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I'm running a distillation reaction at under 100 °C with the heat coming from a hotplate (combined with magnetic stirrer). Is a Florence flask the correct vessel to use on the hotplate?

Given the small contact area of the flask relative to the surface area of the hotplate I'm wondering if it wouldn't be better to use a fully flat-bottomed flask on the hotplate, and if so what best practices are? Regardless, I need to keep a stirrer running during the process.

(I intend to have a Kjeldahl bulb on top of whatever flask before feeding the condenser.)

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    $\begingroup$ Any chance you can use a sand (or other, as appropriate) bath? $\endgroup$ – Todd Minehardt Aug 18 '15 at 14:19
  • $\begingroup$ @ToddMinehardt: Maybe I could set a small layer with a dam on the hotplate? Key is I need to keep the stirrer running during the process. $\endgroup$ – feetwet Aug 18 '15 at 14:29
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    $\begingroup$ I recommend not heating directly off the hot plate. Use some kind of bath. It will be much easier to monitor the temperature, and the distillation pot will heat more uniformly. $\endgroup$ – jerepierre Aug 18 '15 at 14:30
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    $\begingroup$ I'm absolutely with @jerepierre here and I'd use a silicone oil bath. In addition I'd use a round bottom flask with a ground glass joint and a destillation bridge with cooler. $\endgroup$ – Klaus-Dieter Warzecha Aug 18 '15 at 14:51
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At under $100^\circ \mathrm{C}$ The best will be to use water bath and round-bottom flask. Most magnetic stirrers have enough range (magnetic forces do not stop working if vessel is in couple of centimeters from hot plate, though you should use non-magnetic dish for bath itself). Even if magnet in hot plate is weak I would suggest against flasks with flat bottom, because bottom may fall off under heating.

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As others already said, for reaction under 100∘C, water bath with round bottom flask would be ideal and that would also provide greater temp control, distribution and prevents heating over.

You can either use big enough borosilicate evap dish, pyrex glass often found in most local supermarkets or aluminum pot for bath itself.

You could also just use flat bottom erlenmeyer flasks which do not have perfect contact with hotplate but can be used directly and from personal experience I would say that it is decent enough and not that crucial especially when you stir your reaction to prevent any hotspot formation.

In addition you can swap water for oil bath to greatly reduce evaporation rate, especially useful for really long reactions, although your limit of transferred heat increases so overheating with poor temperature control is possible.

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