Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: atomsrus on August 16, 2014, 03:58:54 AM
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I have some white powder.
I'm sure it is either sodium metabisulfite ( Na2S2O5 ) or sodium bisulfite ( NaHSO3 ).
I think there's been a mix up due to the former chemical also being known as sodium disulfite and someone may have confused the "di-" with the "bi-".
Can anyone suggest a simple/easy way to determine if the white powder is "SM" or "SB"?
I don't have any fancy lab equipment, but I may be able to ask a local university to help.
Thanks.
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I would try using iodometry to determine molar mass of the solid.
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Thanks Borek. I'm not sure if your use of the word "try" means the technique may not work, or I may not do it correctly. Probably the latter would be true because I am not a chemist. Let's suppose I can recruit help from chemists with fancy lab machines - would they be able to identify the powder easily or not and which machine would do the job?
Thanks.
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Na2S2O5 in water converts practically to NaHSO3 then slowly evolves SO2, especially on heating. After prolonged warming the pH of solution becomes higher.
Na2S2O5 + H2O = 2NaHSO3
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Thanks Borek. I'm not sure if your use of the word "try" means the technique may not work
It means I am not 100% it will work as expected - while I see no reason why it should not work, chemistry is full of surprises and traps.
Let's suppose I can recruit help from chemists with fancy lab machines - would they be able to identify the powder easily or not and which machine would do the job?
No fancy machines required, just a titration equipment (browse titration.info for details). Difficulty level - high school chemistry course.
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If you want "fancy" try feeding power to one of the automated Sulfur content analyzers. They spit out a ppm / % Sulfur value for you.
You'd have to assume there isn't any crap / filler.
A bit like using a cannon on a mouse.
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They do have different properties. Sodium metabisulfite releases SO2, with its sharp odor, when gently heated or mixed with dilute acid. IIRC, sodium bisulfite is a little more resistant to decomposition, and doesn't release SO2. The sodium metabisulfite I have, for home winemaking, is over powering in odor just by opening the bottle.
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Thanks everyone. Interesting. This stuff I have does smell of SO2 but perhaps that could be because it has been exposed to humid air? I read that sodium bisulfite releases SO2 when mixed with water.
By the way, my interest in using this powder, which I hope is sodium metabisulfite, is to rot tree stumps. As you can read at wiki: "Concentrated sodium metabisulfite can be used to remove tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.". Do you know if sodium bisulfite might give similar results?
Thanks.