Potassium

Potassium,  ₁₉K

Potassium metal in mineral oil, after being cleaned.
General properties
Name, symbol	Potassium, K
Appearance	Silvery-white metal
Potassium in the periodic table
Na ↑ K ↓ Rb Argon ← Potassium → Calcium
Atomic number	19
Standard atomic weight (Ar)	39.0983(1)
Group, block	I; s-block
Period	period 4
Electron configuration	[Ar] 2s1
per shell	2, 8, 8, 1
Physical properties
Silvery-white
Phase	Solid
Melting point	336.7 K ​(63.5 °C, ​​146.3 °F)
Boiling point	1032 K ​(759 °C, ​​1398 °F)
Density near r.t.	0.862 g/cm3
when liquid, at	0.828 g/cm3
Critical point	2223 K, 16 MPa
Heat of fusion	2.33 kJ/mol
Heat of	76.9 kJ/mol
Molar heat capacity	29.6 J/(mol·K)
Atomic properties
Oxidation states	+1, −1 ​(a strongly basic oxide)
Electronegativity	Pauling scale: 0.82
energies	1st: 418.8 kJ/mol 2nd: 3052 kJ/mol 3rd: 4420 kJ/mol (more)
Atomic radius	empirical: 227 pm
Covalent radius	203±12 pm
Van der Waals radius	275 pm
Miscellanea
Crystal structure	​​body-centered cubic (bcc)
Speed of sound thin rod	2000 m/s (at 20 °C)
Thermal expansion	83.3 µm/(m·K) (at 25 °C)
Thermal conductivity	102.5 W/(m·K)
Electrical resistivity	72 Ω·m (at 20 °C)
Magnetic ordering	paramagnetic
Young's modulus	3.53 GPa
Shear modulus	1.3 GPa
Bulk modulus	3.1 GPa
Mohs hardness	0.4
Brinell hardness	0.363 MPa
CAS Registry Number	7440-09-7
Discovery and first isolation	Humphry Davy (1807)

Potassium is a silvery white alkali metal with the symbol K which has limited applications in the home lab due to its highly reactive nature.

Properties

Chemical

Potassium is highly reactive with water, forming hydrogen and potassium hydroxide upon contact.

K + H₂O → KOH + ½ H₂

The reaction is so exothermic, the hydrogen is instantly ignited, forming a burning sphere of potassium. It can be alloyed with sodium to form NaK, which is liquid at room temperature and is sometimes used to dry solvents.

It rapidly oxidizes in air to form potassium oxide and peroxide.

Potassium metal in the presence of carbon tetrachloride and traces of heptane has been shown to form a powerful shock-sensitive explosive material.

Physical

Potassium is a silvery white metal which will tarnish quickly in air. It melts at 63.5 °C and can easily be cut with a butter knife. Potassium has a low boiling point of 759 °C and is therefore often purified industrially by distillation, however this is not viable for the amateur and is extremely dangerous due to high risk of explosion.

Availability

Potassium metal is generally not available from lab suppliers, but stores which cater to element collectors such as GalliumSource and Metallium sell potassium. Potassium bought this way is very expensive and can be up to $10 per gram.

Preparation

Potassium metal can be prepared in a well equipped home lab without too much difficulty. Potassium hydroxide and magnesium turnings or powder are combined in an anhydrous, inert, fully saturated solvent and the mixture is brought to reflux. Although a good choice of solvent is tetralin or Shellsol D70, it's been shown that alkali metals will slowly react with tetralin, though the reaction is more visible when sodium metal is used. Mineral oil has been used by many chemists with good performance, as shown in this video, though it has to be dry for the reaction to work. Kerosene may be used. The reaction may not reflux if these more accessible solvents are used and the temperature must just be maintained at 200 °C. A catalyst of a tertiary alcohol, such as t-butanol or t-amyl alcohol, is then added and, over the course of several hours, spheres of potassium will slowly form. Other tertiary alcohols, and even bulk chain secondary alcohols have been shown to work just as good. The reaction mixture can then be dumped out into toluene and the spheres of potassium taken out and ampouled for storage. It is important to note that without very, very pure reagents, success is unlikely. Even mildly tarnished magnesium turnings/powder may not react well enough to produce potassium.

It's also possible to obtain potassium metal through a thermite-like reaction between magnesium powder and potassium hydroxide. However, it's been noted that if this reaction is done in open air, the resulting potassium will catch fire and be destroyed as soon as it's produced. Thus, while this process works for sodium metal, it is not suitable for potassium, unless it's done in an inert medium.

Another route involves heating sodium metal with potassium chloride in a furnace at 760 °C, which yields sodium chloride and potassium metal vapors, which are then condensed into a separate container, away from open air.

Na + KCl → NaCl + K ↑

If the temperature is not carefully controlled, NaK will be produced instead.

Electrolysis of potassium salts is another possible route. Unlike sodium, potassium metal is soluble in molten potassium hydroxide, thus the Castner process is not very useful in obtaining potassium metal, though it's not impossible.

The Griesheimer process employing the reaction of potassium fluoride with calcium carbide was also used to produce potassium metal. Potassium chloride can also be used:

2 KF + CaC₂ → 2 K + CaF₂ + 2 C

Projects

• Make NaK, an alloy of sodium and potassium that is liquid at room temperature.
• Make potassium superoxide
• Make potassium peroxide
• Make potassium tert-butoxide
• Dry solvents

Handling

Safety

Potassium is highly reactive and may ignite in air or on contact with organic materials (like paper) under the right circumstances. Potassium compounds have little toxicity taken orally, but injecting potassium ions will lead to rapid cardiac arrest and death. People with cardiac problems should limit the consumption of potassium compounds.

NEVER HANDLE POTASSIUM WITH GLOVES! It's easy to tell if your hands are wet, but it is not easy to tell if gloves are wet. If you handle potassium with thick oven mitts and the potassium ignites, it will burn through the gloves in less than two seconds. Use tongs or poke the potassium with a screwdriver or other metal stick, due to the extreme sectile properties of the metal. (If you use a non-metal stick, the potassium will react with it, unless it's glass.)

Storage

Potassium metal is highly reactive with water and care must be taken to prevent contact as this will result in a fire. Potassium must be stored in a flame sealed ampoule if it is to be stored for more than a month, due to a black layer of oxides and superoxides which will build up and may become a shock sensitive explosive. If it is to be stored for less than a month, it can be stored in a tightly closed vial under mineral oil.

Disposal

While it is easy to just chuck a piece in water, the safest method of destruction is the addition of anhydrous isopropyl alcohol to any pieces. This forms potassium isopropoxide, which can be disposed of safely by neutralizing it with a soluble carbonate solution.

Throwing large pieces of potassium in water can cause an explosion and may send hot pieces of potassium metal flying.

References

Relevant Sciencemadness threads

• Make potassium at home
• Potassium metal instead of Sodium from dioxane extraction of KMgO?