Potassium Mirror

Potassium mirrors are frequently used as both high surface area reducing agents and as desiccants for drying solvents, particularly deuterated solvents for NMR. They can also be used to store anhydrous solvents such as pentane, hexane, diethyl ether and toluene.

Potassium reacts violently with water liberating flammable gases that may ignite spontaneously. Ensure that appropriate fire safety protocols are in place!

Step 1: Cycle an ampoule onto the Schlenk line. Flame dry the glassware whilst under vacuum to ensure that it is rigorously dry.

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Flame drying a large ampoule under vacuum.

Step 2: Once cooled to room temperature, backfill the ampoule slowly with inert gas. Remove a chunk of potassium from the mineral oil and cut a 1 x 1 x 0.5 cm piece (approximate dimensions required for a 1 L ampoule) using a scalpel, removing any oxide coating. Wash the potassium in hexane or diethyl ether to remove any residual mineral oil. Add the potassium to the ampoule under a flow of inert gas.

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Backfilling the ampoule with inert gas.

Step 3: Place the ampoule under vacuum and briefly flame dry the glassware to remove any residual solvent, being careful not to heat the potassium. Allow to cool back down to room temperature.

Step 4: Begin carefully heating the potassium with a bunsen burner or blowtorch until it melts. Wait momentarily before continuing to heat the molten metal and the bottom of the ampoule. The potassium will begin to sublime, generating a purple vapour that will begin to rise up the ampoule and condense on the inside walls to slowly form a reflective mirror. Continue heating for 15-30 seconds until no further activity is observed, ensuring that the glassware is not over-heated (avoid orange flames). Allow to cool to room temperature.

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Subliming the potassium by heating under vacuum.

Step 4: Once cooled to room temperature, backfill the flask slowly with inert gas. Ensure that all utensils used to cut and handle the potassium have been safely quenched with toluene/IPA.