Potassium oxide

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Potassium oxide

General
Systematic name	Potassium oxide
Other names	Potassium monoxide
Molecular formula	K₂O
Molar mass	94.196 g/mol
Appearance	gray crystalline solid
CAS number	[12136-45-7]
Properties
Density and phase	2.35 g/cm³
Solubility in water	Decomposes violently, forming KOH
Solubility in ethanol	soluble
Solubility in ether	soluble
Melting point	350 °C decomp. in K2O2 and K
Boiling point	n/a
Related compounds
Other anions	Potassium superoxide Potassium peroxide
Other cations	Lithium oxide Sodium oxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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

4K + O₂ → 2K₂O

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%.