I don’t always do a small scale distillation, but when I do I use the Kugelrohr.

Kugelrohr Labeled

One of the lab’s kugelrohrs.

Distilling 1 mL or less has a number of complications, mostly due to condensation outside of the receiving flask.  In the standard setup the compound of interest boils from a RBF, liquifies over what is essentially a water jacketed condenser, then drips into the final receiving flask.  At small volumes there just isn’t enough liquid to form drops, and instead a fair bit of the yield is lost as dew on the condenser (which will also evaporate and leave the system via the vacuum line).

The kugelrohr condenses compounds directly in the collection flask, with the shortest practical distillation gap (about 2.5 cm/an inch).  Very little sample is lost, (if any), and the entire process generally takes less than 20 minutes or so.

Using the Kugelrohr

Transfer your sample of interest to a small round bottom flask, then attach at least two kugelrohr bulbs (female ground glass joint on top, male on bottom).

Place all but one bulb in the air furnace and chill the last bulb to at least 0C [1].  Slowly spin the bulbs (about 1 rotation per second) and gradually increase the heat.

Compounds will distill slowly, appearing first as a thin film then as single droplets.  When two or more samples are being distilled there will generally be a noticeable gap when the volume in the collection bulb does not increase.  At this point slide one of the bulbs in the air furnace out, and cool.

Repeat as needed.  If necessary collection bulbs can be swapped out for fresh glassware, and 3+ compounds can be collected.

KR - PipetWhen finished, allow the bulbs to cool.  The easiest way to drain out samples is to use a curved pipette tip (~30 degree angle, made by holding the pipette at an angle over a bunsen burner).  Use a bit of solvent to rinse the walls, then condense to final volume on a rotovap or with an inert gas stream.

Disassemble the kugelrohr.  Store glassware in the kugeldrawer.

Finding a Kugelrohr

There are now a number of commercial kugelrohr kits, but the physical requirements of the process lend themselves well to ad hoc construction.  The best examples are immortalized in the pages of J. Chem. Ed.,  in loving hand-drawn detail.  The hardest part to source is the heat source, though the air furnace can be replaced by upside-down variacs or spare rotovap.  For the discerning chemist I recommend the work of Guida and Gawley, as they “have found that commercially available “toaster-ovens” suite the purpose beautifully.” [2]

KR - Bath[1] The mode of cooling is a major point of departure in the kugelrohr guides I’ve seen.  I use a a 20 mL vial filled with crushed dry ice/acetone (~9:1 mix), which is loosely held above the outer bulb with a 3-prong clamp.  With good glass-glass contact this cools the collection bulb to -15C or so, far below what you can get with a stream of acetone or DCM.

[2] Urban legend has it that the first Alrich Kugelrohrs were the remains of stray coffee pots, combined with stripped down windshield wiper motors (bonus low-rent personal design).