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Monopotassium phosphate

Phosphoric acid, monopotassium salt; Potassium biphosphate 83.5g/100mL (90 °C) | NFPA-H = 1 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = Monoammonium phosphate Tripotassium phosphate

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 -150 C, 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 it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to -50 C 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, , at 400 C.

Symmetry	Spacegroup	№	Pearsonsymbol	a
(nm)	b
(nm)	c
(nm)	Z	Density
(g/cm3)	T (°C, °F, K)
doi=10.1002/pssa.2211170234	title=Refinement of the Crystal Structure of KH2PO4 in the Ferroelectric Phase	year=1990	last1=Fukami	first1=T.	journal=Physica Status Solidi A	volume=117	issue=2	pages=K93–K96	bibcode=1990PSSAR.117...93F}}	Fdd2	43	oF48	1.0467	1.0533	0.6926	8	2.37
Tetragonal	I2d	122	tI24	0.744	0.744	0.697	4	2.34	−150 to 190 C
doi=10.1143/JPSJ.39.843	title=X-Ray Study of High-Temperature Phase Transitions in KH2PO4	year=1975	last1=Itoh	first1=Kazuyuki	last2=Matsubayashi	first2=Tetsuo	last3=Nakamura	first3=Eiji	last4=Motegi	first4=Hiroshi	journal=Journal of the Physical Society of Japan	volume=39	issue=3	pages=843	bibcode=1975JPSJ...39..843I }}	P21/c	14	mP48	0.733	1.449	0.747	8	190 to 400 C

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.

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 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.

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

"Potassium dihydrogen phosphate".
"Commission Regulation (EU) No 231/2012 of 9 March 2012 laying down specifications for food additives listed in Annexes II and III to Regulation (EC) No 1333/2008 of the European Parliament and of the Council Text with EEA relevance".
(June 1927). "NOTE". Journal of the American Chemical Society.
(1987). "Phase Transitions in Mixed Crystal System K_1−x(NH₄)_xH₂PO₄". Journal of the Physical Society of Japan.
"Potassium Dihydrogen Phosphate". [[American Elements]].
(2012). "Phosphoric Acid and Phosphates".
(1990). "Refinement of the Crystal Structure of KH₂PO₄ in the Ferroelectric Phase". Physica Status Solidi A.
(1975). "X-Ray Study of High-Temperature Phase Transitions in KH₂PO₄". Journal of the Physical Society of Japan.
(1999). "Producing KDP and DKDP crystals for the NIF laser".
(January 2001). "Rapid growth of KDP-type crystals". Progress in Crystal Growth and Characterization of Materials.
(1996). "Nonlinear Optics: Materials, Fundamentals and Applications".
(October 2005). "Hypophosphatemia: An update on its etiology and treatment". The American Journal of Medicine.

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second-harmonic-generation phosphates potassium-compounds acid-salts nonlinear-optical-materials transparent-materials e-number-additives inorganic-fertilizers

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