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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
LABORATORY CHEMICAL SAFETY SUMMARY: POTASSIUM
Silvery white metal that loses its luster on exposure to air or moisture
bp 765.5 °C, mp 63 °C
Explodes on contact with water
25 °C or below in air or oxygen
Ignites in air and reacts explosively with water; highly corrosive to the skin and eyes.
Potassium reacts with the moisture on skin and other tissues to form highly corrosive potassium hydroxide. Contact of metallic potassium with the skin, eyes, or mucous membranes causes severe burns; thermal burns may also occur due to ignition of the metal and liberated hydrogen.
Flammability and Explosibility
Potassium metal may ignite spontaneously on contact with air at room temperature. Potassium reacts explosively with water to form potassium hydroxide; the heat liberated generally ignites the hydrogen formed and can initiate the combustion of potassium metal itself. Potassium fires must be extinguished with a class D dry chemical extinguisher or by the use of sand, ground limestone, dry clay or graphite, or "Met-L-X®" type solids. Water orCO2,extinguishers must never be used on potassium fires.
Reactivity and Incompatibility
Potassium is one of the most potent reducing reagents known. The metal reacts explosively with water, oxygen, and air to form potassium hydroxide and/or potassium oxide. Potassium reacts violently with many oxidizing agents and organic and inorganic halides and can form unstable and explosive mixtures with elemental halogens. Explosive mixtures form when potassium reacts with halogenated hydrocarbons such as carbon tetrachloride and upon reaction with carbon monoxide, carbon dioxide, and carbon disulfide. Potassium stored under mineral oil can form shock-sensitive peroxides if oxygen is present, so the metal must always be stored and handled under inert gases such as dry nitrogen or argon. It dissolves with such exothermicity in other metals such as mercury that the molten alloy may melt Pyrex glassware. Note that the reactivity of potassium is generally related to its surface area and the cleanliness of the surface at hand; chunks of potassium are less reactive than the very dangerous dispersions and sands.
Storage and Handling
Potassium should be handled in the laboratory using the "basic prudent practices" described in Chapter 5.C, supplemented by the additional precautions for work with flammable (Chapter 5.F) and reactive (Chapter 5.G) substances. Safety glasses, imperme-