Message to the Members

preparation of metallic potassium

1

Equipment:
100 ml Erlenmeyer flask (or 2-neck flask)
Stopper with hole
Glass tube
Disposable syringe (or funnel)
Sand
Cooking pot
Gas stove
Balloon (optional)

Chemicals:
Magnesium chips (even filed)
Potassium hydroxide
Shellsol D70 (or other high boiling point hydrocarbon)
tertiary butanol (Omikron-Online)

Procedure: Into a 100ml flask with 50ml Shellsol D70, 3.11g magnesium filings and 6.12g potassium hydroxide are placed. This is heated slowly in a sand bath for one hour, until the temperature reaches the boiling point of the solvent (about 200degC) under reflux. Care must be taken to avoid contamination with air, and the hydrogen gas which forms during the reaction must be allowed to escape. During the entire time the reaction mixture should be stirred. Do not use Teflon stirring bars, as this could potentially react.

 

0.6 g of tertiary butanol in 0.6g Shellsol D70 was slowly dropped in, over half an hour with continued stirring. This produced more hydrogen. With continued stirring, the formation of metallic potassium is finished after about 4 hours. During this time potassium begins to form small beads that drift around, but soon grow into large balls that settle to the bottom.

 

 

20 minutes after the beginning addition of tert-butanol, shiny metallic potassium pellets become visible, but can be less conspicuous mixed in among the metallic magnesium turnings. These beads will become obvious within 30 minutes.

 

 

 

When the flask is swirled every half hour, the potassium pellets drift through the turbulence, resembling frozen drops of mercury.

 
 

The balls grow in size as the reaction proceeds and more potassium forms.

 
 

After 4 hours, the diameter of the metallic potassium balls approach several centimeters.

 

 

The mixture is left to cool to room temperature in the absence of air, then quickly poured into a new flask. The potassium beads were removed with tweezers and placed into a flask containing Shellsol D70, which acts to protect the metal from air. The metallic balls showed good crystalline surface.

 
The yield was about 70%, not including the tiny globules of metallic potassium.
 
Reactions
Although potassium is generally regarded as a more reactive element than magnesium (metallic potassium can react with magnesium chloride, producing potassium chloride and magnesium metal), under some conditions magnesium can reduce potassium ions.
The explanation is that magnesium can form somewhat covalent bonds with oxygen, which are more favorable to oxide ions, which contain two excess electrons in a relatively small radius. The lattice energy of MgO, compared to K2O could also be an important factor. Similarly magnesium can burn in nitrogen, whereas potassium cannot react with nitrogen gas. (Lithium is the only alkali metal that can burn in nitrogen).
 
 
any mineral will work. In fact, I am told by one online experimenter that kerosene also can be used. But it is probably safer and better to use a higher molecular weight, less volatile, hydrocarbon since the reaction is being heated.
 
The hydrocarbon chains in keresene are typically typically 6-14 carbon atoms long. Those lighter chainer could potentially vaporize out if it is strongly heated, which is generally not a good thing. Shellsol D70 boils at around 200degC, if I remember correctly. It contains about 60% hydrocarbons and 40% cycloalkanes (saturated hydrocarbons in a ring). It often also contains traces of benzene (only around 1-2%). The molecular chains in D70 typically contain baround 14 to 15 carbon atoms. Unlike D70, D80 is more readily available in America and has a slightly higher boiling point.
Unfortunately, the alcohol used is very specific. Isopropanol apparently does not work.
The only alcohols that I am aware of, which have resulted in a successful reaction are tert-butanol or 2-methyl-2-butanol.
Apparently only tertiary alcohols work; the alkoxides of primary and secondary alcohols are presumably not stable enough under such strongly alkaline conditions at 200 C.
Č
ċ
ď
zoom.jpg
(125k)
Anders Hoveland,
Feb 2, 2011, 5:45 PM
ċ
ď
zoom2.jpg
(60k)
Anders Hoveland,
Feb 2, 2011, 5:45 PM
Comments