1. Occurrence and Significance
Potassium (K) is the fourth element in Group IA of the periodic table. It has an atomic number of 19, an atomic weight of 39.10, and a valence of 1. The average abundance of K in the earth’s crust is 1.84%; in soils it has a range of 0.1 to 2.6%; in streams it is 2.3 mg/L, and in groundwaters it has a range of 0.5 to 10 mg/L. Potassium is commonly associated with aluminosilicate minerals such as feldspars. 40K is a naturally occurring radioactive isotope with a half-life of 1.3 × 109 years. Potassium compounds are used in glass, fertilizers, baking powder, soft drinks, explosives, electroplating, and pigments. Potassium is an essential element in both plant and human nutrition, and occurs in groundwaters as a result of mineral dissolution, from decomposing plant material, and from agricultural runoff.
The common aqueous species is K+. Unlike sodium, it does not remain in solution, but is assimilated by plants and is incorporated into a number of clay-mineral structures.