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What does it mean when a substance (powder) is 'dissolved in DMSO and used at $\mathrm{5~\%}$'

Does is mean that the substance is first dissolved in DMSO and then added to another solution where it makes up $\mathrm{5~\%}$ of that total solution? Or does it mean something else?

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    $\begingroup$ When adding a solute to a solvent the only way that a "5% solution" makes sense is by weight percent. When mixing two liquids it could mean "volume percent" (vol. %) or "weight percent" (wt. %) so the writer should specify which is intended. $\endgroup$
    – MaxW
    Dec 31 '15 at 15:21
  • $\begingroup$ See my comment on the answer below $\endgroup$ Jan 3 '16 at 14:32
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Typically, the chemical substance in question is first dissolved by adding 100% DMSO - to some extent for practical reasons, i.e. all the advantages of working with stock solutions. The main reason (to my mind) is, however, that compounds often do not dissolve easily in (aqueous) solutions of DMSO for kinetic reasons, but are perfectly happy when diluting with water afterwards.

For your downstream application, e.g. some biological or biochemical assay, the 100% solution is diluted with the assay buffer to e.g. 5% DMSO. The latter is almost exclusively volume per volume%.

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A good author should express whether percents are weight $(wt./wt.)$, volume ($vl./vl.$) or weight/volume ($wt./vl.$). This unfortunately does not happen often. Typically for solids dissolved in liquids the percent is weight/volume given as : $$\frac{weight}{volume}$$ and expressed as: $$5\% (wt./vl.)$$

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  • $\begingroup$ Thanks but I am confused as to whether 'dissolved in DMSO and used at 5 %' means that the powder is first dissolved in DMSO and then that liquid is used in an aquas solution at 5% (vl/vl) OR if it means the powder makes up 5% (wt/vl) of a DMSO based solution. $\endgroup$ Jan 3 '16 at 14:31

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