Muriate of potash
|Molar mass||74.5513 g·mol−1|
|Appearance||white crystalline solid|
|Melting point||770 °C (1,420 °F; 1,040 K)|
|Boiling point||1,420 °C (2,590 °F; 1,690 K)|
|281 g/L (0 °C)
344 g/L (20 °C)
567 g/L (100 °C)
|Solubility||soluble in glycerol, alkalies
slightly soluble in alcohol, insoluble in ether
Refractive index (nD)
|1.4902 (589 nm)|
|Crystal structure||face centered cubic|
Std enthalpy of
|EU Index||Not listed|
LD50 (Lethal dose)
|2600 mg/kg (oral, rat)|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
The chemical compound potassium chloride (KCl) 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. The solid dissolves readily in water and its solutions have a salt-like taste. 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.
Potash is a mixture of potassium containing compounds with a high KCl content. Unlike white-colorless KCl, 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. Potassium chloride was historically known as "muriate of potash". This name is occasionally still encountered in association with its use as a fertilizer.
|Solubility of KCl in various solvents
(g KCl / 1 kg of solvent at 25 °C)
|Liquid sulfur dioxide||0.41|
In chemistry and physics, it is a common standard, 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. Such solutions serve as a source of chloride ion. When treated with silver salts, one observes the characteristic precipitation of silver chloride:
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)
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
- 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 (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 with other compounds containing potassium, KCl in powdered form gives a lilac flame test result.
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.
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 (KOH) 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
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 (sodium chloride).
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 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.
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 such as zinc selenide.
Potassium chloride has also been used to create 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).
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 (sodium chloride) 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, most of which are extended release oral medicines. With a molecular weight of approximately 75 and a valence of 1, the use of KCl for electrolytes makes 75 mg the equivalent of 1 mEq. Oral and intravenous doses for adults are typically in the range of 8 to 20 mEq, which is 600 to 1500 mg of KCl. 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. Because of the danger of hyperkalemia, the US FDA limits the amount of potassium supplements to 99 mg (about 1.3 mEq) while recommending an adult daily intake of 4700 mg (about 63 mEq).
Prescription potassium citrate (the potassium naturally found in fruits and vegetables) 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 (albeit rarely) 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.
Orally, potassium chloride is toxic in excess; the LD50 is around 2.5 g/kg (meaning that a lethal dose for 50% of people weighing 75 kg (165 lb) is about 190 g (6.7 ounces)). However, this is not far from oral toxicity of sodium chloride (table salt), of 3.75 g/kg, thus potassium chloride is harmless for alimentation (and even good for health, see previous paragraph). 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.
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