Monopotassium phosphate

From Sciencemadness Wiki
Jump to: navigation, search
Monopotassium phosphate
Monopotassium phosphate potassium dihydrogen phosphate sample.jpg
MKP from a pet shop.
IUPAC name
Potassium dihydrogen phosphate
Other names
Potassium phosphate monobasic
Jmol-3D images Image
Molar mass 136.086 g/mol
Appearance White crystalline powder
Odor Odorless
Density 2.338 g/cm3
Melting point 252.6 °C (486.7 °F; 525.8 K)
Boiling point 400 °C (752 °F; 673 K) (decomposes)
22.6 g/100 mL (20 °C)
83.5 g/100 mL (90 °C)
Solubility Slightly soluble in ethanol, methanol
Insoluble in benzene, dichloromethane
Acidity (pKa) 6.86
Safety data sheet ScienceLab
Flash point Non-flammable
Related compounds
Related compounds
Dipotassium phosphate
Tripotassium phosphate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Monopotassium phosphate (MKP) or potassium dihydrogen phosphate (KDP) is a soluble phosphoric acid salt of potassium with the chemical formula KH2PO4.



Monopotassium phosphate is commonly used as a buffer with a pKa of 6.86, and will react with both acids and bases in solution. Reacting MKP with 1 mole of potassium hydroxide will produce dipotassium phosphate, and further adding phosphoric acid will convert it to tripotassium phosphate:

KH2PO4 + KOH → K2HPO4 + H2O
K2HPO4 + KOH → K3PO4 + H2O

Using potassium carbonate for the same reaction stops at dipotassium phosphate due to the lower pKa of potassium carbonate.


Potassium dihydrogen phosphate is a white solid, soluble in water. It decomposes when heated above 400 °C. MPK's density is 2.338 g/cm3 at standard conditions.


MKP is sold in some agricultural stores as fertilizer. Hydroponics stores and companies sell it as a water-soluble nutrient.

It can also be purchased from some pet stores for use in household aquariums.


Potassium dihydrogen phosphate can be made by the partial neutralization of phosphoric acid with potassium hydroxide or potassium carbonate. Excess phosphoric acid is recommended as it will limit the generation of any monohydrogen phosphate species. It can easily be crystallized from solution due to its lower solubility at ambient temperatures.


  • Grow large crystals
  • Make elemental phosphorus
  • pH buffer



MPK is safe to handle and poses little toxicity.


MKP is highly stable with time and doesn't require any particular storage conditions.


MKP is nontoxic and safe to dispose of down a drain or in normal household waste.


Relevant Sciencemadness threads