Two unit cells of MKP viewed close to the b axis
|Systematic IUPAC name
Potassium phosphate monobasic;
Phosphoric acid, monopotassium salt;
3D model (JSmol)
|E number||E340(i) (antioxidants, ...)|
CompTox Dashboard (EPA)
|Appearance||White powder, deliquescent|
|Melting point||252.6 °C (486.7 °F; 525.8 K)|
|Boiling point||400 °C (752 °F; 673 K) , decomposes|
|22.6g/100 mL (20 °C)|
83.5g/100 mL (90 °C)
|Solubility||Slightly soluble in ethanol|
Refractive index (nD)
a = 0.744nm, b = 0.744nm, c = 0.697nm
|Safety data sheet||External MSDS|
|GHS Signal word||Warning|
|P264, P280, P305+351+338, P321, P332+313, P337+313|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|3200mg/kg (rat, oral)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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.
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. Heating to 190 °C (374 °F) changes its structure to monoclinic. When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, , at 400 °C (752 °F).
|T (°C, °F, K)|
|Orthorhombic||Fdd2||43||oF48||1.0467||1.0533||0.6926||8||2.37||< -150 °C, -238 °F, 123 K|
|Tetragonal||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||P21/c||14||mP48||0.733||1.449||0.747||8||190 to 400 °C, 374 to 752 °F, 463 to 673 K|
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.
A large KDP crystal, used in the form of slices at the National Ignition Facility