Vacuum distillation

Vacuum distillation is a process of purifying substances that are difficult to distill at normal pressures, or to save time or energy. This separation technique is based on variations in the boiling points of the components. This technique is used when attempting to boil the desired compound may cause it to break down. Lowering the pressure causes the boiling points of the constituents to decrease. Appropriate vacuum pumps described in this article.


a) Vacuum distillation setup
b) This setup connected to a vacuum trap and water aspirator. Arrows show direction of suction
a) Applying grease to a joint, b) Adequately greased joint, c) Cheking an aspirator’s suction, d) Vacuum hose attached to the vacuum adapter.

Prepare the Apparatus

• Safety note: Carefully examine all glassware to be utilized with the vacuum distillation for stars, cracks, or any other fractures, as these may lead to implosion when the pressure is lowered.

• A stir bar is necessary for the prevention of bumping. Boiling stones are not suitable for vacuum distillations because air held in the pores of the stones is rapidly removed under vacuum, resulting in the stones failing to generate bubbles.

• Even though greasing is a matter of personal preference with straightforward and fractional distillations, all joints should be lubricated in vacuum distillations or the system will leak and not reach a low pressure.

• Put together the apparatus close to the vacuum source. If utilizing a water aspirator, test to ensure that the aspirator works properly since some are more efficient than others. To check an aspirator, affix thick vacuum hosing to the nub on the aspirator, turn on the water and feel for suction at the end of the hose with your finger.

• A Claisen adapter should be incorporated into the apparatus as solutions under vacuum tend to bump severely.

• Connect thick-walled tubing to the vacuum adapter on the distillation apparatus and link to a vacuum trap. A trap suitable for a water aspirator is displayed, but a more substantial trap cooled with dry ice and acetone should be employed with a portable vacuum to prevent solvent vapors from damaging the oil pump.

• Join the trap to the vacuum source (aspirator or vacuum pump). It is best to not bend or stretch the tubing as much as possible, as this may cause a leak in the system.

• Place a wood block or lab jack below the stirring plate to allow for the lowering of the heat source when the distillation is finished.

a) Stir plate with wood board to allow for downing of the apparatus, b) Active distillation, c) Insulating the Claisen and three-way adapters with foil.

Begin the Distillation

• Prior to the heating process, turn on the vacuum supply to initiate a decrease in pressure inside the device. Should any whistling sound be heard, this is an indication of a leak in the system.

• The reduction of pressure before heating is to expel any low-boiling substances (e.g. residual solvent). Otherwise, the low-boiling materials may erupt in the flask when the system is heated.

• When one has access to a manometer, take note of the pressure within the apparatus. This can be utilized to forecast the boiling point of the sample.

• When one is assured that the device is properly emptied and any low-boiling materials have been removed, start to heat the sample.

• If it is difficult to accomplish more than a reflux, the Claisen and three-way adapter should be wrapped in glass wool and then aluminum foil for insulation. This allows the column to maintain warmth and the sample to stay in the gaseous phase for a more extended period. There should be a slight space in the insulation near the distilling flask so that one can “peek in” and make sure the stirring mechanism continues to function correctly.

• Make sure to record the temperature during the collection of material, making sure the value is when the thermometer bulb is entirely immersed in vapors. If a manometer is used, also record the pressure. If no manometer is used, document the vacuum source (e.g. aspirator).

• Noble liquids do not usually distill at a stable temperature when under vacuum, as differences in pressure can quickly occur and affect the boiling temperature. A range of 5 oC is not uncommon for pure liquids. This is especially applicable when the vacuum source is a water aspirator, as variations in water flow can affect the pressure.

• If multiple fractions of distillate are desired, the distillation must be terminated prior to replacing the receiving flask. If available, a “cow” or “spider” receiving flask can be employed to gather various fractions without ending the vacuum.

a) Сow-shaped flask utilized for gathering multiple fractions: when a fresh fraction is distilling, the flask is rotated to permit the distillate to accumulate into an unfilled section of the “udder”, b) Chilling the flask with a faucet water bath, c) Unsealing the vacuum on the trap prior to switching off the vacuum, d) Different approach to open the apparatus before deactivating the vacuum.

Distillation Stop

• To end the distillation, first eliminate the heat source, cool the flask to room temperature then further chill in a tap water bath.

• Gradually introduce the atmospheric pressure into the flask by opening the pinch shutoff at the vacuum trap, or by disconnecting the rubber tubing at the vacuum adapter or aspirator. You will know the system is exposed to the atmosphere when there is an increase in water flow at the aspirator, or if a hissing sound is heard. Then turn off the vacuum source.

• It is important to first cool the system before permitting air back in as the superheated residue in the flask may respond unpredictably with oxygen in the air.

• It is also essential to first allow air back into the system prior to switching off the vacuum source. If the vacuum is switched off first, occasionally changes in pressure inside the apparatus (as it cools) cause back-suction. If a water aspirator is used, this could cause water from the sink to be pulled into the vacuum line. The vacuum trap avoids this back suction from spoiling the distillate.

• Disassemble and clean up the distillation apparatus as promptly as is feasible, as the joints can sometimes harden if left connected for lengthy periods.


Potential dangers arise from pressure buildup, commonly used flammable materials, and the use of heat to vaporize the chemicals involved. Careful design and construction of the distillations system is required to accomplish effective separation and avoid leaks that can lead to fires or contamination of the work area.

It is necessary to ensure smooth boiling during the separation process and avoid bumping, which can blow apart the distillation apparatus. Stirring the distillation mixture is the best method to avoid bumping. Boiling stones are only effective for distillations at atmospheric pressure. Use fresh boiling stones when a liquid is boiled without stirring. Do not add boiling stones or any other material to a liquid that is near its boiling point, because this may cause it to boil over spontaneously.

An electric mantle heater, a ceramic cavity heater, steam coils, or a nonflammable liquid bath are the best to provide even heating. Silicone oil or another suitable high-boiling-temperature oil can be used on a hot plate. Hot water or steam may also be used in some cases. An extra thermometer inserted at the center bottom of the distilling flask will warn of dangerously high temperatures that could indicate exothermic decomposition. Do not distill or evaporate organic compounds to dryness unless they are known to be free of peroxides.

Because superheating and bumping occur frequently during distillation using reduced pressure, it is important that the distillation assembly is secure and the heat distributed more evenly than is possible with a flame. Evacuate the assembly gradually to minimize the possibility of bumping. Stirring, or using an air or nitrogen bleed tube, provides good vaporization without overheating and decomposition.

Put a standing shield in place for protection in the event of implosion. After finishing a reduced-pressure distillation, cool the system, and then slowly bleed in air so as not to induce an explosion in a hot system. Pure nitrogen is preferred to air and can be used even before cooling the system. Use a face shield when working directly with a distillation unit.