Potassium metaphosphate

Potassium metaphosphate
Diagram of the cyclic trimetaphosphate
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.029.284 Edit this at Wikidata
EC Number
  • 232-212-6
KEGG
UNII
  • InChI=1S/K.HO3P/c;1-4(2)3/h;(H,1,2,3)/q+1;/p-1
    Key: OQZCJRJRGMMSGK-UHFFFAOYSA-M
  • [O-]P(=O)=O.[K+]
Properties
KPO3
Molar mass 118.069 g·mol−1
Appearance white
Odor odorless
Density 2.45 g/cm3
Melting point 807 °C (1,485 °F; 1,080 K)
practically insoluble
Solubility soluble in dilute sodium salt solutions
readily soluble in ammonium citrate solution
Structure
monoclinic (trimetaphosphate)
P 1 21/n 1 (No. 14)
a = 11.074 Å, b = 11.965 Å, c = 7.35 Å
α = 90°, β = 102.18°, γ = 90°
952 Å3
4 units per cell
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Potassium metaphosphate is an inorganic compound with the empirical formula KPO3. It is a white, odorless, practically insoluble solid. A cyclic trimetaphosphate has been characterized. The polymeric form (KPO3)n is known as potassium Kurrol's salt or potassium polyphosphate.

Properties

Although the compound is practically insoluble in water, it forms a viscous solution in the presence of other alkali metal cations such as Na+ as well as ammonium. This is attributed to ion exchange where the compact structure of the crystal is disturbed, permitting dissolution. Additionally, if sodium metaphosphate and potassium metaphosphate (both individually insoluble) are mixed, the mixture dissolves due to mutual ion exchange. The compound can also be solublized by passing an aquous suspension through a colloid mill.

Structure

The trimetaphosphate forms monoclinic crystals (space group P 1 21/n 1). It is isotypic with ammonium trimetaphosphate.

Four modifications of the polymeric form have been characterized. All are polyphosphates composed of long chain anions of corner-sharing PO4 tetrahedra. All three transformations are reversible, fast, and show no hysteresis.

Modifications of potassium polyphosphate, (KPO3)n
ModificationStability rangeCrystal systemSpace groupa (Å)b (Å)c (Å)β (°)V (Å3)Z (KPO3)Dx (g·cm−3)Measured atRef.
T (Tief, low) below 275 °CmonoclinicP21/a14.02(3)4.54(1)10.28(2)101.5(2)641(2)82.45(1)20 °C
Z (Zwischen, intermediate) 275–460 °CmonoclinicP21/a14.02(3)4.54(1)11.56(2)64.7(1)665(2)82.36(1)400 °C
H (Hoch, high) 460–650 °CorthorhombicPbnm12.80(3)4.54(1)5.89(1)90342(1)42.29(1)510 °C
HT (Höchst, highest) above 650 °CorthorhombicBbmm12.94(3)4.54(2)5.92(2)90349(3)42.22(2)660 °C

Preparation

Potassium metaphosphate can be prepared by reacting phosphorus pentoxide with potassium chloride at high temperature.

Potassium dihydrogen phosphate dehydrates rapidly at 205–230 °C (401–446 °F; 478–503 K) forming the polyphosphate. Temperatures above 150 °C (302 °F; 423 K) are also known to result in varying chain lengths. The product appears to have a fibrous structure much like asbestos.

The compound may also be prepared by direct action of potassium chloride on phosphoric acid:

KCl + 2H3PO4 ⇌ KH5(PO4)2 + HCl
KH5(PO4)2 ⇌ KH2PO4 + H3PO4
KH2PO4 ⇌ (KPO3)n + H2O

The degree of polymerization increases with temperature and time of heating. Upon heating at 260 °C (500 °F; 533 K) for 215-280 hr the degree of polymerization is 2300; at 445 °C (833 °F; 718 K) for 115-240 hr, it is 11,700. The common product has a molecular weight range of 250,000 to several million.

Uses

Potassium metaphosphate finds use as a buffer in pharmaceuticals. In foods, it used as a binder, fat emulsifier, moisture-retaining agent, alkaline agent, and raising agent. It is also used in micro-encapsulation systems.

The compound was considered an experimental agricultural fertiliser. It is of relatively minor importance in this application. It is a slow-release source of phosphorus as it hydrolyzes to give more water-soluble orthophosphates; the rate is affected by moisture content, temperature, and acidity of the mix.

See also

References

  1. 1 2 Ash, Michael (2008). Handbook of green chemicals. Irene Ash (2nd ed.). Endicott, NY: Synapse Information Resources. p. 845. ISBN 978-1-890595-79-1.
  2. 1 2 Gowariker, Vasant R.; Krishnamurthy, V. N., eds. (2009). The fertilizer encyclopedia. Hoboken, N.J: John Wiley & Sons. p. 523. doi:10.1002/9780470431771. ISBN 978-0-470-43176-4.
  3. 1 2 Bagieu-Beucher, M.; Tordjman, I.; Durif, A.; Guitel, J. C. (1976-05-15). "Structure cristalline du trimétaphosphate de potassium K3P3O9". Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry (in French). 32 (5). International Union of Crystallography: 1427–1430. doi:10.1107/S0567740876005505. ISSN 0567-7408.
  4. 1 2 3 4 5 6 7 Bailar, J.-C. (2013). Comprehensive Inorganic Chemistry (1st ed.). Chantilly: Elsevier Science & Technology. pp. 524–525. ISBN 978-1-4832-8313-5.
  5. 1 2 3 4 5 Ellinger, R. H. (2017). Phosphates As Food Ingredients. Milton: CRC Press LLC. p. 11. ISBN 978-1-351-08396-6.
  6. 1 2 3 Jost, K. H.; Schulze, H. J. (1969). "Zur Phasentransformation des Kaliumpolyphosphates (KPO3)x". Acta Crystallographica B. 25: 1110–1118. doi:10.1107/S0567740869003608.
  7. ↑ Jost, K. H.; Schulze, H. J. (1971). "Mechanismen der Phasentransformationen des KPO3 und die Struktur der Höchsttemperatur-Modifikation des KPO3". Acta Crystallographica B. 27: 1345–1353. doi:10.1107/S0567740871003972.
  8. ↑ Egan, Edward P.; Wakefield, Zachary T. (December 1, 1960). "Low Temperature Heat Capacity and Entropy of Potassium Metaphosphate". The Journal of Physical Chemistry. 64 (12): 1955–1956. doi:10.1021/j100841a506.
  9. ↑ Mengel, Konrad; Kirkby, Ernest A.; Kosegarten, Harald; Appel, Thomas, eds. (2001). Principles of plant nutrition (5th ed.). Dordrecht: Springer Science+Business Media, B. V. p. 510. ISBN 978-94-010-1009-2.
  10. ↑ Bunt, B. R. (2012). Media and Mixes for Container-Grown Plants: A Manual on the Preparation and Use of Growing Media for Pot Plants. Springer London, Limited. pp. 126–127. ISBN 978-94-011-7904-1.

Further reading