Potassium chloride, chemical formula KCl, is a chemical compound: specifically, it is a metal halide salt which is made by combining potassium and chlorine. It is odourless, with a white or colourless vitreous crystal appearance. It dissolves quickly within water and tastes strikingly similar to salt. Its applications include agricultural uses (specifically, as a fertiliser), as a medicine, and also within the scientific and food processing industries (wherein it has an E number additive E508).

The chemical is also used, in some places in the United States, to cause cardiac arrest; as such, it is the last component of 'three-drug cocktails' used in lethal injections. It occurs naturally as the mineral slyvite and in combination with sodium chloride as sylvinite.

This agent was used in Syphon Filter. It is also presumably seen in Syphon Filter 2 and The Omega Strain.


As a fertiliser

The majority of the potassium chloride produced is used for making fertiliser, called potash, since the growth of many plants is limited by potassium availability. The two main types of potash are muriate of potash (MOP, potassium chloride) and sulphate of potash (SOP, potassium sulphate). While SOP typically sells at a premium to MOP, the vast majority of potash fertiliser worldwide is sold as MOP.

As a medicine

Potassium is vital in the human body, and potassium chloride orally delivered is the common means to treat low blood potassium, although it can also be given intravenously. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system. Lethal injections where potassium chloride is the last component are used in induced abortions and executions.

As a culinary product

It can be used as a salt substitute for food, but due to its weak, bitter, unsalty flavor, it is often mixed with ordinary table salt (sodium chloride) to improve the taste to form low sodium salt. The addition of 1 ppm of thaumatin considerably reduces this bitterness. Complaints of bitterness or a chemical or metallic taste are also consistent with potassium chloride used in food.

As an industrial product

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 amount 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 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 produce heat packs which employ exothermic chemical reactions,[12] 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.

Side effects

The typical amounts of potassium chloride found in the diet appear to be generally safe.[13] In larger quantities, however, potassium chloride is toxic. The LD50 of orally ingested potassium chloride is approximately 2.5 g/kg, or 190 grams (6.7 oz) for a body mass of 75 kilograms (165 lb). In comparison, the LD50 of sodium chloride (table salt) is 3.75 g/kg.

Intravenously, the LD50 of potassium chloride is far smaller, at about 57.2 mg/kg to 66.7 mg/kg; this is found by dividing the lethal concentration of positive potassium ions (about 30 to 35 mg/kg)[14] by the proportion by mass of potassium ions in potassium chloride (about .52445 mg K+/mg KCl)[15]. In such quantities, it has severe consequences on the cardiac muscles, potentially causing cardiac arrest and rapid death. For this reason, it is used in surgical abortion, and as the third and final drug delivered in the lethal injection process.

Chemical properties


KCl is soluble in a variety of polar solvents.

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.

Physical properties

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.

Solvent Solubility

(g/kg of solvent at 25 °C)

H2O 360
Liquid ammonia 0.4
Liquid sulfur dioxide 0.41
Methanol 5.3
Formic acid 192
Sulfolane 0.04
Acetonitrile 0.024
Acetone 0.00091
Formamide 62
Acetamide 24.5
Dimethylformamide 0.17–0.5

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 emits a lilac flame.


Potassium chloride is extracted from minerals sylvite, carnallite, and potash. 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 production of nitric acid from potassium nitrate and hydrochloric acid.

The vast majority of potassium chloride is produced as agricultural and industrial grade potash in Saskatchewan, Canada, as well as Russia and Belarus. Saskatchewan alone accounted for over 25% of the world's potash production in 2017.

Laboratory methods

Potassium chloride is inexpensively available and is rarely prepared intentionally in the laboratory. It can be generated by treating potassium hydroxide (or other potassium bases) 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

Applications in the series

Syphon Filter

By persuading Gabe Logan that he was on a mission to 'cure test subjects' who had been infected with the Syphon Filter Virus, Thomas Markinson was secretly able to have his operative terminate the guinea pigs. Lian Xing would later tell Gabe that '[Syphon Filter] has no universal cure' because the antidote required depends on each victim's specific DNA structure. Her analysis of the serum that was issued by Markinson to Logan eventually revealed that it was no vaccine, but highly concentrated potassium chloride that was injected into the test subjects.[1]

Syphon Filter 2

Although not seen first-hand, KCI was apparently used by Dr. Elsa Weissenger to kill the CBDC operative Dobson, who was captured during an attempt to stop Dillon Morgan and Derek Falkan from catching Lian Xing at the PHARCOM Warehouses. He was flown to McKenzie Air Force Base with Lian Xing, the Agency having set up some operations in the area. When Elsa Weissenger extracted plasma from Lian (and Phagan, who was being held comatose), she presumably had interrogated Dobson and personally injected him with the compound. Lian finds his corpse and laments that she told him he should have stayed back.[2]

Syphon Filter: The Omega Strain

It is not directly mentioned, but one could assume that KCI was also a part of the serum issued by Dr. Elsa Weissenger to the IPCA operative Cobra during the second destruction of the SS Lorelei. The agent was instructed to administer the serum to Dr. Nikolai Jandran in order for the latter, who had been infected with a strain of the Syphon Filter Virus that was presumably harmless to himself but harmful to others, to be brought into the IPCA for interrogation. However, Jandran began exhibiting 'adverse reactions to the serum', and was killed before he could be questioned. Elsa Weissenger later confessed to killing her mentor in order for her to change the world.[3]

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