Potassium oxide

Potassium oxide is a compound of potassium and oxygen used mainly as an intermediate in inorganic synthesis. It is formed by reacting metallic potassium with a limited supply of oxygen. This is done by only oxidizing a small portion of the metal with dry air at a time to prevent peroxide formation. Potassium oxide can also be synthesized by heating appropriate quantities of potassium nitrate with metallic potassium in a vacuum. As the potassium nitrate is heated, it decomposes into potassium nitrite and oxygen. In this manner, the amount of oxygen produced can be carefully controlled and an intimate mixture of the two reagents ensures that the oxygen will react evenly with the potassium. In both cases, an excess of potassium is used and is subsequently vacuum distilled at high temperatures to separate it from the potassium oxide. The two most common synthesis methods are outlined below.

1. 4K + O₂ (dry air) → 2K₂O
2. 2KNO₃ → 2KNO₂ + O₂

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Potassium oxide is a very basic oxide and reacts with water violently to produce the caustic potassium hydroxide. It is deliquescent in air and will absorb water from the atmosphere, initiating this vigorous reaction. It is therefore both toxic and corrosive to human tissue.

Metallic potassium is sometimes used to eliminate the last traces of oxygen in a vacuum. Potassium oxide is the main by-product of this reaction.

The chemical formula K₂O is used in the N-P-K (nitrogen-phosphorus-potassium) numbers on the labels of fertilizers. Although K₂O is the correct formula for potassium oxide, potassium oxide is not used as a fertilizer in these products. Normally, potassium chloride, potassium sulfate, or potassium carbonate is used as a fertilizer source for potassium. The percentage of K₂O given on the label only represents the amount of potassium in the fertilizer if it was in the form of potassium oxide. Potassium oxide is about 83% potassium by weight, but potassium chloride, for instance, is only 52% potassium by weight. Potassium chloride provides less potassium than an equal amount of potassium oxide. Thus, if a fertilizer is 30% potassium chloride by weight, its standard potassium rating, based on potassium oxide, would be only 19%.

References

Lange's Handbook of Chemistry (14th Edition), McGraw-Hill, 1992; Section 1; Table 1.15
The Condensed Chemical Dictionary (10th Edition), Gesner G. Hawley
Handbook of Preparative Inorganic Chemistry: Volume 1 (2nd Edition), Georg Brauer, 1963
http://scifun.chem.wisc.edu/CHEMWEEK/AgriFert/agrifert.html
http://www.inchem.org/documents/icsc/icsc/eics0769.htm
http://www.webelements.com/webelements/compounds/text/K/K2O1-12136457.html