Potassium metal in mineral oil, after being cleaned.
|Name, symbol||Potassium, K|
|Potassium in the periodic table|
|Standard atomic weight (Ar)||39.0983(1)|
|Group, block||I; s-block|
|Electron configuration||[Ar] 2s1|
|2, 8, 8, 1|
|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)|
|Oxidation states||+1, −1 (a strongly basic oxide)|
|Electronegativity||Pauling scale: 0.82|
1st: 418.8 kJ/mol |
2nd: 3052 kJ/mol
3rd: 4420 kJ/mol
|Atomic radius||empirical: 227 pm|
|Covalent radius||203±12 pm|
|Van der Waals radius||275 pm|
|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)|
|Young's modulus||3.53 GPa|
|Shear modulus||1.3 GPa|
|Bulk modulus||3.1 GPa|
|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.
- 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 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.
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.
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.
- 2 KF + CaC2 → 2 K + CaF2 + 2 C
- 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
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.)
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.
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.