Two unit cells of MKP viewed close to the b axis
|Jmol-3D images||Image 1|
|Molar mass||136.086 g/mol|
|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|
|Refractive index (nD)||1.4864|
|Lattice constant||a = 0.744 nm, b = 0.744 nm, c = 0.697 nm|
|EU Index||Not listed|
|LD50||3200 mg/kg (rat, oral)|
|Other cations||Monosodium phosphate
|Related compounds||Dipotassium phosphate
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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.
Monopotassium phosphate can exist 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. Heating to 190 °C changes its structure to monoclinic. When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, KPO3, at 400 °C (752 °F).
|a (nm)||b (nm)||c (nm)||Z||density,
|Tetragonal||I42d||122||tI24||0.744||0.744||0.697||4||2.34||−150 to 190|
|Monoclinic||P21/c||14||mP48||0.733||1.449||0.747||8||190 to 400|
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.
- 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.
- 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.
- 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.
|Wikimedia Commons has media related to Monopotassium phosphate.|
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