Monopotassium phosphate (MKP) (also, potassium dihydrogen phosphate, KDP, or monobasic potassium phosphate) is the

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula KH₂PO₄. Together with dipotassium phosphate (K₂HPO₄^.(H₂O)_x) it is often used as a fertilizer,

food additive Food additives are substances added to food to preserve flavor or enhance taste, appearance, or other sensory qualities. Some additives, such as vinegar ( pickling), salt ( salting), smoke ( smoking) and sugar ( crystallization), have been used f ...

, and

buffering agent A buffer solution is a solution where the pH does not change significantly on dilution or if an acid or base is added at constant temperature. Its pH changes very little when a small amount of strong acid or base is added to it. Buffer solution ...

. The salt often cocrystallizes with the dipotassium salt as well as with

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

Single crystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no Grain boundary, grain bound ...

s are paraelectric at room temperature. At temperatures below , they become ferroelectric.

Structure

Monopotassium phosphate can exist in several polymorphs. At room temperature it forms paraelectric crystals with tetragonal symmetry. Upon cooling to it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to when hydrogen is replaced by deuterium. Heating to changes its structure to monoclinic. When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, , at .

Manufacturing

Monopotassium phosphate is produced by the action of

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.

Applications

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 A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

source in the

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trade and in

hydroponics Hydroponics is a type of horticulture and a subset of #Passive sub-irrigation, hydroculture which involves growing plants, usually crops or medicinal plants, without soil, by using water-based mineral Plant nutrition, nutrient Solution (chemi ...

Crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s of MKP are used in optical modulators and for non-linear optics such as

second-harmonic generation Second-harmonic generation (SHG), also known as frequency doubling, is the lowest-order wave-wave nonlinear interaction that occurs in various systems, including optical, radio, atmospheric, and magnetohydrodynamic systems. As a prototype behav ...

(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.

Gallery

Image:KDP crystal.jpg, A large KDP crystal, used in the form of slices at the National Ignition Facility Image:Monocristal dsc03676.jpg, Alternate angle, covered

References

External links

International Chemical Safety Card 1608EPA: Potassium dihydrogen phosphate Fact Sheet
{{Potassium compounds Second-harmonic generation Phosphates Potassium compounds Acid salts Nonlinear optical materials Transparent materials E-number additives Inorganic fertilizers