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Questions And Answer

• Introduction

  Potassium was first isolated as a free metal in 1807 by Sir Humphry Davy. It was the first alkali metal to be discovered, produced by electrolysis of potassium carbonate (potash). The element was earlier called Kalium, derived from the Arabic word qili, meaning grass wort, the ash of which was a source of potash. The element derived its symbol K from Kalium. The English name potassium came from potash (pot ash), the carbonate salt of the metal.
  Potassium is distributed widely in nature. The metal is too reactive to occur in native elemental form. It is the seventh most abundant element on earth, constituting 2.40% by weight of the earth’s crust. It is abundantly present in sea water. Oceans contain 0.07% (wt to volume) potassium chloride.
  Potassium occurs in many igneous rocks, such as, feldspar (potassium aluminum silicate), KAlSi3O8 (leucite) and mica, KH2Al3(SiO4)3. Disintegration of these rocks adds potassium to soil and water. Deposits of potassium chloride are found in practically all salt beds, associated with sodium chloride. Some important potassium minerals are leucite, KAlSi2O6; glauconite (a complex silicoaluminate structure of varying compositions); sylvite, KCl; carnallite, KCl•MgCl2•6H2O; langbeinite, K2SO4•2MgSO4; and polyhalite, K2SO4•MgSO4•2CaSO4•2H2O.
  Potassium, along with nitrogen and phosphorus, is an essential element needed for plant growth. In plants, it occurs mostly as K+ ion in cell juice. It is found in fruit or seed. Deficiency can cause curling leaves, yellow or brown coloration of leaves, weak stalk and diminished root growth. Potassium deficiency has been associated with several common animal ailments. Potassium is in extracellular fluid in animals at lower concentrations than sodium. ;

• Potassium and health

  Potassium in its ionic form, K+, is the most abundant positive ion in human and animal cells. As an electrolytic solution, K+ ions are pumped through the blood to all vital organs. Potassium's importance to the physiological system cannot be overstated: It plays a crucial role in electrical pulse transmission along nerve fibers; protein synthesis; acid-base balance; and formation of collagen, elastin, and muscle.
  
  Potassium is highly soluble in water and regulates flow across semipermeable membranes like cell walls. It is this feature that makes a deficiency or an excess of potassium hazardous to health. Either extreme can have undesirable and even disastrous consequences. ;

• Physical Properties

  Silvery metal; body-centered cubic structure; imparts crimson-red color to flame; density 0.862g/cm3 at 20ºC; melts at 63.25ºC; density of liquid potassium at 100ºC is 0.819 g/cm3 and 0.771g/cm3 at 300ºC; vaporizes at 760ºC; vapor pressure 123 torr at 587ºC; electrical resistivity 6.1 microhm-cm at 0ºC and 15.31 microhm-cm at 100ºC; viscosity 0.25 centipoise at 250ºC; surface tension 86 dynes/cm at 100ºC; thermal neutron absorption cross section 2.07 barns; reacts violently with water and acids; reacts with alcohol; dissolves in liquid ammonia and mercury. ;

• Production

  Potassium can be produced by several methods that may be classified under three distinct types: (1) electrolysis, (2) chemical reduction, and (3) thermal decomposition.
  Electrolysis processes have been known since Davy first isolated the metal in 1807. Electrolysis, however, suffers from certain disadvantages. A major problem involves miscibility of the metal with its fused salts. Because of this molten potassium chloride, unlike sodium chloride, cannot be used to produce the metal. Fused mixtures of potassium hydroxide and potassium carbonate or chloride have been used as electrolytes with limited success. Chemical reduction processes are employed nowadays in commercial, as well as, laboratory preparation of potassium. In one such process, molten potassium chloride is reduced with sodium at 760 to 880ºC and the free metal is separated by fractionation:
  KCl + Na → K + NaCl
  Potassium is obtained at over 99.5% purity. The metal, alternatively, may be alloyed with sodium for further applications.
  Reduction of potassium fluoride with calcium carbide at 1,000 to 1,100ºC (Greisheim process) is an effective production method (Greer, J.S., Madaus, J.H and J.W. Mausteller. 1982. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed. p. 914, New York: Wiley Interscience):
  2KF + CaC2 → CaF2 + 2C + 2K
  Some other chemical reduction methods that may be applied for laboratory generation of small quantities of potassium from its salts at high temperatures require a suitable reducing agent such as carbon, calcium, or calcium carbide:
  K2CO3 + 2C → 3CO +2K
  2KCl + Ca → CaCl2 + 2K
  2KCl + CaC2 → CaCl2 + 2C + 2K
  2K2CO3 + +3Si + 3CaO → 4K + 2C + 3CaSiO3
  2K2SiO3 + Si + 3 CaO → 4K + 3CaSiO3
  Potassium can be produced by thermal decomposition of potassium azide:
  2KN3 → 2K + 3N2
  High purity metal may be produced by distillation of technical grade metal. Potassium (technical grade) may be packed under nitrogen. Argon should be used for packing high purity metal. Metal is shipped in stainless steel or carbon containers. In small quantities potassium is transported in glass or metal ampules. ;

• Reactions

  Potassium reacts with oxygen or air forming three oxides: potassium monoxide, K2O; potassium peroxide, K2O2; and potassium superoxide, KO2. The nature of the product depends on oxygen supply. In limited supply of oxygen potassium monoxide is formed, while in excess oxygen, superoxide is obtained:
  4K + O2 → 2K2O
  2K + O2 → K2O2
  K + O2 → KO2
  Potassium reacts violently with water, forming potassium hydroxide:
  2K + 2H2O → 2KOH + H2
  Potassium reacts with hydrogen at about 350ºC to form potassium hydride:
  2K + H2 → 2KH
  Reactions with halogens, fluorine, chlorine and bromine occur with explosive violence. Thus, in contact with liquid bromine it explodes forming potassium bromide:
  2K + Br2 → 2KBr
  Potassium ignites in iodine vapor forming potassium iodide.
  Violent reactions can occur with many metal halides. For example, with zinc halides or iron halides, single replacement reactions take place. Such potassium-metal halide mixtures can react violently when subjected to mechanical shock.
  At ordinary temperatures, potassium does not combine with nitrogen but with an electric charge, potassium azide is formed.
  Reaction with carbon (graphite) at above 400ºC produces a series of carbides, such as KC4, KC8, and KC24. With carbon monoxide, an unstable explosive carbonyl forms:
  K + CO → KCO
  Potassium reduces carbon dioxide to carbon, carbon monoxide and potassium carbonate:
  6K + 5CO2 → CO + C + 3K2CO3
  Potassium reacts with ammonia gas to form potassium amide with liberation of hydrogen:
  2K + 2NH3 → 2KNH2 + H2
  Reactions with phosphorus, arsenic and antimony form phosphide, arsenide, and antimonide of potassium, respectively:
  K + As → K3As
  Reaction with sulfur forms three sulfides. When reactants are in molten state, the product is K2S, but in liquid ammonia K2S2 and KS2 are the main products.
  Potassium reacts explosively with sulfuric acid, forming potassium sulfate with evolution of hydrogen:
  K + H2SO4 → K2SO4 + H2
  Potassium liberates hydrogen from ethanol forming potassium ethoxide:
  2K + 2C2H5OH → 2C2H5OK + H2
  Reaction with potassium nitrate yields potassium monoxide and nitrogen:
  10K + 2KNO3 → 6K2O + N2 ;

• Hazard

  Potassium metal can be dangerous to handle if proper precautions are not taken. Many of its reactions at ordinary temperatures can proceed to explosive violence (see Reactions). Also, it liberates flammable hydrogen gas when combined with water, acids, and alcohols. ;

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