Oil turning from light to dark

When running oil suspended in ethanol it looks like this

but after rotovaping oil comes out pretty dark. Is this natural? Am I heating oil in rotovap too fast? Any advice to keep oil light color when rotovaping?

Are you carbon scrubbing your crude during filtering?

How hot is the roto. And yes, when things are diluted they will appear to be lighter in color. When you spread it out, on parchment or ptfe paper, does it still appear dark or is it back to the lighter color. Is it winterized prior to roto

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Using bleaching clay, Charcoal, and diatomaceous earth for filtering. Yes it is winterized before roto. Currently have not spread out on parchment, have just been putting it in smaller beaker and into oven to back off left over alcohol. Roto is on 95-100f oven is set for 110f.

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Heat will darken a concentrate

Before evap and after

When you’re removing solvent from a solution that contains light absorbing compounds, you’re effectively concentrating those compounds and the resulting solution appears darker.

Heat and oxidation can cause darkening of pigments as well. If your rotovap is using a vacuum (it should be) you shouldn’t need very much heat for the water bath (not more than 50 C).

I think the likely source in this case is the presence of plant pigment that concentrates as the solvent is removed from solution. Recall the Beer-Lambert Law says that absorbance is proportional to concentration of an absorbing species, so as the pigments increase in concentration more light is absorbed and the solution appears darker.

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