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 chemist ...

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 In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

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 people, French chemist. He is known for his ''Dictionnaire de chymie'' (1766). He was also involved in practical applications, to medicine and industry, such as ...

(1718–1784) first reported the preparation of potassium ferrocyanide, which he achieved by reacting

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...

(iron(III) ferrocyanide) with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

Modern production

Potassium ferrocyanide is produced industrially from

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...

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 as ...

, 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 gl ...

to give the tetrapotassium salt.

Historical production

Historically, the compound was manufactured from organic compounds containing nitrogen, iron filings, and potassium carbonate. Common

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

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 Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

gives H₂ e(NO)(CN)₅ After neutralization of this intermediate with

sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

, red crystals of

sodium nitroprusside Sodium nitroprusside (SNP), sold under the brand name Nitropress among others, is a medication used to lower blood pressure. This may be done if the blood pressure is very high and resulting in symptoms, in certain types of heart failure, and d ...

can be selectively crystallized. Upon treatment with

chlorine Chlorine is a chemical element with the Symbol (chemistry), 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 betwee ...

gas, potassium ferrocyanide converts to

potassium ferricyanide Potassium ferricyanide is the chemical compound with the formula K3 e(CN)6 This bright red salt contains the octahedrally coordinated 3−.html" ;"title="e(CN)6sup>3−">e(CN)6sup>3− ion. It is soluble in water and its solution shows some g ...

: :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

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_Applications

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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 X-gal (also abbreviated BCIG for 5-bromo-4-chloro-3-indolyl-β--galactopyranoside) is an organic compound consisting of galactose linked to a substituted indole. The compound was synthesized by Jerome Horwitz and collaborators in 1964. The formal c ...

, 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 The Castner process is a process for manufacturing sodium metal by electrolysis of molten sodium hydroxide at approximately 330 °C. Below that temperature, the melt would solidify; above that temperature, the molten sodium would start to dissol ...

, 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 Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications includ ...

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

ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...

s.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 In toxicology, the lethal dose (LD) is an indication of the lethal toxicity of a given substance or type of radiation. Because resistance varies from one individual to another, the "lethal dose" represents a dose (usually recorded as dose per kilog ...

(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