Potassium ferrocyanide is the

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...

with formula K₄ e(CN)₆�3H₂O. It is the potassium salt of the

coordination complex A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

e(CN)₆sup>4−. This salt forms lemon-yellow monoclinic crystals. Potassium-ferrocyanide-trihydrate-sample

Synthesis

In 1752, the French chemist

Pierre Joseph Macquer Pierre-Joseph Macquer (9 October 1718 – 15 February 1784) was an influential French chemist. He is known for his ''Dictionnaire de chymie'' (1766). He was also involved in practical applications, to medicine and industry, such as the French de ...

(1718–1784) first reported the preparation of potassium ferrocyanide, which he achieved by reacting Prussian blue (iron(III) ferrocyanide) with potassium hydroxide.

Modern production

Potassium ferrocyanide is produced industrially from hydrogen cyanide,

ferrous chloride Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a paramagnetic solid with a high melting point. The compound is white, but typical samples are often off-white. FeCl2 crystallizes from water ...

, and

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...

, the combination of which affords Ca₂ e(CN)₆�11H₂O. This solution is then treated with potassium salts to precipitate the mixed calcium-potassium salt CaK₂ e(CN)₆ which in turn is treated with

potassium carbonate Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and ...

to give the tetrapotassium salt.

Historical production

Historically, the compound was manufactured from organic compounds containing nitrogen, iron filings, and potassium carbonate. Common nitrogen and carbon sources were torrified horn, leather scrap,

offal Offal (), also called variety meats, pluck or organ meats, is the organs of a butchered animal. The word does not refer to a particular list of edible organs, which varies by culture and region, but usually excludes muscle. Offal may also refe ...

, 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 e(NO)(CN)₅ After neutralization of this intermediate with sodium carbonate, red crystals of sodium nitroprusside can be selectively crystallized. Upon treatment with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is ...

gas, potassium ferrocyanide converts to

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: :2 K₄ e(CN)₆+ Cl₂ → 2 K₃ e(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 Fe CN).html"_;"title="Cyanide.html"_;"title="e(Cyanide">CN)">Cyanide.html"_;"title="e(Cyanide">CN)_this_insoluble_but_deeply_coloured_material_is_the_blue_of_ CN).html"_;"title="Cyanide.html"_;"title="e(Cyanide">CN)">Cyanide.html"_;"title="e(Cyanide">CN)_this_insoluble_but_deeply_coloured_material_is_the_blue_of_blueprint">Cyanide">CN).html"_;"title="Cyanide.html"_;"title="e(Cyanide">CN)">Cyanide.html"_;"title="e(Cyanide">CN)_this_insoluble_but_deeply_coloured_material_is_the_blue_of_blueprinting.

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Applications

Potassium ferrocyanide finds many niche applications in industry. It and Sodium ferrocyanide">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. In the EU, ferrocyanides (E 535–538) were, as of 2017, solely authorised in two food categories as salt additives. The kidneys are the organ for ferrocyanide toxicity. It can also be used in animal feed. 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 β-Galactosidase (EC 3.2.1.23, lactase, beta-gal or β-gal; systematic name β-D-galactoside galactohydrolase), is a glycoside hydrolase enzyme that catalyzes hydrolysis of terminal non-reducing β-D-galactose residues in β-D-galactosides. β ...

, 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 The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

cyanides Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...

. In this historical process, potassium cyanide was produced by decomposing potassium ferrocyanide: K₄ e(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 e(CN)₆sup>4− centers crosslinked with K⁺ ions that are bound to the CN ligands.Willans, M.J.; Wasylishen, R.E.; McDonald, R. "Polymorphism of potassium ferrocyanide trihydrate as studied by solid-state multinuclear NMR spectroscopy and X-ray diffraction" Inorganic Chemistry 2009, volume 48, p4342-4353 The K⁺---NC linkages break when the solid is dissolved in water.

Toxicity

Potassium ferrocyanide is nontoxic, and is not decomposed to cyanide in the body. The toxicity in rats is low, with lethal dose (LD₅₀) at 6400 mg/kg.

References

External links

* * {{DEFAULTSORT:Potassium Ferrocyanide Potassium compounds Iron(II) compounds Cyano complexes Coordination complexes E-number additives Nephrotoxins Iron complexes