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 Teflon tapped ampoule with a 90° side-arm, equipped with a stir bar, is cycled onto the Schlenk Line using a suitable O-ring or ground-glass adapter.
Step 2: Once cycled onto the Schlenk line, the ampoule is back-filled with inert gas. The solvent/liquid, together with the appropriate desiccant if necessary, is then carefully added to the ampoule under a positive flow of inert gas. The solvent/liquid should be dried over the desiccant for at least 12-24 hours prior to distillation.
Step 3: The Teflon tap is closed, and the ampoule is slowly submerged in a dewar of liquid nitrogen. Once completely frozen, the flask is placed under vacuum and the Teflon tap is opened whilst still submerged in the liquid nitrogen. After 5-10 minutes under dynamic vacuum to evacuate the headspace, the Teflon tap is closed and the ampoule is removed from the liquid nitrogen to thaw. Repeat the freeze-pump-thaw process.
Step 4: During the second thawing cycle, a second ampoule with a 90° side-arm is connected using a vacuum distillation/transfer bridge, and thoroughly dried under dynamic vacuum.
Step 5: Once the ampoule has fully thawed and warmed back to room temperature, it is re-submerged in the dewar of liquid nitrogen and allowed to completely freeze. The Teflon tap on the transfer ampoule is opened to dynamic vacuum to evacuate the headspace, and to build the static vacuum within the distillation setup.
Step 6: After 5-10 minutes under dynamic vacuum, the Teflon tap on the distillation bridge is closed, the transfer ampoule is removed from the liquid nitrogen and the receiving ampoule is submerged in the dewar of liquid nitrogen. As the solvent/liquid slowly thaws, it will distill under the reduced pressure and condense in the receiving ampoule.
It may be necessary to refresh the static vacuum periodically by closing the Teflon tap on the transfer ampoule and opening the Teflon tap on the distillation bridge to dynamic vacuum. After 3-5 minutes, the Teflon tap on the distillation bridge is closed and the Teflon tap on the transfer ampoule is slowly opened to continue the distillation.
Step 7: Once the distillation is complete, the Teflon taps on both ampoules are closed, and the receiving flask is removed from the liquid nitrogen. Once thawed, the vacuum within the system is slowly quenched and back-refilled with inert gas. The receiving amouple is sealed under inert gas, disconnected from the distillation bridge and cycled onto the Schlenk line. The freshly distilled solvent/liquid is then 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 glass wool in the vacuum distillation bridge to prevent it entering the receiving ampoule. 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.
- For further details of suitable desiccants and procedures for purifying and drying solvents/reagents, consult the latest edition of Purification of Laboratory Chemicals.