Potassium chloride

The chemical compound potassium chloride (KCl) is a metal halide 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 also commonly known as "Muriate of Potash". 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.

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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.

Manufacture/Extraction

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, as a sodium-free substitute for table salt (sodium chloride), and in judicial execution through lethal injection. It is sometimes used in water as a completion fluid in oil and gas operations. 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 allegedly pet and vegetarian friendly "Ice Melt" though inferior in melting quality to calcium chloride (0° F v. -25° F).

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 [1]). 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.

Physical properties

Potassium chloride has a crystalline structure like many other salts. Structure: face-centered cubic. Lattice Constant: roughly 6.3 angstroms.

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 it is toxic in excess; the LD₅₀ is around 2500 mg/kg (meaning that a person weighing 75 kg (165 lb) would have to consume about 190 g (6.7 oz); regular salt is about as toxic). Intravenously this is reduced to just over 100 mg/kg but of more concern are its severe effects on cardiac muscles; high doses can cause cardiac arrest and rapid death. A massive overdose of intravenous potassium chloride is used to stop the heart in execution by lethal injection.

References

1. ^ Bernard Martel. Chemical Risk Analysis: A Practical Handbook. Kogan, 2004, page 369. ISBN 1903996651.

• Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
• N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984. ISBN 0-08-022057-6