# Potassium ferrocyanide

Potassium hexacyanidoferrate(II) is the inorganic compound with formula K$_4$[Fe(CN)$_6$]·3H$_2$O. 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.

### Modern production

Potassium hexacyanidoferrate(II) is produced industrially from hydrogen cyanide, iron(II) chloride, and calcium hydroxide, the combination of which affords Ca$_2$[Fe(CN)$_6$]·11H$_2$O. This solution is then treated with potassium salts to precipitate the mixed calcium-potassium salt CaK$_2$[Fe(CN)$_6$], which in turn is treated with potassium carbonate to give the tetrapotassium salt.

### Historical production

Historically, the compound was manufactured from nitrogenous organic material, iron filings, and potassium carbonate. 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 H$_2$[Fe(NO)(CN)$_5$]. After neutralization of this intermediate with sodium carbonate, red crystals of sodium nitroprusside can be selectively crystallized.
Upon treatment with chlorine gas, potassium hexacyanidoferrate(II) converts to potassium hexacyanidoferrate(III):
    2 K$_4$[Fe(CN)$_6$] + Cl$_2$ → 2 K$_3$[Fe(CN)$_6$] + 2 KCl
This reaction can be used to remove potassium hexacyanidoferrate(II) from a solution.
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 K$_4$[Fe(CN)$_6$] + 4 FeCl$_3$ → Fe$_4$[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.
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.
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.
Prior to 1900, before the invention of the Castner process, potassium hexacyanidoferrate(II) was the most important source of alkali metal cyanides. In this historical process, potassium cyanide was produced by decomposing potassium hexacyanidoferrate(II):
K$_4$[Fe(CN)$_6$] → 4 KCN + FeC$_2$ + N$_2$
Potassium ferrocyanide is a component of white gunpowder or Augendre's powder, a blasting powder of high energy. It comprises 28 parts yellow potassium ferrocyanide, 23 parts cane sugar, and 49 parts of potassium chlorate, mixed under absolute alcohol to prevent premature detonation.

## 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. The K$^+$---NC linkages break when the solid is dissolved in water.

### Toxicity

The toxicity in rats is low, with lethal dose (LD$_{50}$) at 1.6—3.2g/kg. The kidneys are the organ for ferrocyanide toxicity.