Static vacuum distillations are frequently employed for the purification and/or drying of small volumes of solvents or liquid reagents with relatively low boiling points (<150 °C).
Step 1: A J. Young’s Schlenk flask/ampoule with a 90° side-arm, equipped with a stir bar, is connected to the Schlenk Line via O-ring adapter and flame-dried under dynamic vacuum.
Step 2: Once cooled to room temperature, the vacuum is slowly quenched and the flask is refilled with inert gas. The solvent/reagent, together with the appropriate desiccant if necessary, is then carefully added to the flask under a positive flow of inert gas.
Step 3: The J. Young’s valve is sealed, and the flask is slowly submerged in a dewar of liquid nitrogen. Once completely frozen, the flask is placed under vacuum and the J. Young’s tap is opened whilst still submerged in the liquid nitrogen. After a few minutes under dynamic vacuum to evacuate the headspace, the J. Young’s tap is closed and the flask is removed from the liquid nitrogen to thaw.
Step 4: Whilst the transfer flask is thawing, a second flask is connected via a vacuum distillation bridge, and flame-dried under dynamic vacuum.
Step 5: Once the transfer flask has fully thawed and warmed back to room temperature, it is slowly resubmerged in a dewar of liquid nitrogen and allowed to completely freeze. The J. Young’s tap on the transfer flask is opened to dynamic vacuum to evacuate the headspace, and to build the static vacuum within the distillation setup.
Step 6: After a few minutes under dynamic vacuum, the J. Young’s tap on the vacuum transfer side-arm is closed, the transfer flask is removed from the liquid nitrogen and the receiving flask is submerged in the dewar of liquid nitrogen. As the solvent/reagent slowly thaws, it will distill under the reduced pressure and condense in the receiving flask.
It may be necessary to refresh the static vacuum periodically by closing the J. Young’s tap on the transfer flask and opening the J. Young’s tap on the vacuum transfer side-arm to dynamic vacuum. After several minutes, the J. Young’s tap on the vacuum transfer side-arm is closed and the J. Young’s tap on the transfer flask is slowly opened to continue the distillation.
Step 7: Once the distillation is complete, the J. Young’s taps on both flasks are closed, and the receiving flask is removed from the liquid nitrogen. Once thawed, the vacuum within the system is slowly quenched and refilled with inert gas. The receiving flask is sealed under inert gas, disconnected from the distillation side-arm and reconnected to the Schlenk line via O-ring adapter with three evacuate-refill cycles. The freshly distilled solvent/reagent is finally transferred to a flame-dried ampoule via cannula, sealed under inert gas and labelled.
Hints and tips:
- If a desiccant is used during the distillation, it is advised to place a small piece of dry cotton wool in the vacuum distillation/transfer side-arm to prevent it entering the receiving flask. Distillation bridges with built in glass sinter frits are also available. Alternatively, the desiccant can be removed via filter cannula before or after the distillation.
- Wrapping a small piece of PTFE tape around the rubber O-rings can help ensure a good seal and vacuum within the distillation setup.
- For further details of suitable desiccants and procedures for purifying and drying solvents/reagents, consult the latest edition of Purification of Laboratory Chemicals.