Degassing Liquids with Freeze-Pump-Thaw Cycling

1. First, place the desired solvent or solution into a Schlenk flask and close the stopcock (Figure 2a). Seal all other openings on the Schlenk flask. Caution: do not use more than 50% of the volume of the flask and inspect the flask for cracks or fractures.  An overfilled or broken flask may shatter during the process.
2. Attach the flask to a Schlenk line and keep the corresponding valve on the Schlenk line closed. Freeze the liquid completely by submerging the flask into a Dewar containing liquid nitrogen or a dry ice slurry. (Figure 2b). Caution: before freezing, flush the Schlenk flask with nitrogen gas to ensure the environment is completely free of oxygen.
3.  When the solvent is frozen, open the stopcock on the Schlenk flask and the valve on the Schlenk line to vacuum (Figure 2c). Keep the flask under vacuum and inside the cooling bath for about 10 min.  Seal the flask by closing the stopcock (Figure 2d).
4. Thaw the solvent until it melts using a warm water bath. During this procedure gas bubbles visibly evolve from the solvent (Figures 2e, 2f). Allow the frozen solvent to thaw by itself slowly and do not to disturb the liquid.
5. Once the solvent thawed, replace the warm water bath with the cooling bath and refreeze the solvent.
6. Repeat steps 3–5 until you no longer see the evolution of gas as the solution thaws (Figure 2g). A minimum of three cycles is recommended to minimize the percentage of dissolved gas present.
7. After the completion of three cycles, the Schlenk flask should be sealed under nitrogen before use (Figure 2h). Open the nitrogen gas valve on the Schlenk line and open the stopcock of the flask to expose the solvent to a nitrogen atmosphere. Once the Schlenk flask is filled with nitrogen, finally close the valve to the flask.
8. The solvent is now degassed and ready for use.

Figure 2. Detail photos of the free-pump-thaw steps: (a) step 1, place solvent in flask; (b) step 2, freeze the solvent in dry ice (or alternatively with liquid nitrogen); (c) step 3, introduce vacuum; (d) step 4, seal the flask under vacuum; (e), (f), step 5, thaw the solvent and observe evolution of gas bubbles; (g) step 7 repeat freeze thaw process (three cycles recommended); (h) step 8, seal the solvent under nitrogen.