Potassium

Potassium is a chemical element. It has the symbol K and atomic number 19. The name “potassium” comes from the word “potash”, as potassium was first isolated from potash. Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals. It oxidizes rapidly in air and is very reactive, especially towards water. In many respects, potassium and sodium are chemically similar, although organisms in general, and animal cells in particular, treat them very differently.

Notable characteristics

Physical

Potassium is the second least dense metal; only lithium is less dense. It is a soft, low-melting solid that can easily be cut with a knife. Freshly cut potassium is silvery in appearance, but in air it begins to tarnish toward grey immediately. Potassium must be protected from air for storage to prevent disintegration of the metal from oxide and hydroxide corrosion. Often samples are maintained under a reducing medium such as kerosene.

Like the other alkali metals, potassium reacts violently with water producing hydrogen. The reaction is notably more violent than that of lithium or sodium with water, and is sufficiently exothermic that the evolved hydrogen gas ignites.

2K(s) + 2H₂O(l) ? H₂(g) + 2KOH(aq)

Because potassium reacts quickly with even traces of water, and its reaction products are nonvolatile, it is sometimes used alone, or as NaK (an alloy with sodium which is liquid at room temperature) to dry solvents prior to distillation. In this role, it serves as a potent desiccant

Potassium and its compounds emit a violet color in a flame. This fact is the basis of the flame test for the presence of potassium in a sample.

Potassium compounds generally have excellent water solubility, due to the high hydration energy of the K⁺ ion. The potassium ion is colorless in water.

Potassium concentration in solution is commonly determined by flame photometry, atomic absorption spectrophotometry, inductively coupled plasma, or ion selective electrodes. Methods of separating potassium by precipitation, sometimes used for gravimetric analysis, include the use of sodium tetraphenyl boron, dihydrogen hexachloroplatinate (IV) hexahydrate, and sodium cobaltinitrite.

Biochemical

Potassium is important in nerve function and in influencing osmotic balance between cells and the interstitiual fluid.

Potassium may be detected by taste because it triggers three of the five types of tastebuds, according to concentration. Dilute solutions of potassium ion taste sweet (allowing moderate concentrations in milk and juices), while higher concentrations become increasingly bitter/alkaline, and finally also salty to the taste. The combined bitterness and saltiness of high potassium content solutions makes high-dose potassium supplementation by liquid drinks a palatability challenge.