I came across a protocol saying:
Homogenize 50 g powdered plant material (accurately weighed and recorded) in 2 vol (w/v) acidified methanol.
What does 2 vol (w/v) mean?
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2 Answers
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Weight per volume percent. It's basically the density.
$$ w/v = \frac{mass_{solute}}{volume_{solution}} * 100 $$
Usually it includes units too, but you didn't specify. I'm guessing it means grams per liter?
For more, see http://www.ausetute.com.au/wtvol.html
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1$\begingroup$ Yes, that is what "w/v" means. But on it's own it is not a satisfactory answer, as that doesn't address the "2 vol" bit, or give any indication of how the OP should interpret the quoted protocol. $\endgroup$– R.M.Commented Jul 12, 2017 at 22:06
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It is actually a poor protocol telling you that the mass of your solute (or disperded material) should go in a double volumetric amount of liquid. Strictly speaking you do not have the knowledge of what you are supposed to do, as units are missing. I am experienced in lab and personally I wouldn't be able to interpret the protocol without guessing, and I'll use 50 times 2 = 100 millilitres of solvent.
As additional comment, the w/v concentration should be used with caution as it is susceptible of being practically meaningless (volume contraction / dilation upon solution preparation) without further knowledge. Also, if the attained solution has to be used as a source of solute, its density must be known (unless it is aqueous and very diluted).
answered Jul 12, 2017 at 20:32