[nimbus8]

Quick question. How do you calculate for heat loss from a hot plate. For example lets theoretically say caffeine has a sublimation temperature of 120 C. And you are using a suction flask and a cold finger set ontop a hot plate for the experiment. What temp should you set the hot plate to, since setting it to exactly to 120 would not be enough because of heat loss from the hot plate to the flask.


Edit
I changed how to hot in title
bill

[Arkcon]

Typically, what you've described isn't done.  In fact, hot plates often don't even have temperature ratings, just a value from low to high.  If you need precise temperature control, you can put a vessel of water between the hot plate and the reaction flask with the thermometer.  Or put a thermometer in your reaction flask.  Then, adjust the heat source, as needed, to get the proper temperature.

[nimbus8]

Yeah the only problem with that is is that if you accidentally go too high you might sublimate some impurities.

I have a digital hotplate that can be adjusted to any temperature precisely. But i saw this guy on another forum saying that if the substance sublimates at 120 you would set the plate at like 160 in order to compensate for energy loss. I was just wondering if there was a more precise way to go about applying heat to the flask without having to compensate for so much energy loss.

A digital heat gun that goes to lower temps like 80 C might be an option. What do professional labs use?

[Arkcon]

What do professional labs use?

Skill.

 

Naw, I'm not going to leave it at that.  But you know you can't rely on that guy's comment, you have no idea how much heat is being lost between hot plate and flask.  Between a water bath and a flask, there is at most 1 or 2 ° C, and only if the temperature changes rapidly.  A thermometer in the subliming vapor would be best, but by all means, you could use the digital heat gun.

[Corribus]

A temperature setting on a hot plate means nothing unless there's a feedback mechanism for the hotplate to adjust heat output relative to a real temperature in your flask.  Different substances heat up at different rate based on heat capacity and mass; the hotplate does not know how to correspond energy output to the desired temperature.

Best way would be to use an oil bath with a thermometer or something similar.  It will give you much more control, will reduce the chance of overheating and once you learn the settings on your hotplate you can easily set it every time.