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.
Single crystals are paraelectric at room temperature. At temperatures below -150C, they become ferroelectric.
Monopotassium phosphate can exist in several polymorphs. At room temperature it forms paraelectric crystals with tetragonal symmetry. Upon cooling to -150C it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to -50C when hydrogen is replaced by deuterium. Heating to 190C changes its structure to monoclinic. When heated further, MKP decomposes, by loss of water, to potassium metaphosphate,, at 400C.
| Symmetry | Space group | Pearson symbol | a (nm) | b (nm) | c (nm) | Z | Density (g/cm3) | T (°C, °F, K) | ||
|---|---|---|---|---|---|---|---|---|---|---|
| Orthorhombic[1] | Fdd2 | 43 | oF48 | 1.0467 | 1.0533 | 0.6926 | 8 | 2.37 | < -150C | |
| Tetragonal | I2d | 122 | tI24 | 0.744 | 0.744 | 0.697 | 4 | 2.34 | NaNC | |
| Monoclinic[2] | P21/c | 14 | mP48 | 0.733 | 1.449 | 0.747 | 8 | NaNC |
Monopotassium phosphate is produced by the action of phosphoric acid on potassium carbonate.
Fertilizer-grade MKP powder contains the equivalent of 52% and 34%, and is labeled NPK0-52-34. 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.[3]