[atomsrus]

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.

[Borek]

I would try using iodometry to determine molar mass of the solid.

[atomsrus]

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.

[AWK]

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

[Borek]

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.

[curiouscat]

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.

[Arkcon]

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.

[atomsrus]

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.