[spyro]

Hi!
    I need to make vacuum distillation (a little bit over 100 mmHg) of a moringa oleifera extract (solvent: 96% ethanol), so i can boil the solvent under 40°C to preserve most of the medicinal properties of the plant's extract.

    I have a heating mantle, so i can use round flasks, and I have an aspirator pump. But i was thinking about the possibility of purchasing flat bottom round flasks as this could bring up some versatility, 'cause I could heat the flasks on the hot plate also. I'm aware the all round (spherical) flasks are more resistant to the stress of making vacuum as their shape distributes almost evenly the force now exerted by the atmospheric pressure, but, how about the flat bottom ones?

    Are the flat bottom ones significantly less resistant to the vacuum stress and heat than the all round ones? I was thinking the hot plate maybe creates a steeper temperature gradient, as the heat comes only from the bottom. Do this temperature gradient induces an uneven response to stress that could break the flask if i heat it on the hot plate? That would be one of the reasons i think of why the mantle is better, because the heat exchange occurs in a wider surface area.

    The flat bottom flasks are almost all spherical, except for the small flat region at the bottom. Do this represents a critical weakness for the flask when doing vacuum distillation? Is heating it on the hot plate a bad idea?

    Thanks for your advice. Best regards!

[Corribus]

Depends on the pressure of course but I'd stick with round bottom flasks for that reason and because you get better even heat distribution with a round bottom immersed in sand/oil/water than a flat bottom sitting right on a hot plate. Buy a cork ring if you want to sit it on the table.

[wildfyr]

Usually suggested round bottom because it distributes heat and pressure better.

Granted, I've never actually seen a problem myself. You're probably fine.