Potassium chloride

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This article is about the chemical compound. For other uses, see KCL.
Potassium chloride
Potassium chloride.jpg
Potassium-chloride-3D-ionic.png
Names
Other names
Sylvite
Muriate of potash
Identifiers
7447-40-7 YesY
ATC code A12BA01
B05XA01
ChEBI CHEBI:32588 YesY
ChEMBL ChEMBL1200731 N
ChemSpider 4707 YesY
DrugBank DB00761 YesY
Jmol-3D images Image
KEGG D02060 YesY
PubChem 4873
RTECS number TS8050000
UNII 660YQ98I10 YesY
Properties
KCl
Molar mass 74.5513 g·mol−1
Appearance white crystalline solid
Odor odorless
Density 1.984 g/cm3
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[1]
Acidity (pKa) ~7
1.4902 (589 nm)
Structure
face centered cubic
Thermochemistry
83 J·mol−1·K−1[2]
−436 kJ·mol−1[2]
Hazards
Safety data sheet ICSC 1450
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
2600 mg/kg (oral, rat)[3]
Related compounds
Other anions
Potassium fluoride
Potassium bromide
Potassium iodide
Other cations
Lithium chloride
Sodium chloride
Rubidium chloride
Caesium chloride
Related compounds
Potassium chlorate
Potassium perchlorate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
 N verify (what isYesY/N?)
Infobox references

The chemical compound potassium chloride (KCl) is a metal halide salt composed of potassium and chloride. 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.[4]

Chemical properties[edit]

Solubility[edit]

KCl is soluble in a variety of polar solvents.

Solubility[5]
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

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[edit]

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.[4]

Physical properties[edit]

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.

Production[edit]

Sylvite
Sylvinite

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.

Laboratory methods[edit]

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

Uses[edit]

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 for persons concerned about its health effects.

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 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,[6] 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).[citation needed]

Potassium chloride is used as a scotophor with designation P10 in dark-trace CRTs, e.g. in the Skiatron.

Biological and medical properties[edit]

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.[7] Complaints of bitterness or a chemical or metallic taste are also reported with potassium chloride used in food.[8]

Medically, it is used in the treatment of hypokalemia and associated conditions as an electrolyte replenisher.[9] 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.[10] 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.[11]

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.[12][13] 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.[14]

Precautions[edit]

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)). The oral toxicity of sodium chloride (table salt) is about the same, 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.[15] 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[edit]

  1. ^ "Potassium chloride (PIM 430)". International Programme on Chemical Safety. 3.3.1 Properties of the substance. Retrieved 2011-01-17. 
  2. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X. 
  3. ^ http://chem.sis.nlm.nih.gov/chemidplus/rn/7447-40-7[full citation needed]
  4. ^ a b Burkhardt, Elizabeth R. (2006). "Potassium and Potassium Alloys". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a22_031.pub2. 
  5. ^ Burgess, J. (1978). Metal Ions in Solution. New York: Ellis Horwood. ISBN 0-85312-027-7. [page needed]
  6. ^ U.S. Patent 3,874,504
  7. ^ Lorient, Denis; Linden, G. (1999). New ingredients in food processing: biochemistry and agriculture. Boca Raton: CRC Press. p. 357. ISBN 1-85573-443-5. ... in dietary food containing potassium chloride, thaumatin added in the ratio of 1 ppm considerably reduces the sensation of bitterness. ... 
  8. ^ Sinopoli, Dominique A.; Lawless, Harry T. (2012). "Taste Properties of Potassium Chloride Alone and in Mixtures with Sodium Chloride Using a Check-All-That-Apply Method". Journal of Food Science 77 (9): S319–22. doi:10.1111/j.1750-3841.2012.02862.x. PMID 22901084. 
  9. ^ Hypokalemia in Emergency Medicine~treatment at eMedicine
  10. ^ Hyperkalemia in Emergency Medicine at eMedicine
  11. ^ He, F. J.; Markandu, N. D.; Coltart, R.; Barron, J.; MacGregor, G. A. (2005). "Effect of Short-Term Supplementation of Potassium Chloride and Potassium Citrate on Blood Pressure in Hypertensives". Hypertension 45 (4): 571–4. doi:10.1161/01.HYP.0000158264.36590.19. PMID 15723964. 
  12. ^ Stubblefield, Phillip G.; Carr-Ellis, Sacheen; Borgatta, Lynn (2004). "Methods for Induced Abortion". Obstetrics & Gynecology 104 (1): 174–85. doi:10.1097/01.AOG.0000130842.21897.53. PMID 15229018. 
  13. ^ Types of Abortion Procedures. Americanpregnancy.org (2011-09-20). Retrieved on 2012-02-16.
  14. ^ Boyes, Roger (March 29, 2008). "Death for hire – suicide machine lets you push final button". The Times. 
  15. ^ Bhatkhande, C.Y.; Joglekar, V.D. (1977). "Fatal poisoning by potassium in human and rabbit". Forensic Science 9 (1): 33–6. doi:10.1016/0300-9432(77)90062-0. PMID 838413. 

Further reading[edit]