Potassium oxide is a compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive species. Some materials of commerce, such as fertilizers and cements, are assayed assuming the percent composition that would be equivalent to K₂O.

Production

Potassium oxide is produced from the reaction of oxygen and potassium; this reaction affords potassium peroxide, K₂O₂. Treatment of the peroxide with potassium produces the oxide:

Alternatively and more conveniently, K₂O is synthesized by heating potassium nitrate with metallic potassium:
Potassium hydroxide cannot be further dehydrated to the oxide.

Properties and reactions

K₂O crystallises in the antifluorite structure. In this motif the positions of the anions and cations are reversed relative to their positions in CaF₂, with potassium ions coordinated to 4 oxide ions and oxide ions coordinated to 8 potassium. K₂O is a basic oxide and reacts with water violently to produce the caustic potassium hydroxide. It is deliquescent and will absorb water from the atmosphere, initiating this vigorous reaction.

Potassium oxide in fertilizers

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%.