POTASSIUM

Potassium
(Kalium)
CAS 7440-09-7

K

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

Odorless

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.

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 or CO₂ extinguishers must never be used on potassium fires.

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.

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, impermeable gloves, and a fire-retardant laboratory coat should be worn at all times when working with potassium, and the metal should be handled under the surface of an inert liquid such as mineral oil, xylene, or toluene. Potassium should be used only in areas free of ignition sources and should be stored under mineral oil in tightly sealed metal containers under an inert gas such as argon. Potassium metal that has formed a yellow oxide coating should be disposed of immediately; do not attempt to cut such samples with a knife since the oxide coating may be explosive.

In the event of skin contact, remove contaminated clothing and any metal particles and immediately wash with soap and water. In case of eye contact, promptly wash with copious amounts of water for 15 min (lifting upper and lower lids occasionally) and obtain medical attention. If potassium is ingested, obtain medical attention immediately.

In the event of a spill, remove all ignition sources, quench the resulting potassium fire with a dry chemical extinguishing medium, sweep up, place in an appropriate container under an inert atmosphere, and dispose of properly. Respiratory protection may be necessary in the event of a spill or release in a confined area.

Excess potassium and waste material containing this substance should be placed in an appropriate container under an inert atmosphere, clearly labeled, and handled according to your institution's waste disposal guidelines. Experienced personnel can destroy small scraps of potassium by carefully adding t-butanol or n-butanol to a beaker containing the metal scraps covered in an inert solvent such as xylene or toluene.

The information in this LCSS has been compiled by a committee of the National Research Council from literature sources and Material Safety Data Sheets and is believed to be accurate as of July 1994. This summary is intended for use by trained laboratory personnel in conjunction with the NRC report Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. This LCSS presents a concise summary of safety information that should be adequate for most laboratory uses of the title substance, but in some cases it may be advisable to consult more comprehensive references. This information should not be used as a guide to the nonlaboratory use of this chemical.

Copyright 1995 National Academy of Sciences. All rights reserved.