Quenching stills (LiAlH4, CaH2, Na) Safely
On occasion, we need to distill reagents from reactive hydrides or sodium and then need to quench the stills after the distillation. Here is a procedure for that process. Most fires and many injuries in chemistry labs come from incompetent quenching of stills. This is a dangerous process, but can be done safely every time if you follow these rules.
Never do this by yourself for the first time. Always ask your colleagues or advisor to help if you have not done it before.
Always do this when others are present.
Always wear eye protection, gloves, and a lab coat, and perform all procedures in the hood.
Never distill anything (hydride still or otherwise) to dryness.
Allow several hours to quench the still.
Allow the distillation apparatus to come to room temperature. Do not use a water or ice bath to cool or at any stage of this procedure.
Make sure there is an appropriately sized stir bar in the flask.
Place the flask in an empty crystallizing dish (or similar secondary container) on a stir plate, and be sure the stir bar is spinning.
No condenser is necessary, and do NOT seal the flask (stopper, etc.). You can use an addition funnel with a pressure-‐equilibrating tube if desired. Only use this if you/it are capable of adding one drop at a time.
Add isopropanol dropwise. At the beginning, this means 1 drop every 15-‐30 sec. DO
NOT RUSH. You will see vigorous gas evolution in the beginning. When gas evolution stops, stir the mixture vigorously for 30 min. This is the most dangerous time. Often people assume everything is quenched, only to find that there were insoluble clumps that cause a problem when they come in contact with water.
After stirring for 30 min, add MeOH dropwise. Use an equal volume as you used IPA.
Add water dropwise. Use an equal volume as you used IPA and MeOH. Stir 1h.
Dispose as mixed aqueous/organic waste.
Quenching sodium stills:
Follow same procedure as above, but stir overnight before introduction of water. Stir several hours after introduction of water.