Monopotassium phosphite
IUPAC name
Potassium hydrogen phosphonate
Other names
Potassium dihydrogen phosphite; Mono potassium phosphite; Monopotassium dihydrogen phosphite; Potassium phosphite monobasic
3D model (JSmol)
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  • InChI=1S/K.H2O3P/c;1-4(2)3/h;1-2H/q+1;-1
  • OP(O)[O-].[K+]
Molar mass 120.085 g·mol−1
Appearance white crystals
Density 2.14 g/cm3
2200 g/L
Solubility soluble in ethanol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Monopotassium phosphite is an inorganic compound with the formula KH2PO3. A compositionally related compound has the formula H3PO3.2(KH2PO3). Both are white solids that consist of salts of the phosphite anion H2PO3, the conjugate base of phosphorous acid.[1]

Phosphites of potassium are used as fungicides (in a loose sense) in agriculture to combat water mold infection. Confusingly, they have also been marketed as fertilizers to avoid a regulatory burden. While perfectly capable to supply potassium to the plant, the phosphorus in phosphite form is unavailable to plants, and may even inhibit the uptake of the normal phosphate form if used in excess.[2]


  1. ^ J. Loub (1991). "Crystal Chemistry of Inorganic Phosphites". Acta Crystallogr. B47: 468–473. doi:10.1107/S0108768191002380.
  2. ^ Thao; Yamakawa (2008). "Phosphite (phosphorous acid): Fungicide, fertilizer or bio-stimulator?". Soil Science and Plant Nutrition. 55 (2): 228–234. doi:10.1111/j.1747-0765.2009.00365.x. S2CID 95723306.