Potassium

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Potassium is a soft, silvery-white metal which is extremely reactive and so has few uses in its pure state. Potassium is used as a component in plant fertilizer (as K2O or KCl), as a laboratory reagent; in the synthesis of inorganic potassium compounds, in organic syntheses involving condensations, dehalogenation, reduction, and polymerization reactions, as the super oxide KO2 to produce oxygen in gas masks, and in the seeding of combustion gases in magnetohydrodynamic generators. Potassium is also used in an alloy with sodium as a heat transfer medium.

TYPICAL PROPERTIES :

Crystal Structure: Body-centered cubic, type A2; a = 0.5344 nm at 20 oC

Atomic Weight: 39.09

Density: 0.855 at 20 oC

Volume Change on Freezing: 2.41% contraction

Boiling Point: 756.5 oC

Melting Point: 63.2 oC

Coefficient of Thermal Expansion:

Linear (0 oC - 95 oC): 83 µm/m ⋅ K

Volumetric (Liquid)(63.2 oC - 1250 oC):

V/V = 1 + 2.58 × 10-4 T + 13.08 × 10-8 T2 + 1.98 × 10-12 T3 (where T is in oC)

Specific Heat: 0.770 kJ/kg ⋅ K at 20 oC

Specific Heat vs Temperature:

Solid (-172 oC - 63.2 oC): Cp = 538.07 + 0.8004 T J/kg ⋅ K where T is in K.
Liquid (63.2 oC - 1150 oC): Cp = 839.14 - 0.3675 T + 4.594  × 10-4 T2 J/kg ⋅ K, (where T is inoC)

Latent Heat of Fusion: 59.45 kJ/kg

Latent Heat of Vaporization: 1985 kJ/kg

Thermal Conductivity: 108.3 W/m ⋅ K at 293 K

Vapor Pressure: 15 kPa at 590 oC, 200 kPa at 850 oC, 4000 kPa at 1420 oC

Electrical Resistivity:

oC                   nΩ ⋅ m

 0                     64.9

20                     72

63                  92.2 (solid)

63                 139.5 (liquid)

77                    146.4

Thermoelectric Potential: Versus Platinum: 1.83 × 10-7 V/K (25 oC); 1.988 × 10-6 V/K (250 oC);
                                           1.0168 × 10-5 V/K (800 oC)

Electrochemical Equivalent: For K+: 0.4052 mg/C

Electrolytic Solution Potential: Versus H2: -2.922 V

Hall Coefficient: -4.9 aV ⋅ m/An ⋅ T

Work Function: 2.24 eV (0.359 aJ)

Magnetic Susceptibility:

Volume (mks units): at 30 oC, 4.94 × 10-6; at 100 oC, 4.72 × 10-6; (at 250 oC, 4.61 × 10-6)

Refractive Index: 0.392 for λ = 313 nm; 0.924 for λ = 134 nm; 0.964 for λ = 128 nm

Elastic Constants (295 oC): C11, 3.715 GPa; C12, 3.153 GPa; C44, 1.88 GPa

Kinematic Liquid Viscosity: 0.00628 mm2/s at 69.6 oC; 0.00328 mm2/s at 250 oC;
                                             0.00254 mm2/s at 400 oC

Liquid Surface Tension: σ = 0.1157 - 6.4 × 10-5 T, where T is in oC and σ is in N/m

Speed of Sound: In liquid: speed of sound at melting point, 1880 m/s. Speed of sound over
                            liquid range:v=1880-0.53(T - Tm), where Tm is the melting point.

General Corrosion Behavior:

Potassium is a highly reactive metal and consequently is found only in a combined state. It reacts vigorously with water to form the hydroxide and for this reason must be kept in a moisture-free environment. Potassium reacts with many other materials, including hydrogen, oxygen, sulfur, nitrogen, bromine, and graphite. It also forms alloys with many metals.

Hazards:

Dangerous fire risk; reacts with moisture to form potassium hydroxide and hydrogen. The reaction evolves much heat causing the potassium to melt and splatter. It also ignites the hydrogen. Burning potassium is difficult to extinguish; dry powdered soda ash or graphite or special mixtures of dry chemical is recommended. It can ignite spontaneously in moist air. Moderate explosion risk by chemical reaction. Potassium metal will form the peroxide and the superoxide at room temperature even when stored under mineral oil; may explode violently when handled or cut. Oxide-coated potassium should be destroyed by burning. Store in inert atmospheres, such as argon or nitrogen, or under liquids which are oxygen-free, such as toluene or kerosene, or in glass capsules which have been filled under vacuum or inert atmosphere.