DISTILLATION

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Q: Why set the thermowell on an iron ring instead of on the benchtop?

A: Primarily so that it can be lowered if it were necessary to stop the distillation abruptly.

 

Q: How fast does the water in the condenser need to flow?

A: Very slowly. If you notice a sink filling up, let the instructor know.

 

Q: Why do we need to anchor the drain hose?

A: So the water drains in the trough only. We have flooded the optics lab downstairs in the past.

 

Q: Should I distill as much as I can?

A: No. Stop when there is still a milliliter or so in the pot, so the flask does not get too hot. (In the research lab, you may have reactions in which there is a significant amount of higher molecular weigh byproducts from which you are distilling and there will be considerably more residue than will be our experience in the teaching lab.)

 

Q: When I look up boiling points, I find different values for the same compound with superscripted numbers - what does this mean?

A: Those are boiling points at different pressures, and the superscripts refer to pressure in mm of Hg. The reason for this is that we often do distillation under vacuum to lower the boiling point, and thus prevent decomposition. In Chem 113A we will not be doing vacuum distillation.

 

Q: What are the most common experimental errors in distillation?

A: •Placing the thermometer too high will lead to a low value for boiling point - be sure all the mercury in the bulb is below the sidearm of the still head.

•Inadequately clamping the apparatus can be dangerous. Over clamping can be a problem, since it causes stress in the apparatus. Refer to the Distillation section for detail on clamping the apparatus in place and clamping the pieces together. It is especially important to tighten the nut which keeps the utility clamp from rotating.

•Check that the joints are well greased and that all the joints are seated before turning on the thermowell.

•Remember to turn on the cooling water.

•Remember to use boiling chips.

 

Q: I see my distilland boiling and going down in volume but there is no distillate appearing - what is wrong?

A: Check that the cooling water is on - if it is then you must have a leak in the system. Stop the distillation immediately (turn off and lower the thermowell) and locate the problem (either a crack or a loose joint).

 

Q: How do I clean the fractionating column after use?

A: Rinse it through with some acetone and suck air through it using the vacuum tap to dry it. Please don't use soap and water as soap residue leads to foaming for the next user.

 

Q: In what order do I set up the apparatus?

A: Start by clamping the pot securely atop the thermowell (supported in turn on an iron ring) and build from there one piece at a time.

 

Q: Is this the same kind of "still" used to make bootleg spirits?

A: Pretty much. Take a look at this commercial moonshine still and you'll see all the same elements (pot, column, condenser) only in metal.

©2001,2002 Daniel A. Straus

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