Potassium Ferricyanide Alternative names: Red Prussiate

Description

Potassium ferricyanide (K3[Fe(CN)6]) also known as red prussiate, Prussian red or potassium hexacyanoferrate(III), is a coordination compound that is stable at room temperature and pressure and forms ruby red crystals and powder. It is soluble in water and its solution shows some green-yellow fluorescence.It is manufactured by passing chlorine gas through solutions of potassium ferrocyanide. Potassium ferricyanide then separates out of the solution.

Applications

The compound has widespread use in blueprint drawing and in photography. 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 with sodium thiosulfate (hypo) to reduce the density of negative where the mixture is known as Farmer's reducer. 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 Fe3+ ions, and which can therefore be used to detect rust. It is possible to calculate the number of moles of Fe3+ ions by using a colorimeter, because 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 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).

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