Q: What is reflux used for?

A: Temperature control; the liquid or solution stays at its boiling point. One can choose a solvent according to the desired temperature.

Q: Won't the vapor be lost out the to of the condenser.

A: Not much escapes. One can reflux solutions for days on end. If using a very low boiling solvent like ethyl ether of dichloromethane, one may need to use ice-cooled water in the condenser, or an especially efficient condenser design or both. It is important not to plug the top of the condenser - heating a closed system is dangerous.

Q: Why have boiling chips in the flask?

A: A few of these porous, inert chips will promote smooth boiling by presenting sites on which gas bubbles can begin to form. Without them the solution might "bump," superheating and releasing a lot of gas at once, and perhaps shooting out of the condenser.

Q: How high should I turn the temperature?

A: Just enough that slow dripping of the condensate is seen. One nice thing about reflux is that you do not need to fine-tune the temperature - if it is set a little higher the vapors will travel further up the condenser tube, and the drip rate will increase. (Only if the temperature were way too high would vapors escape the condenser. One can see the upper limit traveled by the vapors as sort of a "mirage," called the reflux ring, and caused by a difference in the index of refraction for air versus the solvent vapor.)

©2001,2002 Daniel A. Straus