Monopotassium phosphate

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Monopotassium phosphate
Two unit cells of MKP viewed close to the b axis
CAS number 7778-77-0 YesY
PubChem 516951
ChemSpider 22914 N
EC number 231-913-4
RTECS number TC6615500
Jmol-3D images Image 1
Molecular formula KH2PO4
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)
Solubility in water 22.6 g/100 mL (20 °C)
83.5 g/100 mL (90 °C)
Solubility slightly soluble in ethanol
Acidity (pKa) 7.2
Basicity (pKb) 11.9
Refractive index (nD) 1.4864
Crystal structure tetragonal[1]
Space group I42d
Lattice constant a = 0.744 nm, b = 0.744 nm, c = 0.697 nm
MSDS External MSDS
EU Index Not listed
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 1: Exposure would cause irritation but only minor residual injury. E.g., turpentine Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point Non-flammable
LD50 3200 mg/kg (rat, oral)
Related compounds
Other cations Monosodium phosphate
Monoammonium phosphate
Related compounds Dipotassium phosphate
Tripotassium phosphate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

Monopotassium phosphate, MKP, (also potassium dihydrogen phosphate, KDP, or monobasic potassium phosphate), KH2PO4, is a soluble salt of potassium and the dihydrogen phosphate ion which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent. When used in fertilizer mixtures with urea and ammonium phosphates, it minimizes escape of ammonia by keeping the pH at a relatively low level.

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


Monopotassium phosphate can exists in several polymorphs. At room temperature it forms paraelectric crystals with tetragonal symmetry. Upon cooling to −150 °C it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to −50 °C to when hydrogen is replaced by deuterium.[2] Heating to 190 °C changes its structure to monoclinic.[3] When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, KPO3, at 400 °C (752 °F).

Symmetry Space
No Pearson
a (nm) b (nm) c (nm) Z density,
T (°C)
Orthorhombic[2] Fdd2 43 oF48 1.0467 1.0533 0.6926 8 2.37 < −150
Tetragonal[1] I42d 122 tI24 0.744 0.744 0.697 4 2.34 −150 to 190
Monoclinic[3] P21/c 14 mP48 0.733 1.449 0.747 8 190 to 400


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


Fertilizer-grade MKP powder contains the equivalent of 52% P2O5 and 34% K2O, and is labeled 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%/cm of thickness while highly deuterated KDP has absorbances of typically less than 0.8%/cm.



  1. ^ a b Ono, Yasuhiro; Hikita, Tomoyuki; Ikeda, Takuro (1987). "Phase Transitions in Mixed Crystal System K1-x(NH4)xH2PO4". Journal of the Physics Society Japan 56 (2): 577. doi:10.1143/JPSJ.56.577. 
  2. ^ a b 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. 
  3. ^ 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. doi:10.1143/JPSJ.39.843. 

External links[edit]