The solid dissolves readily in water, and its solutions have a salt-like taste. It could be obtained from ancient dried lake deposits. KCl is used as a fertilizer, in medicine, in scientific applications, and in food processing, where it may be known as E number additive E508.
In a few states of the United States, it is used to cause cardiac arrest, as the third drug in the "three drug cocktail" for executions by lethal injection. It occurs naturally as the mineral sylvite, and in combination with sodium chloride as sylvinite.
Solutions of KCl are common standards, for example for calibration of the electrical conductivity of (ionic) solutions, since KCl solutions are stable, allowing for reproducible measurements. In aqueous solution, it is essentially fully ionized into solvated K+ and Cl– ions.
Redox and the conversion to potassium metal
Although potassium is more electropositive than sodium, KCl can be reduced to the metal by reaction with metallic sodium at 850 °C because the more volatile potassium can be removed by distillation (see Le Chatelier's principle):
KCl(l) + Na(l) ⇌ NaCl(l) + K(g)
This method is the main method for producing metallic potassium. Electrolysis (used for sodium) fails because of the high solubility of potassium in molten KCl.
"Raise banana yields using Israeli potassium chloride!", an ad above a highway in a banana-growing district of Hekou County, Yunnan, China
The crystal structure of potassium chloride is like that of NaCl. It adopts a face-centered cubic structure. Its lattice constant is roughly 6.3 Å. Crystals cleave easily in three directions.
Some other properties are
Transmission range: 210 nm to 20 µm
Transmissivity = 92% at 450 nm and rises linearly to 94% at 16 µm
Refractive index = 1.456 at 10 µm
Reflection loss = 6.8% at 10 µm (two surfaces)
dN/dT (expansion coefficient)= −33.2×10−6/°C
dL/dT (refractive index gradient)= 40×10−6/°C
Thermal conductivity = 0.036 W/(cm·K)
Damage threshold (Newman and Novak): 4 GW/cm2 or 2 J/cm2 (0.5 or 1 ns pulse rate); 4.2 J/cm2 (1.7 ns pulse rate Kovalev and Faizullov)
As a chemical feedstock, it is used for the manufacture of potassium hydroxide and potassium
metal. It is also used in medicine, lethal injections, scientific applications, food processing,
soaps, and as a sodium-free substitute for table salt for people concerned about the health effects of sodium.
It is used as a supplement in animal feed to boost the number of nutrients in the feed, which in turn promotes healthy growth in animals. As an added benefit, it is known to increase milk production.
It is sometimes used in water as a completion fluid in petroleum and natural gas operations, as well as being an alternative to sodium chloride in household water softener units.
Glass manufacturers use granular potash as a flux, lowering the temperature at which a mixture melts. Because potash confers excellent clarity to glass, it is commonly used in eyeglasses, glassware, televisions, and computer monitors.
KCl is useful as a beta radiation source for calibration
of radiation monitoring equipment, because natural potassium contains 0.0118% of the isotope 40K. One kilogram of KCl yields 16350 becquerels
of radiation consisting of 89.28% beta and 10.72%
gamma with 1.46083 MeV. In order to use off the shelf materials it needs to be crystallized sequentially using controlled temperature in order to extract KCl, which is the subject of ongoing research. There are also a small number of 511 keV gamma rays from positron annihilation which can be used to calibrate medical scanners.
Potassium chloride is used in some de-icing products that are designed to be safer for pets and plants, though these are inferior in melting quality to calcium chloride [lowest usable temperature 12 °F (−11 °C) v. −25 °F (−32 °C)]. It is also used in various brands of bottled water, as well as in bulk quantities for fossil fuel drilling purposes.
Potassium chloride was once used as a fire extinguishing agent, used in portable and wheeled fire extinguishers. Known as Super-K dry chemical, it was more effective than sodium bicarbonate-based dry chemicals and was compatible with protein foam. This agent fell out of favor with the introduction of potassium bicarbonate (Purple-K) dry chemical in the late 1960s, which was much less corrosive and more effective. It is rated for B and C fires.
Along with sodium chloride and lithium chloride, potassium chloride is used as a flux for the gas welding of aluminium.
Potassium chloride is also an optical crystal with a wide transmission range from 210 nm to 20 µm. While cheap, KCl crystals are hygroscopic. This limits its application to protected environments or short-term uses such as prototyping. Exposed to free air, KCl optics will "rot". Whereas KCl components were formerly used for infrared optics, it has been entirely replaced by much tougher crystals such as zinc selenide.
Potassium chloride has also been used to produce heat packs which employ exothermic chemical reactions, but these have mostly been discontinued with the advent of cheaper and more efficient methods, such as the oxidation of metals ('Hot Hands' one-time-use products) or the crystallization of sodium acetate (multiple-use products).
Potassium chloride is used as a scotophor with designation P10 in dark-trace CRTs, e.g. in the Skiatron.