Potassium
Potassium is a silvery white alkali metal which has limited applications in the home lab due to its highly reactive nature.
Contents
Properties
Chemical
Potassium is highly reactive with water, forming hydrogen and potassium hydroxide upon contact. 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.
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 are combined in an anhydrous, inert, fully saturated solvent and the mixture is brought to reflux. A good choice of solvent is tetralin or Shellsol D70, however these are difficult to find and mineral oil or kerosene may be used, though success with these alternatives has yet to be substantially demonstrated. The reaction may not reflux if these more accessible solvents are used and the temperature must just be maintained at 200°C instead. 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. 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.
Projects
- Make NaK, an alloy of sodium and potassium that is liquid at room temperature.
- Make potassium superoxide
- Make potassium peroxide
- 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. Handle potassium with your bare hands, counterintuitive as it may seem. A better method, however, is to 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.
