Monopotassium phosphate

Monopotassium phosphate

Two unit cells of MKP viewed close to the b axis
Names
IUPAC name Potassium dihydrogen phosphate
Other names Potassium phosphate monobasic Phosphoric acid, monopotassium salt
Identifiers
CAS Number	7778-77-0
ChEMBL	ChEMBL1200925
ChemSpider	22914
EC Number	231-913-4
Jmol interactive 3D	Image
PubChem	516951
RTECS number	TC6615500
UNII	4J9FJ0HL51
InChI InChI=1S/3K.H3O4P/c;;;1-5(2,3)4/h;;;(H3,1,2,3,4)/q3*+1;/p-3 Key: LWIHDJKSTIGBAC-UHFFFAOYSA-K InChI=1/3K.H3O4P/c;;;1-5(2,3)4/h;;;(H3,1,2,3,4)/q3*+1;/p-3 Key: LWIHDJKSTIGBAC-DFZHHIFOAX
SMILES [K+].OP(O)([O-])=O
Properties
Chemical formula	KH2PO4
Molar mass	136.086 g/mol
Appearance	White powder deliquescent
Odor	odorless
Density	2.338 g/cm^3
Melting point	252.6 °C (486.7 °F; 525.8 K)
Boiling point	400 °C (752 °F; 673 K) (decomposes)
Solubility in water	22.6 g/100 mL (20 °C) 83.5 g/100 mL (90 °C)
Solubility	slightly soluble in ethanol
Acidity (pKa)	6.86[1]
Basicity (pKb)	11.9
Refractive index (nD)	1.4864
Structure
Crystal structure	tetragonal[2]
Space group	I42d
Lattice constant	a = 0.744 nm, b = 0.744 nm, c = 0.697 nm
Hazards
Safety data sheet	External MSDS
NFPA 704	0 1 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (Median dose)	3200 mg/kg (rat, oral)
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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Infobox references

Monopotassium phosphate, MKP, (also potassium dihydrogen phosphate, KDP, or monobasic potassium phosphate), KH₂PO₄, is a soluble salt of potassium and the dihydrogen phosphate ion which is used as a fertilizer, a food additive and a fungicide. It is a source of phosphorus and potassium. It is also a buffering agent. When used in fertilizer mixtures with urea and ammonium phosphates, it minimizes escape of ammonia by keeping the pH at a relatively low level.

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 it transforms to a ferroelectric phase of orthorhombic symmetry, and the transition temperature shifts up to −50 °C to when hydrogen is replaced by deuterium.^[3] Heating to 190 °C changes its structure to monoclinic.^[4] When heated further, MKP decomposes, by loss of water, to potassium metaphosphate, KPO₃, at 400 °C (752 °F).

Symmetry	Space group	No	Pearson symbol	a (nm)	b (nm)	c (nm)	Z	density, g/cm^3	T (°C)
Orthorhombic[3]	Fdd2	43	oF48	1.0467	1.0533	0.6926	8	2.37	< −150
Tetragonal[2]	I42d	122	tI24	0.744	0.744	0.697	4	2.34	−150 to 190
Monoclinic[4]	P21/c	14	mP48	0.733	1.449	0.747	8		190 to 400

Manufacturing

Monopotassium phosphate is produced by the action of phosphoric acid on potassium carbonate.

Applications

Fertilizer-grade MKP powder contains the equivalent of 52% P₂O₅ and 34% K₂O, and is labeled NPK 0-52-34. 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. 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 ~1064 nm of neodymium-based lasers. Regular KDP has absorbances at this wavelength of approximately 4.7–6.3%/cm of thickness while highly deuterated KDP has absorbances of typically less than 0.8%/cm.

Gallery

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References

1. Mathews, Christopher K., K. E. Van Holde, Ean R. Appling, and Spencer J. Anthony-Cahill. Biochemistry. Redwood City, CA: Benjamin/Cummings Pub., 1990. Print.
2. Ono, Yasuhiro; Hikita, Tomoyuki; Ikeda, Takuro (1987). "Phase Transitions in Mixed Crystal System K_1-x(NH₄)_xH₂PO₄". Journal of the Physics Society Japan 56 (2): 577. doi:10.1143/JPSJ.56.577.
3. Fukami, T. (1990). "Refinement of the Crystal Structure of KH₂PO₄ in the Ferroelectric Phase". Physica status solidi (a) 117 (2): K93. doi:10.1002/pssa.2211170234.
4. Itoh, Kazuyuki; Matsubayashi, Tetsuo; Nakamura, Eiji; Motegi, Hiroshi (1975). "X-Ray Study of High-Temperature Phase Transitions in KH₂PO₄". Journal of the Physical Society of Japan 39 (3): 843. doi:10.1143/JPSJ.39.843.

External links

• International Chemical Safety Card 1608
• EPA: Potassium dihydrogen phosphate Fact Sheet
• Potassium Phosphate – a Hydroculture Salt