Monopotassium phosphate
Chemical compound
From Wikipedia, the free encyclopedia
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]
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 Two unit cells of MKP viewed close to the b axis | |
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| Names | |
|---|---|
| IUPAC names
Potassium dihydrogen phosphate[1] | |
| Other names
Potassium phosphate monobasic; Phosphoric acid, monopotassium salt; Potassium biphosphate | |
| Identifiers | |
3D model (JSmol) |
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| ChEMBL | |
| ChemSpider | |
| ECHA InfoCard | 100.029.012 |
| EC Number |
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| E number | E340(i) (antioxidants, ...) |
PubChem CID |
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| RTECS number |
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| UNII | |
CompTox Dashboard (EPA) |
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| Properties | |
| KH 2PO 4 | |
| 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) [3] |
| 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[4] |
| Basicity (pKb) | 11.9 |
Refractive index (nD) |
1.4864 |
| Structure | |
| Tetragonal[5] | |
| I42d | |
a = 0.744 nm, b = 0.744 nm, c = 0.697 nm | |
| Hazards | |
| GHS labelling: | |
 [6] | |
| Warning[6] | |
| H315, H319[6] | |
| P264, P280, P305+P351+P338, P321, P332+P313, P337+P313[6] | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) |
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).
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Single crystals are paraelectric at room temperature. At temperatures below −150 °C (−238 °F), they become ferroelectric.
Structure
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
3, at 400 °C (752 °F).
| Symmetry | Space group |
№ | Pearson symbol |
a (nm) |
b (nm) |
c (nm) |
Z | Density (g/cm3) |
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 |
Manufacturing
Monopotassium phosphate is produced by the action of phosphoric acid on potassium carbonate. It can then be crystallized into boules, large crystals by dissolving the KDP in hot water and salt, creating a growth solution, placing a seed crystal in the solution and then cooling the solution, done in a holden-type crystallizer, in what is known as solution growth.[10][11][12]
Applications
Fertilizer-grade MKP powder contains the equivalent of 52% P
2O
5 and 34% K
2O, and is labeled NPK 0-52-34. MKP powder is often used as a nutrient source in the greenhouse trade and in hydroponics.
Crystals of MKP are used in optical modulators and for non-linear optics such as second-harmonic generation (SHG). Potassium dideuterium phosphate (KDP), with slightly different properties, is also used in nonlinear frequency conversion of laser light. 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.[13]
Gallery
A large KDP crystal, used in the form of slices at the National Ignition Facility
Alternate angle, covered
