Do I need to dry my solvent if I evaporate it completely to have my extract?

Question

All the procedures tell that we must use drying agent in the solvent to remove water. But why it is so important? I evaporate all my solvent then I put my extract in solution with methanol and then measure concentration. I could understand that impurities in water could affect my experiment if I'd take the mass of my extract but, in my case, only the concentration of a compound X.

Thanks

I evaporate completely the solvent and I'm not weighing the compound that I extracted. I only put it in solution in methanol and then analyse it by HPLC.

Answer 1

Anhydrous magnesium sulfate is used to dry the organic solvent phase isolated after an extraction of your solute (compound) from an aqueous phase. After isolating the organic phase, there is a certain amount of water present (solubility factor, suspended microdroplets of water, how careful you were at separating the layers). While it is relatively easy to remove the solvent (ie: ether, ethyl acetate, hexane) by evaporation, it is more difficult to remove traces of water. Often water becomes hydrogen bonded to your solute resulting in a sticky mess that will affect your weighing measurements. Water is especially hard to get rid of if your compound is a liquid. Weighing errors will lead to inaccurate concentration measurements. Inaccurate concentration measurements lead to less than an "A" grade on your experiment.

The magnesium sulfate hydrates rapidly (forming both magnesium sulfate monohydrate and magnesium sulfate heptahydrate) and is then easily separated from the organic phase containing your dissolved solute by simple filtration. Voila, the water is now gone, you have a "dry" organic phase which, when evaporated, will give you a dry compound.

Answer 2

depends on the compound. some react with water, but if you're only using it for extractions you should be able to get away with not drying the solvent. if the extraction mixture involves water you need to dry it to get rid of the excess water because to evaporate water you need high temps and that can kill your product.