Monopotassium phosphate

Monopotassium phosphate, MKP, (also potassium dihydrogenphosphate, KDP, or monobasic potassium phosphate) is the inorganic compound with the formula KH2PO4. Together with dipotassium phosphate (K2HPO4.(H2O)x) it is often used as a fertilizer, food additive, and buffering agent. The salt often cocrystallizes with the dipotassium salt as well as with phosphoric acid.[4]

Monopotassium phosphate

Two unit cells of MKP viewed close to the b axis
IUPAC names
Potassium dihydrogenphosphate
Potassium dihydrogen(tetraoxidophosphate)(1−)
Systematic IUPAC name
Potassium dihydroxidodioxidophosphate(1−)
Other names
Potassium phosphate monobasic;
Phosphoric acid, monopotassium salt;
potassium biphosphate
3D model (JSmol)
ECHA InfoCard 100.029.012
EC Number
  • 231-913-4
E number E340(i) (antioxidants, ...)
RTECS number
  • TC6615500
Molar mass 136.086 g/mol
Appearance White 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
Acidity (pKa) 6.86[1]
Basicity (pKb) 11.9
a = 0.744 nm, b = 0.744 nm, c = 0.697 nm
Safety data sheet External MSDS
GHS pictograms [3]
GHS Signal word Warning[3]
H315, H319[3]
P264, P280, P305+351+338, P321, P332+313, P337+313[3]
NFPA 704 (fire diamond)
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
3200 mg/kg (rat, oral)
Related compounds
Other cations
Monosodium phosphate
Monoammonium phosphate
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).
N verify (what is YN ?)
Infobox references

Single crystals are paraelectric at room temperature. At temperatures below −150 °C (−238 °F), they become ferroelectric.


Monopotassium phosphate can exist in several polymorphs. At room temperature it forms paraelectric crystals with tetragonal symmetry. Upon cooling to −150 °C (−238 °F) it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to −50 °C (−58 °F) when hydrogen is replaced by deuterium.[5] Heating to 190 °C (374 °F) changes its structure to monoclinic.[6] When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, KPO
, at 400 °C (752 °F).

Symmetry Space
No Pearson
a (nm) b (nm) c (nm) Z density,
T (°C, °F, K)
Orthorhombic[5] Fdd2 43 oF48 1.0467 1.0533 0.6926 8 2.37 < (−150 °C, −238 °F, 123 K)
Tetragonal[2] I42d 122 tI24 0.744 0.744 0.697 4 2.34 (−150 to 190 °C, −238 to 374 °F, 123 to 463 K)
Monoclinic[6] P21/c 14 mP48 0.733 1.449 0.747 8 (190 to 400 °C, 374 to 752 °F, 463 to 673 K)


Monopotassium phosphate is produced by the action of phosphoric acid on potassium carbonate.


Fertilizer-grade MKP powder contains the equivalent of 52% P
and 34% K
, and is labeled NPK 0-52-34. MKP powder is often used as a nutrient source in the greenhouse trade and in hydroponics.

As a crystal, MKP is noted for its non-linear optical properties. Used in optical modulators and for non-linear optics such as second-harmonic generation (SHG).

Also to be noted is KD*P, potassium dideuterium phosphate, with slightly different properties. Highly deuterated KDP is used in nonlinear frequency conversion of laser light instead of protonated (regular) KDP due to the fact that the replacement of protons with deuterons in the crystal shifts the third overtone of the strong OH molecular stretch to longer wavelengths, moving it mostly out of the range of the fundamental line at ~1064 nm of neodymium-based lasers. Regular KDP has absorbances at this wavelength of approximately 4.7–6.3% per cm of thickness while highly deuterated KDP has absorbances of typically less than 0.8% per cm.


  1. Mathews, Christopher K., K. E. Van Holde, Ean R. Appling, and Spencer J. Anthony-Cahill. Biochemistry. Redwood City, CA: Benjamin/Cummings Pub., 1990. Print.
  2. Ono, Yasuhiro; Hikita, Tomoyuki; Ikeda, Takuro (1987). "Phase Transitions in Mixed Crystal System K1−x(NH4)xH2PO4". Journal of the Physical Society of Japan. 56 (2): 577. doi:10.1143/JPSJ.56.577.
  3. "Potassium Dihydrogen Phosphate". American Elements. Retrieved October 30, 2018.
  4. Klaus Schrödter; Gerhard Bettermann; Thomas Staffel; Friedrich Wahl; Thomas Klein; Thomas Hofmann (2012). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3.
  5. Fukami, T. (1990). "Refinement of the Crystal Structure of KH2PO4 in the Ferroelectric Phase". Physica Status Solidi A. 117 (2): K93. doi:10.1002/pssa.2211170234.
  6. Itoh, Kazuyuki; Matsubayashi, Tetsuo; Nakamura, Eiji; Motegi, Hiroshi (1975). "X-Ray Study of High-Temperature Phase Transitions in KH2PO4". Journal of the Physical Society of Japan. 39 (3): 843. doi:10.1143/JPSJ.39.843.
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