The chemical compound potassium chloride is a metal halide salt composed of potassium
and chlorine. In its pure state, it is odorless and has a white or colorless vitreous crystal
appearance, with a crystal structure that cleaves easily in three directions. Potassium
chloride crystals are face-centered cubic. Potassium chloride was historically known
as "muriate of potash". This name is occasionally still encountered in association with its
use as a fertilizer. Potash varies in color from pink or red to white depending on the
mining and recovery process used. White potash, sometimes referred to as soluble potash, is
usually higher in analysis and is used primarily for making liquid starter fertilizers. KCl
is used in medicine, scientific applications, and food processing. It occurs naturally as
the mineral sylvite and in combination with sodium chloride as sylvinite.
Chemical properties
In chemistry and physics, it is a very commonly used standard, for example as a calibration
standard solution in measuring electrical conductivity of solutions, since carefully
prepared KCl solutions have well-reproducible and well-repeatable measurable properties.
Potassium chloride can react as a source of chloride ion. As with any other soluble ionic
chloride, it will precipitate insoluble chloride salts when added to a solution of an appropriate
metal ion: KCl(aq) + AgNO3(aq) → AgCl(s) + KNO3(aq)
Although potassium is more electropositive than sodium, KCl can be reduced to the metal
by reaction with metallic sodium at 850°C because the potassium is removed by distillation:
KCl(l) + Na(l) ⇌ NaCl(l) + K(g) This method is the main method for producing
metallic potassium. Electrolysis fails because of the high solubility of potassium in molten
KCl. As with other compounds containing potassium,
KCl in powdered form gives a lilac flame test result.
Physical properties Potassium chloride has a crystalline structure
like many other salts. Its structure is face-centered cubic. Its lattice constant is roughly 6.3Å.
Some other properties are Transmission range: 210 nm to 20 µm
Transmittivity = 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
dN/dT= −33.2×10−6/°C dL/dT= 40×10−6/°C
Thermal conductivity = 0.036 W/(cm·K) Damage threshold: 4 GW/cm2 or 2 J/cm2; 4.2
J/cm2 Synthesis and production
Potassium chloride occurs naturally as sylvite, carnallite, and potash, and it can be extracted
from these ores. It is also extracted from salt water and can be manufactured by crystallization
from solution, flotation or electrostatic separation from suitable minerals. It is a
by-product of the making of nitric acid from potassium nitrate and hydrochloric acid.
Synthesis Potassium chloride is inexpensively available
and is rarely prepared intentionally in the laboratory. It can be generated by two routes
that are of instructive but not practical value: One way is to treat potassium hydroxide
with hydrochloric acid: KOH + HCl → KCl + H2O
This conversion is an acid-base neutralization reaction. The resulting salt can then be purified
by recrystallization. Another method would be to allow potassium to burn in the presence
of chlorine gas, also a very exothermic reaction: 2 K + Cl2 → 2 KCl
Uses The majority of the potassium chloride produced
is used for making fertilizer, since the growth of many plants is limited by their potassium
intake. 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, and as a sodium-free substitute for table salt.
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.
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. Potassium chloride is used in some deicing products that are designed to be safer for
pets and plants, though these are inferior in melting quality to calcium chloride v.
−25 °F). 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
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 crystal is 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 like zinc selenide. Potassium chloride has also been used to create
heat packs which employ exothermic chemical reactions, but these are no longer being created
due to cheaper and more efficient methods, such as the oxidation of metals or the crystallization
of sodium acetate. Potassium chloride is used as a scotophor
with designation P10 in dark-trace CRTs, e.g. in the Skiatron.
Biological and medical properties Potassium is vital in the human body, and
oral potassium chloride is the common means to replenish it, although it can also be diluted
and given intravenously. It can be used as a salt substitute for food, but due to its
weak, bitter, unsalty flavor, it is usually mixed with ordinary table salt for this purpose
to improve the taste. The addition of 1 ppm of thaumatin considerably reduces this bitterness.
Complaints of bitterness or a chemical or metallic taste are also reported with potassium
chloride used in food. Medically, it is used in the treatment of
hypokalemia and associated conditions as an electrolyte replenisher. Brand names include
K-Dur, Klor-Con, Micro-K, Slow-K, Sando-K and Kaon Cl. Side effects can include gastrointestinal
discomfort including nausea and vomiting, diarrhea and bleeding of the digestive tract.
Overdoses cause hyperkalemia, which can lead to paresthesia, cardiac conduction blocks,
fibrillation, arrhythmias, and sclerosis. Prescription potassium citrate can be prescribed
as an alternative to potassium chloride. Slow-K is a 1950s development where the medicine
is formulated to enter the bloodstream at delayed intervals. It was first only prescribed
to British military forces to balance their diets while serving in Korea.
Some cardiac surgery procedures cannot be carried out on the beating heart. For these
procedures, the surgical team will bypass the heart with a heart-lung machine and inject
potassium chloride into the heart muscle to stop the heartbeat.
The lethal effects of potassium chloride overdoses have led to its use in lethal injection, as
the third of a three-drug combination. Additionally, KCl is used in fetal intracardiac injections
in second- and third-trimester induced abortions. Jack Kevorkian's thanatron machine injected
a lethal dose of potassium chloride into the patient, which caused the heart to stop functioning,
after a sodium thiopental-induced coma was achieved.
Precautions Orally, potassium chloride is toxic in excess;
the LD50 is around 2.5 g/kg is about 190 g). However, this is not far from oral toxicity
of sodium chloride, of 3.75 g/kg, thus potassium chloride is harmless for alimentation. But
intravenously, without the step of digestive absorption, this is reduced to just over 30 mg/kg.
Most concerns are its severe effects on the cardiac muscles: high doses can cause cardiac
arrest and rapid death, thus the aforementioned use as the third and final drug delivered
in the lethal injection process. References
Further reading Lide, D. R., ed.. CRC Handbook of Chemistry
and Physics. Boca Raton: CRC Press. ISBN 0-8493-0486-5. Greenwood, Norman N.; Earnshaw, Alan. Chemistry
of the Elements. Oxford: Pergamon Press. ISBN 0-08-022057-6.