Purple-K is a firefighting dry chemical. Although bicarbonate of soda had been used to fight fires for many years prior to its discovery, in the 1950s the U.S. Naval Research Laboratory discovered that switching potassium in for the sodium in bicarbonate increased its ability to quench flame by a factor of at least two.
Fire requires four components to burn, sometimes known as the 'fire tetrahedron': heat, oxygen, fuel, and a chemical chain reaction. Purple-K is one of the few that directly attacks the chemical chain reaction, efficiently interrupting said chain reaction and hence stopping combustion. It consists, in its industrial preparation, of potassium bicarbonate and sodium bicarbonate (approximately 80% and 15% by weight), mica, Fuller's Earth and silica making up the remainder.
It is one of the most effective tools in fighting flammable liquid fires. As a result, it is used heavily in chemical and petroleum refineries, at airports and fuel storage depots and at other locations containing large quantities of flammable liquid. It can be mixed with other firefighting agents such as carbon dioxide or other smothering agents for improved effectiveness.
It is available in portable fire extinguishers, usually propelled as a powder spray by carbon dioxide or another propellant gas. In large quantities, it can be sprayed from special nozzles, either from built-in fire systems or from special delivery systems built into trucks. It can also, in emergencies, be dumped onto fires from above with reasonable effectiveness. Although it loses effectiveness if it is heavily wetted since it must be powdered in order to spread quickly into a flame, it can be mixed into light foam and delivered as a component of said foam.
Its purple color comes from a violet dye used to mark areas where the powder has been used. It is highly alkali, corrosively so, but is non-toxic to humans even in relatively large quantities - a handy characteristic for firefighting. Under high heat conditions, however, it will decompose into carbon monoxide and carbon dioxide as well as silicon oxides; the former is toxic to humans, but is usually found in fire situations anyway.
Sources:
Naval Research Laboratory
Badger Fire Protection
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