Potassium ferrocyanide

Potassium ferrocyanide is the inorganic compound with formula K4[Fe(CN)6]·3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4−. This salt forms lemon-yellow monoclinic crystals.

Sample of potassium ferrocyanide
Not to be confused with potassium ferricyanide.
Potassium ferrocyanide
Potassium ferrocyanide trihydrate
Names
IUPAC name
Potassium hexacyanidoferrate(II)
Other names
(Yellow) Prussiate of Potash,[1]
Potassium hexacyanoferrate (II) trihydrate,
Tetrapotassium ferrocyanide trihydrate,
Ferrate hexacyano tetrapotassium trihydrate[2]
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.034.279
EC Number
  • 237-722-2
E number E536 (acidity regulators, ...)
UNII
  • InChI=1S/6CN.Fe.4K.3H2O/c6*1-2;;;;;;;;/h;;;;;;;;;;;3*1H2/q6*-1;+2;4*+1;;;
    Key: UTYXJYFJPBYDKY-UHFFFAOYSA-N
  • [K+].[K+].N#C[Fe-4](C#N)(C#N)(C#N)(C#N)C#N.[K+].[K+]
Properties
K4[Fe(CN)6]
Molar mass 368.35 g/mol (anhydrous)
422.388 g/mol (trihydrate)
Appearance Light yellow, crystalline granules
Density 1.85 g/cm3 (trihydrate)
Boiling point (decomposes)
trihydrate
28.9 g/100 mL (20 °C)
Solubility insoluble in ethanol, ether
−130.0·10−6 cm3/mol
Hazards
GHS labelling:
GHS09: Environmental hazard
Warning
H411
NFPA 704 (fire diamond)
NFPA 704 four-colored diamond
1
0
0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
6400 mg/kg (oral, rat)[3]
Related compounds
Other anions
 Potassium ferricyanide
Other cations
 Sodium ferrocyanide
Prussian blue
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

Synthesis

In 1752, the French chemist Pierre Joseph Macquer (1718–1784) first reported the preparation of potassium ferrocyanide, which he achieved by reacting Prussian blue (iron(III) ferrocyanide) with potassium hydroxide.[4][5]

Modern production

Potassium ferrocyanide is produced industrially from hydrogen cyanide, iron(II) chloride, and calcium hydroxide, the combination of which affords Ca2[Fe(CN)6]·11H2O. This solution is then treated with potassium salts to precipitate the mixed calcium-potassium salt CaK2[Fe(CN)6], which in turn is treated with potassium carbonate to give the tetrapotassium salt.[6]

Historical production

Historically, the compound was manufactured from organic compounds containing nitrogen, iron filings, and potassium carbonate.[7] Common nitrogen and carbon sources were torrified horn, leather scrap, offal, or dried blood. It was also obtained commercially from gasworks spent oxide (purification of city gas from hydrogen cyanide).

Chemical reactions

Treatment of potassium ferrocyanide with nitric acid gives H2[Fe(NO)(CN)5]. After neutralization of this intermediate with sodium carbonate, red crystals of sodium nitroprusside can be selectively crystallized.[8]

Upon treatment with chlorine gas, potassium ferrocyanide converts to potassium ferricyanide:

2 K4[Fe(CN)6] + Cl2 → 2 K3[Fe(CN)6] + 2 KCl

This reaction can be used to remove potassium ferrocyanide from a solution.

A famous reaction involves treatment with ferric salts to give Prussian blue. With the composition FeIII
4[FeII
(CN)
6]
3, this insoluble but deeply coloured material is the blue of blueprinting.

Applications

Potassium ferrocyanide finds many niche applications in industry. It and the related sodium salt are widely used as anticaking agents for both road salt and table salt. The potassium and sodium ferrocyanides are also used in the purification of tin and the separation of copper from molybdenum ores. Potassium ferrocyanide is used in the production of wine and citric acid.[6]

In the EU, ferrocyanides (E 535–538) were, as of 2017, solely authorised in two food categories as salt additives.

It can also be used in animal feed.[9]

In the laboratory, potassium ferrocyanide is used to determine the concentration of potassium permanganate, a compound often used in titrations based on redox reactions. Potassium ferrocyanide is used in a mixture with potassium ferricyanide and phosphate buffered solution to provide a buffer for beta-galactosidase, which is used to cleave X-Gal, giving a bright blue visualization where an antibody (or other molecule), conjugated to Beta-gal, has bonded to its target. On reacting with Fe(3) it gives a Prussian blue colour. Thus it is used as an identifying reagent for iron in labs.

Potassium ferrocyanide can be used as a fertilizer for plants.

Prior to 1900, before the invention of the Castner process, potassium ferrocyanide was the most important source of alkali metal cyanides.[6] In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide:[7]

K4[Fe(CN)6] → 4 KCN + FeC2 + N2

Structure

Like other metal cyanides, solid potassium ferrocyanide, both as the hydrate and anhydrous salts, has a complicated polymeric structure. The polymer consists of octahedral [Fe(CN)6]4− centers crosslinked with K+ ions that are bound to the CN ligands.[10] The K+---NC linkages break when the solid is dissolved in water.

Toxicity

Potassium ferrocyanide is nontoxic, and does not decompose into cyanide in the body. The toxicity in rats is low, with lethal dose (LD50) at 6400 mg/kg.[2] The kidneys are the organ for ferrocyanide toxicity.[11]

See also

References
  1. Five Hundred Useful and Amusing Experiments in Chemistry, and in the Arts and Manufactures: With Observations on the Properties Employed, and Their Application to Useful Purposes. Thomas Tegg. 1825.
  2. "POTASSIUM FERROCYANIDE MSDS Number: P5763 - Effective Date: 12/08/96". J. T. Baker Inc. Archived from the original on 2015-11-21. Retrieved 2012-04-08.
  3. http://chem.sis.nlm.nih.gov/chemidplus/rn/13943-58-3
  4. Macquer (1752). "Éxamen chymique de bleu de Prusse" [Chemical examination of Prussian blue]. Histoire de l'Académie Royale des Sciences …, § Mémoires de l'Académie royale des Sciences (in French): 60–77. From pp. 63-64: "Après avoir essayé ainsi inutilement de décomposer le bleu de Prusse par les acides, … n'avoit plus qu'une couleur jaune un peu rousse." (After having tried so vainly to decompose Prussian blue by acids, I made recourse to alkalies. I put a half ounce of this [Prussian] blue in a flask, and I poured on it ten ounces of a solution of nitre fixed by tartar [i.e., potassium nitrate (nitre) which is mixed with crude cream of tartar and then ignited, producing potassium carbonate]. As soon as these two substances had been mixed together, I saw with astonishment that, without the aid of heat, the blue color had entirely disappeared; the powder [i.e., precipitate] at the bottom of the flask had only a rather gray color: having put this vessel on a sand bath in order to heat the solution until it simmered, this gray color also disappeared entirely, and all that was contained in the flask, both the powder [i.e., precipitate] and the solution, had only a yellow color [that was] a little red.)
  5. Munroe, Charles E.; Chatard, Thomas M. (1902). "Manufactures: Chemicals and Allied Products". Twelfth Census of the United States: Bulletins (210): 1–306.; see p. 31.
  6. Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M.; Kellens, R.; Reddy, J.; Steier, N.; Hasenpusch, W. (October 2011). "Cyano Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a08_159.pub3. ISBN 978-3527306732.
  7. Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477.
  8. Seel, F. (1965). "Sodium nitrosyl cyanoferrate". In Brauer, G. (ed.). Handbook of Preparative Inorganic Chemistry. Vol. 2 (2nd ed.). New York: Academic Press. p. 1768. LCCN 63-14307. Archived from the original on 2010-03-07. Retrieved 2017-09-10.
  9. "EuSalt Expert Meeting on E 535 and E 536 as Feed Additives". EUSalt. Archived from the original on 2019-05-12. Retrieved 2018-12-06.
  10. Willans, Mathew J.; Wasylishen, Roderick E.; McDonald, Robert (2009-05-18). "Polymorphism of Potassium Ferrocyanide Trihydrate as Studied by Solid-State Multinuclear NMR Spectroscopy and X-ray Diffraction". Inorganic Chemistry. 48 (10): 4342–4353. doi:10.1021/ic802134j. ISSN 0020-1669. PMID 19425611.
  11. Peter Aggett, Fernando Aguilar, Riccardo Crebelli, Birgit Dusemund, Metka Filipič, Maria Jose Frutos, Pierre Galtier, David Gott, Ursula Gundert‐Remy, Gunter Georg Kuhnle, Claude Lambré, Jean‐Charles Leblanc, Inger Therese Lillegaard, Peter Moldeus, Alicja Mortensen, Agneta Oskarsson, Ivan Stankovic, Ine Waalkens‐Berendsen, Rudolf Antonius Woutersen, Matthew Wright and Maged Younes. (2018). "Re‐evaluation of sodium ferrocyanide (E 535), potassium ferrocyanide (E 536) and calcium ferrocyanide (E 538) as food additives". EFSA Journal. 16 (7): 5374. doi:10.2903/j.efsa.2018.5374. PMC 7009536. PMID 32626000.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. Kosugi, Nobuhiro; Yokoyama, Toshihiko; Kuroda, Haruo (May 1986). "Polarization dependence of XANES of square-planar Ni(CN)2−4 ion. A comparison with octahedral Fe(CN)4−6 and Fe(CN)3−6 ions". Chemical Physics. 104 (3): 449–453. doi:10.1016/0301-0104(86)85034-0. ISSN 0301-0104.
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