In my chemistry classes in high school, I was teached never to distill to dryness because of the possibility that instable peroxides could form. Is this also a risk with low boiling solvents like petroleum ether? I would like to recover and speed up the evaporation by distilling the solvent fully off in a hot water bath. Of course this could give some flask cleaning challenges but that’s accounted for.

Petroleum ether should not form any explosive peroxides if it is comprised solely of alkanes ("paraffins"). If it had a significant content of alkenes ("olefins") there would be a risk of peroxide formation. For example, cyclohexene is known to form explosive peroxides on storage. Once the solvent has been used - which is surely so in your case as you mention recovery of the solvent - then it is important to consider what might possibly remain in the solvent and whether these compounds might be dangerous, or be capable of forming dangerous reaction products including but not limited to explosive peroxides.

The tiny amount of additional (and relatively cheap) solvent that you'll recover by distilling to dryness is hardly worth the effort and/or cost of cleaning or disposing of the ensuing mess in the glassware. Furthermore, the charring of the residue will very likely end up contaminating the tail end of your recovery distillation anyhow. Cut it short for a happier life and consider how much you've saved by leaving a pourable/pipettable sacrifice to the glassware gods.

Thanks. The recycling is not that important. The extract will be processed further in the same flask It’s more of a concern that I don’t want to have any petroleum ether left in it or unstable situations

I was looking back through some old threads about distillation a couple of days ago and I noticed one thread where it was mentioned that you might want to pre-distil your petroleum ether before using it in an extraction where it will be evaporated off to such an extent. This is because the heavier fractions will boil off last - the result being that if your pet. ether has a wide boiling range there's a high risk DMT will start co-distilling with heavier naphtha fractions. Using a solvent with a narrow boiling range prevents this scenario - assuming it's the solvent fractions with a low enough boiling point that are being used, of course.

That’s indeed one of the reasons do distill as well: to make sure the initial petroleum ether has no residues in it when evaporating. I have 40-60 petroleum ether so the boiling point should be low already.
In the meantime I received a 500ml Soxhlet as well. I got this to experiment with extraction of baicalin and artemisinin but also wondering if the following for cactus could work:
- do a couple of petr ether cycles on dry cactus to defat
- replace the flask with ethanol and do a couple of cycles to het the alkaloids out.
- as the remaining extract should be low on fats, the idea is to dissolve it directly in a small amount of 5% Hcl to convert the natural alkaloids to Hcl and evap off the Hcl.
- clean up the extract direcly with ice cold IPA and/or Acetone.

The general consensus seems to be that the pressure cooker method saves a lot of time, but the a/b or stb is a quite messy procedure with a lot of transfers and pulls. So albeit more time, I could imagine that the extraction is a lot less messy and most steps take place in the same apparatus.

Could this work or is it doomed to fail?

Yes, this could work, but it is better to make cactus alkaline before extraction with ethanol.
But don't expect pure crystals, ethanol is not very selective, most likely you'll end up with resinous outcome.

If you're doing cactus extraction you might want to try ethyl acetate instead. I'd be hesitant to suggest using that solvent in a soxhlet, however. The problem with this is the likely co-reactivity of mescaline with the ethyl acetate, so if you decide to try this do it with a small amount first. I'd hate to be responsible for a ruined extraction - but the experimental data would be interesting.

@downwardsfromzero Thanks, I’ll try that new ethyl acetate tech soon as I have some ethyl acetate and conc. sulfuric. Seems like a easy one without too much steps.

@doubledog: thanks, but I want to try specifically without the alkaline addition as it has to be neutralized leaving residual neutralization reaction salts which can’t be filtered out properly without doing an basification , extraction and salting again right? As the fats should be mostly absorbed by the first cycles of petr ether I hope the remaining ethanol extract converted to Hcl could be cleaned to near pure crystals with a few water free ipa / acetone washes.

Please note that with the ethyl acetate you'll best be using citric acid. Sulfuric is too strong and will likely hydrolyse the solvent as well as being problematic if added in excess. I mean, unless of course you're happy to add to scientific knowledge. It's just that citric acid obviates the need for the titration that is a must when using sulfuric.