Make Potassium Metal (Catalyzed Magnesium Reduction Method) |
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Description: We make potassium metal using a lesser known chemical approach of reduction with magnesium. Warning: Making potassium is extremely dangerous. Full fire safety protocols must be in place including protective clothing, goggles and face shields. Burning potassium cannot be extinguished by conventional means and water only serves to increase its danger. This experiment should only be performed by an experienced chemist in a fume hood. In a flask place 2.4g of magnesium turnings or powder, 5g of potassium hydroxide, and 20 mL of tetahydronapthalene or paraffin/mineral oil. If using oil the boiling point should be greater than 200 degrees Celsius. Fit a reflux condenser onto the flask and prevent air access by covering the top of the condenser with foil. Alternatively a bubbler or a punctured balloon may be used. Heat the reaction to reflux or +200 degrees Celsius. Color change of the solvent may occur at this time. Inject of 0.4 mL of a tertiary alcohol (t-butanol and t-amyl alcohol have been confirmed successful) through the top of the condenser. Add in an additional 0.1mL of alcohol every 10 minutes for a total of 0.6 mL of additional alcohol (1.0 mL overall). Continue to reflux until potassium is produced. If potassium goes dark, add an additional 0.1 mL of alcohol. What's happening is the magnesium reacts with the potassium hydroxide to produce potassium metal, hydrogen gas and magnesium oxide. The alcohol catalyzes this reaction. If you're using a high-density solvent where the potassium spheres float, then coalescence of the spheres into larger spheres should occur automatically. If they are not, then an additional 0.1 mL shot of alcohol may be necessary. If using a lower density solvent, then coalescence may be achieved using this video: https://www.youtube.com/watch?v=xzkajgxVbyI The reaction is done when there is no more magnesium. Turn off the heating and allow to cool. Dump the cooled products into toluene and retrieve the metallic potassium, washing with additional toluene. Place the potassium into a clean vial with mineral oil for short term storage. All apparatus must be rendered safe before water-washing by first adding isopropanol to all containers that handled potassium. Any potassium residues should start bubbling. Only when bubbling has ceased can the wastes be disposed of as organic waste and containers subsequently washed with water. DO NOT use water washing before alcohol washing as metallic potassium residues will ignite. Additional Notes: Alcohol *may* be added entirely at the beginning of the reaction but reproducibility is an issue since a competing reaction of alcohol decomposition sometimes occurs. It is difficult to predict how all reagents and solvent grades will behave so it is recommended to first attempt the experiment with staggered addition as outlined here. A special note about long term storage safety. Do not leave potassium in storage for too long. Potassium forms shock sensitive explosive coatings on prolonged storage even under mineral oil. It is recommended that potassium be used or destroyed within a few months. If long-term storage is required then sealing in a glass ampoule or storage in an inert atmosphere glove box is recommended. Further information about the dangers can be found at: http://pubs.acs.org/doi/pdf/10.1021/ed085p634.2 Occasionally stirring the reaction (every 30-45 minutes) greatly improves reaction rate and yield. Remember to allow reaction to cool before removing the condenser. The quality of magnesium is important for the reaction. Highly oxidized magnesium didn't work to produce potassium. Buying new magnesium, or grinding it off from a magnesium ingot is recommended. Alternatively, low quality magnesium maybe activated by placing it into the solvent first without the potassium hydroxide and adding 0.1mL of alcohol and boiling for several minutes. The alcohol helps to etch the surface and restore activity. Note, this method is not absolutely reliable. There have been reports of sodium being made by this method but I have never been able to successfully make it. Further reading at: http://www.sciencemadness.org/talk/viewthread.php?tid=14970 
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