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Potassium chloride
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The chemical compound potassium chloride (KCl) is a metal halide salt composed of potassium and chlorine. In its pure state it is odorless. It has a white or colorless vitreous crystal, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are face-centered cubic. Potassium chloride is occasionally known as "muriate of potash," particularly when used as a fertilizer. Potash varies in color from pink or red to white depending on the mining and recovery process used.
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The chemical compound potassium chloride (KCl) is a metal halide salt composed of potassium and chlorine. In its pure state it is odorless. It has a white or colorless vitreous crystal, with a crystal structure that cleaves easily in three directions. Potassium chloride crystals are face-centered cubic. Potassium chloride is occasionally known as "muriate of potash," particularly when used 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, food processing and in judicial execution through lethal injection. It occurs naturally as the mineral sylvite and in combination with sodium chloride as sylvinite.
Chemical 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 (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.
As with other compounds containing potassium, KCl in powdered form gives a lilac flame test result.
Production
Potassium chloride occurs naturally as sylvite, and it can be extracted from sylvinite. 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.
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, scientific applications, food processing, and as a sodium-free substitute for table salt (sodium chloride).
Potassium chloride is also used as the third of a three drug combination in lethal injection. Additionally, KCl is used to terminate the life of the unborn fetus in induced abortion procedures by lethal injection to the heart, which induces cardiac arrest.
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 makes up 70% of Ace Hardware's pet and vegetation-friendly "Ice Melt" though inferior in melting quality to calcium chloride (0 °F 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 (Purple-K) dry chemical in the late 60s, which was much less corrosive and more effective. Rated for B and C fires.
Potassium chloride is also an optical crystal with a wide transmission range from 210 nm to 20 µm. It was often used in the infrared spectrum range and it is still used some time.
KCl crystal is hygroscopic and cheap. This limits its application to protected environment or for short term uses (prototyping). Exposed to free air KCl optics will "rot".
Today other crystals much more tough like ZnSe has overcome KCl (for the IR spectral range).
Optical data:
Transmission range: 210 nm to 20 µm
Transmittivity = 92% @ 450 nm and rises linearly to 94% @ 16 µm
Refractive Index = 1.456 @ 10 µm
Reflection Loss = 6.8% @ 10 µm (two surfaces)
dN/dT (expansion coefficient)= -33.2×10-6/°C
dL/dT (refractive index gradient)= 40×10-6/°C
Coefficient of absorption: 0.001 cm-1
Thermal conductivity = 0.036 W/(cm·K):
Damage threshold (Newman & Novak): 4 GW/cm2 or 2 J/cm2 (0.5 or 1 ns pulse rate)
Damage threshold (Kovalev & Faizullov)= 4.2 J/cm2 (1.7 ns pulse rate)
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 ('Hot Hands', one time use products) or the crystallization of sodium acetate (multiple use products).
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 (of course, in concentrations much lower than those used in executions). It can be used as a salt substitute for food, but due to its weak, bitter, unsalty flavor, it is usually mixed with regular salt (sodium chloride), for this purpose to improve the taste (for example, in Morton Lite Salt). Medically it is used in the treatment of hypokalemia and associated conditions, for digitalis poisoning, and as an electrolyte replenisher. Brand names include K-Dur, Klor-Con, Micro-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.
Dr. 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. A similar device, the German 'Perfusor', also uses potassium chloride as a suicide aid.
Physical properties
Potassium chloride has a crystalline structure like many other salts. Its structure is face-centered cubic. Its lattice constant is roughly 630 picometers.
In chemistry and physics it is a very commonly used as a standard, for example as a calibration standard solution in measuring electrical conductivity of (ionic) solutions, since carefully prepared KCl solutions have well-reproducible and well-repeatable measurable properties.
Precautions
Orally, KCl is toxic in excess; the LD50 is around 2500 mg/kg (meaning that a lethal dose for 50% of people weighing 75 kg (165 lb) is about 190 g (6.7 ounces), or about 38 teaspoons). Table salt is about as toxic. Intravenously this is reduced to just over 100 mg/kg, but of more concern are its severe effects on the cardiac muscles; high doses can cause cardiac arrest and rapid death, ergo its aforementioned use as the third and final drug delivered in the lethal injection process.
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