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Potassium hexacyanidoferrate(II)
	Potassium ferrocyanide trihydrate
Names
	IUPAC name Potassium hexacyanidoferrate(II)
	Other names (Yellow) Prussiate of Potash; Potassium hexacyanoferrate (II) trihydrate; Tetrapotassium ferrocyanide trihydrate; Ferrate hexacyano tetrapotassium trihydrate;
Identifiers
CAS Number	13943-58-3 (anhydrous) ; 14459-95-1 (trihydrate) ;
3D model (JSmol)	Interactive image;
ChemSpider	20162028;
ECHA InfoCard	100.034.279
EC Number	237-722-2;
E number	E536 (acidity regulators, ...)
PubChem CID	161067;
UNII	GTP1P30292 (anhydrous) ; 961WP42S65 (trihydrate) ;
CompTox Dashboard (EPA)	DTXSID60892423 ;
	InChI InChI=1S/6CN.Fe.4K.3H2O/c61-2;;;;;;;;/h;;;;;;;;;;;31H2/q6-1;+2;4+1;;; Key: UTYXJYFJPBYDKY-UHFFFAOYSA-N;
	SMILES [K+].[K+].N#C[Fe-4](C#N)(C#N)(C#N)(C#N)C#N.[K+].[K+];
Properties
Chemical formula	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)
Solubility in water	trihydrate ; 28.9 g/100 mL (20 °C)
Solubility	insoluble in ethanol, ether
Magnetic susceptibility (χ)	−130.0·10−6 cm3/mol
Hazards
	GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H411
NFPA 704 (fire diamond)	1 0 0
Flash point	Non-flammable
	Lethal dose or concentration (LD, LC):
LD50 (median dose)	6400 mg/kg (oral, rat)
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). verify (what is ?) Infobox references

Potassium hexacyanidoferrate(II) 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.

Synthesis

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

Modern production

Potassium hexacyanidoferrate(II) 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 nitrogenous organic material, 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 hexacyanidoferrate(II) 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 hexacyanidoferrate(II) converts to potassium hexacyanidoferrate(III):

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

This reaction can be used to remove potassium hexacyanidoferrate(II) from a solution.^{[citation needed]}

A famous reaction involves treatment with ferric salts, most commonly Iron(III) chloride, to give Prussian blue. In the reaction with Iron(III) chloride, producing Potassium chloride as a side-product:

3 K₄[Fe(CN)₆] + 4 FeCl₃ → Fe₄[Fe(CN)₆]₃ + 12 KCl

With the composition Fe^III
₄[Fe^II
(CN)
₆]
₃, this insoluble but deeply coloured material is the blue of blueprinting, as well as on many famous paintings such as The Great Wave off Kanagawa and The Starry Night.

Applications

Potassium hexacyanidoferrate(II) 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 hexacyanidoferrates(II) are also used in the purification of tin and the separation of copper from molybdenum ores. Potassium hexacyanidoferrate(II) is used in the production of wine and citric acid.^[6]

In the EU, hexacyanidoferrates(II) (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 hexacyanidoferrate(II) is used to determine the concentration of potassium permanganate, a compound often used in titrations based on redox reactions. Potassium hexacyanidoferrate(II) 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 hexacyanidoferrate(II) can be used as a fertilizer for plants.^{[citation needed]}

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

K₄[Fe(CN)₆] → 4 KCN + FeC₂ + N₂

Structure

Like other metal cyanides, solid potassium hexacyanidoferrate(II), 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.^{[clarification needed]}^{[citation needed]}

Toxicity

The toxicity in rats is low, with lethal dose (LD₅₀) at 6400 mg/kg.^[2]^{[better source needed]} The kidneys are the organ for ferrocyanide toxicity.^[11]

References

^ 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.
^ ^a ^b "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.
^ https://chem.nlm.nih.gov/chemidplus/rn/13943-58-3 ^{[dead link‍]}
^ 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.)
^ 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.
^ ^a ^b ^c 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.
^ ^a ^b Von Wagner, Rudolf (1897). Manual of chemical technology. New York: D. Appleton & Co. p. 474 & 477.
^ 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.
^ "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.
^ 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.
^ 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)
^ 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.