Monopotassium phosphate

Two unit cells of MKP viewed close to the b axis
IUPAC names
Potassium dihydrogen phosphate[1]
Other names
Potassium phosphate monobasic;
Phosphoric acid, monopotassium salt;
Potassium biphosphate
3D model (JSmol)
ECHA InfoCard 100.029.012 Edit this at Wikidata
EC Number
  • 231-913-4
E number E340(i) (antioxidants, ...)
RTECS number
  • TC6615500
  • InChI=1S/3K.H3O4P/c;;;1-5(2,3)4/h;;;(H3,1,2,3,4)/q3*+1;/p-3 checkY
  • InChI=1/3K.H3O4P/c;;;1-5(2,3)4/h;;;(H3,1,2,3,4)/q3*+1;/p-3
  • [K+].OP(O)([O-])=O
Molar mass 136.086 g/mol
Appearance Colourless crystals or white granular or crystalline powder[2]
Odor Odorless [2]
Density 2.338 g/cm3
Melting point 252.6 °C (486.7 °F; 525.8 K) [4]
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[3]
Basicity (pKb) 11.9
a = 0.744 nm, b = 0.744 nm, c = 0.697 nm
GHS labelling:
GHS07: Exclamation mark[6]
H315, H319[6]
P264, P280, P305+P351+P338, P321, P332+P313, P337+P313[6]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
3200 mg/kg (rat, oral)
Safety data sheet (SDS) External MSDS
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 checkY☒N ?)

Monopotassium phosphate (MKP) (also, potassium dihydrogen phosphate, 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.[7]

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.[8] Heating to 190 °C (374 °F) changes its structure to monoclinic.[9] When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, KPO
, at 400 °C (752 °F).

Symmetry Space
Z Density
T (°C, °F, K)
Orthorhombic[8] Fdd2 43 oF48 1.0467 1.0533 0.6926 8 2.37 < −150 °C, −238 °F, 123 K
Tetragonal[5] 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[9] 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. It is 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 approximately 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.

Monopotassium phosphate is also used as an ingredient in sports drinks such as Gatorade and Powerade.

In medicine, monopotassium phosphate is used for phosphate substitution in hypophosphatemia.[10]



  1. ^ "Potassium dihydrogen phosphate".
  2. ^ a b "Commission Regulation (EU) No 231/2012 of 9 March 2012 laying down specifications for food additives listed in Annexes II and III to Regulation (EC) No 1333/2008 of the European Parliament and of the Council Text with EEA relevance". EUR-Lex. E 340 (i) MONOPOTASSIUM PHOSPHATE. Retrieved 19 January 2023.((cite web)): CS1 maint: location (link)
  3. ^ Mathews, Christopher K., K. E. Van Holde, Ean R. Appling, and Spencer J. Anthony-Cahill. Biochemistry. Redwood City, CA: Benjamin/Cummings Pub., 1990. Print.
  4. ^ King, Harold S. J. Am. Chem. Soc. 1927, 49, 6, 1511–1512
  5. ^ a b 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. Bibcode:1987JPSJ...56..577O. doi:10.1143/JPSJ.56.577.
  6. ^ a b c d "Potassium Dihydrogen Phosphate". American Elements. Retrieved October 30, 2018.
  7. ^ 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. ISBN 978-3527306732.
  8. ^ a b Fukami, T. (1990). "Refinement of the Crystal Structure of KH2PO4 in the Ferroelectric Phase". Physica Status Solidi A. 117 (2): K93–K96. Bibcode:1990PSSAR.117...93F. doi:10.1002/pssa.2211170234.
  9. ^ a b 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. Bibcode:1975JPSJ...39..843I. doi:10.1143/JPSJ.39.843.
  10. ^ Gaasbeek, André; Meinders, A. Edo (October 2005). "Hypophosphatemia: an update on its etiology and treatment". The American Journal of Medicine. 118 (10): 1094–1101. doi:10.1016/j.amjmed.2005.02.014. ISSN 0002-9343. PMID 16194637.