 10:35


No, I need it pure because I want to heat it because then it decomposes to sodium oxide. Any impurity will hinder it from most uses I need it for.
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6-2-2013 at 11:27
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[*] posted on 6-2-2013 at 11:32


Quote: Originally posted by APO  
No, I need it pure because I want to heat it because then it decomposes to sodium oxide. Any impurity will hinder it from most uses I need it for.

Do you have a cite for that? I can't imagine it would decompose to Na2O at a temperature lower than sodium carbonate would do the same thing. You'd be much more likely to get Na2S2O5.

It's never going to dissolve in toluene, either.
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[*] posted on 6-2-2013 at 17:40


http://en.wikipedia.org/wiki/Sodium_metabisulfite

Na2S2O5 → Na2O + 2 SO2

I've done it with a normal stove before, so It will definetly make sodium oxide.
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[*] posted on 19-10-2015 at 18:09
Not Quite


Quote: Originally posted by APO  
http://en.wikipedia.org/wiki/Sodium_metabisulfite

Na2S2O5 → Na2O + 2 SO2

I've done it with a normal stove before, so It will definetly make sodium oxide.


This article states the decomposition as : Na2S2O5 > Na2SO3 +SO2
How does using Wikipedia as the only resource count as diligent scientific research? Dig harder!




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[*] posted on 21-10-2015 at 12:47


Here is a reference that appears to suggest heating in air is not the way to go:

"ABSTRACT
Thermal decomposition of sodium metabisulfite in air involves two major weight changes before the final formation of sodium sulfate. The first is an endothermic process, in which sulfur dioxide is evolved, leaving a residue of sodium sulfite. The second change leads to the formation of sodium sulfate by oxidation of the sulfite. There are, however, intermediate decomposition reactions, occurring at the second change, which lead to the formation of elemental sulfur and to incomplete recovery of the original sulfur in the final sulfate product. "

Source: "Thermal analysis of sodium metabisulfite" by Myron Malanchuk at http://www.researchgate.net/publication/18038266_Thermal_ana...

Now, sodium sulfite, itself, is said to decompose at 600 C, which is lower than Na2CO3 (851 C), and around 600 to 700 C for NaNO2 (see http://pubs.acs.org/doi/abs/10.1021/j150545a005 but to 2 NaNO2 → Na2O + NO + NO2 per page 150 of "High Temperature Properties and Thermal Decomposition of Inorganic Salts ..." by Kurt H. Stern, or is it, Na2O2 + N2 + O2 per the source https://books.google.com/books?id=_AucCAAAQBAJ&pg=PA312&... ).

[Edited on 21-10-2015 by AJKOER]
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