Structural Formula Vector Image
Title: Potassium
CAS Registry Number: 7440-09-7
Additional Names: Kalium
Literature References: K; at. wt 39.0983; at. no. 19; valence 1. Group IA (1). Alkali metal. Occurrence in earth's crust: 2.59% by wt. Naturally occurring isotopes: 39 (93.26%); 40 (0.012%); 41 (6.73%); 40K is radioactive: T½ 1.27 ´ 109 years; known isotopes range in mass number from 35 to 54. Found mainly as the chloride (sylvite); also in the aluminosilicates orthoclase, and microcline (KAlSi3O8), and as carnallite (KCl.MgCl2.6H2O). Major essential element for plant growth. First prepd in free form by Davy in 1807 by electrolysis of fused potassium hydroxide. Produced industrially by chemical reduction. Prepns: Hackspill, Helv. Chim. Acta 11, 1003 (1928); Jackson, Werner, US 2480655 (1949 to Mine Safety Appliances Co.). NMR spectrum of potassium anion (K-): P. P. Edwards et al., Nature 317, 242 (1985). Reviews: Whaley, "Sodium, Potassium, Rubidium, Cesium, and Francium" in Comprehensive Inorganic Chemistry vol. 1, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 369-529; Chemistry of the Elements N. N. Greenwood, A. Earnshaw, Eds. (Pergamon Press, New York, 1984) pp 75-116; K.-W. Chiu in Kirk-Othmer Encyclopedia of Chemical Technology vol. 19 (Wiley-Interscience, New York, 4th ed., 1996) pp 1047-1057.
Properties: Soft, silvery-white metal; body-centered cubic structure; tarnishes on exposure to air; becomes brittle at low temps; mp 63.2°. bp 765.5°. d20 0.856. Specific heat (0°): 0.176 cal/g deg. Thermal conductivity (cal/sec °C cm): 0.23 (21°); 0.956 (400°). Sol in liq ammonia, ethylenediamine, aniline; sol in several metals; forms liquid alloys with other alkali metals. Emits characteristic violet color (766.5 nm) in flame. One of the most active metals; E0 (aq) K/K+ 2.922 V. Reacts vigorously with oxygen; with water even at -100°; with acids; with the halogens, igniting with bromine and iodine. Molten metal reacts with sulfur; with hydrogen sulfide. Reacts with hydrogen slowly at 200°, rapidly at 350-400°. Reacts slowly with anhydr hydrogen halides at room temp; molten metal ignites in the reaction. Reduces silicates, sulfates, nitrates, carbonates, phosphates, oxides and hydroxides of the heavy metals, often with the separation of the metal. Reacts with organic compds containing active groups. Inert to saturated aliphatic and to aromatic hydrocarbons. Keep under liquid containing no oxygen, e.g., liquid petrolatum, petroleum, etc.
Melting point: mp 63.2°
Boiling point: bp 765.5°
Density: d20 0.856
CAUTION: Direct contact with metal may be corrosive and cause skin and eye burns. See: Fire Protection Guide to Hazardous Materials (National Fire Protection Assoc., Quincy, MA, 12th ed., 1997) Section 49, p 109.
Use: In synthesis of inorganic potassium compds; in organic syntheses involving condensation, dehalogenation, reduction, and polymerization reactions. As heat transfer medium together with sodium: Chem. Eng. News 33, 648 (1955). Radioactive decay of 40K to 40Ar used as tool for geological dating.

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