Monopotassium 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 Template:Convert, they become ferroelectric.

Structure

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

Symmetry	Space group	№	Pearson symbol	a (nm)	b (nm)	c (nm)	Z	Density (g/cm3)	T (°C, °F, K)
Orthorhombic	Fdd2	43	oF48	1.0467	1.0533	0.6926	8	2.37	< Template:Convert
Tetragonal	I42d	122	tI24	0.744	0.744	0.697	4	2.34	Template:Convert
Monoclinic	P21/c	14	mP48	0.733	1.449	0.747	8		Template:Convert

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% Template:Chem and 34% Template:Chem, and is labeled NPKTemplate:Nbsp0-52-34. 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

• A large KDP crystal, used in the form of slices at the National Ignition Facility
  A large KDP crystal, used in the form of slices at the National Ignition Facility
• Alternate angle, covered
  Alternate angle, covered

External links

• International Chemical Safety Card 1608
• EPA: Potassium dihydrogen phosphate Fact Sheet
• Potassium PhosphateTemplate:Spaced ndasha Hydroculture Salt