Potassium ferricyanide

Overview
Potassium ferricyanide is the chemical compound with the formula K3[Fe(CN)6]. This bright red salt consists of the coordination compound [Fe(CN)6]3−. It is soluble in water and its solution shows some green-yellow fluorescence.

Preparation
Potassium ferricyanide is manufactured by passing chlorine through a solution of potassium ferrocyanide. Potassium ferricyanide separates from the solution:
 * 2K4[Fe(CN)6] +  Cl2  &rarr;  2K3[Fe(CN)6]  +  2KCl

Applications
The compound has widespread use in blueprint drawing and in photography (Cyanotype process). Iron and copper toning involve the use of potassium ferricyanide. Potassium ferricyanide is used as an oxidizing agent to remove silver from negatives and positives, a process called dot etching. In color photography, potassium ferricyanide is used to reduce the size of color dots without reducing their number, as a kind of manual color correction. The compound is also used to temper iron and steel, in electroplating, dyeing wool, as a laboratory reagent, and as a mild oxidizing agent in organic chemistry. It is also used in photography with sodium thiosulfate (hypo) to reduce the density of a negative where the mixture is known as Farmer's reducer; this can help offset problems from overexposure. Variants of Farmer's reducer can also be used as the intermediate step in reversal photography to dissolve the silver image produced by the first development.

Potassium ferricyanide is also one of two compounds present in ferroxyl indicator solution (along with phenolphthalein) which turns blue (Prussian blue) in the presence of Fe2+ ions, and which can therefore be used to detect metal oxidation that will lead to rust. It is possible to calculate the number of moles of Fe2+ ions by using a colorimeter, because of the very intense color of Prussian blue Fe4[Fe(CN)6]3.

Potassium ferricyanide is often used in physiology experiments as a means of increasing a solution's redox potential (Eo' ~ 436 mV at pH 7). Sodium dithionite is usually used as a reducing chemical in such experiments (Eo' ~ −420 mV at pH 7).

Potassium ferricyanide is the main component of Murakami's etchant for cemented carbides.

Prussian blue
Prussian blue, the deep blue pigment in blue printing, is generated by the reaction of K3[Fe(CN)6] with ferrous (Fe2+) ions.

In histology, potassium ferricyanide is used to detect ferrous iron in biological tissue. In this reaction, potassium ferricyanide reacts with ferrous iron in acidic solution to produce an insoluble blue pigment, and both the stain and the pigment are commonly referred to as Turnbull's blue. To detect ferric (Fe3+) iron, potassium ferrocyanide is used instead; the stain and pigment produced are commonly known as Prussian blue. It has been found that the compound formed in the Turnbull's blue reaction and the compound formed in the Prussian blue reaction are the same unique compound, Prussian blue.