Potassium Ferricyanide

What is Potassium Ferricyanide?

Potassium ferricyanide is a coordination compound whose chemical formula is K₃[Fe(CN)₆]. This chemical compound exists as a bright red salt under standard conditions for temperature and pressure (usually abbreviated to STP).

Each potassium ferricyanide molecule consists of three positively charged potassium cations and one ferricyanide anion. It can be noted that the coordination structure of the ferricyanide anion is octahedral.

Potassium ferricyanide is known to be soluble in water. Furthermore, aqueous solutions of this coordination compound in water are also known to exhibit certain levels of green or greenish-yellow fluorescence.

The discovery of this coordination compound is credited to the German chemist Leopold Gmelin. Potassium ferricyanide is believed to have been discovered in the year 1822. Initially, the primary application of this compound was in the dye industry for the preparation of ultramarine dyes.

How is Potassium Ferricyanide Prepared?

Potassium ferricyanide can be prepared on an industrial scale by obtaining a solution of potassium ferrocyanide and passing chlorine gas through it. When the chlorine gas is passed through the potassium ferrocyanide solution, the red-coloured potassium ferricyanide is formed. This compound goes on to separate itself from the potassium ferrocyanide solution. The chemical equation for this reaction is provided below.

2K₄[Fe(CN)₆] + Cl₂ → 2KCl + 2K₃[Fe(CN)₆]

Therefore, it can be understood that two molar equivalents of potassium chloride are obtained for each molar equivalent of chlorine gas passed through the potassium ferrocyanide solution.

Structure of Potassium Ferricyanide

Potassium ferricyanide is known to have an extremely complicated polymeric structure (as is usually the case with most metal cyanides). A simplified structure of potassium ferricyanide molecules is illustrated below.

It is important to note that the ferricyanide anion, which is sometimes referred to as the hexacyanoferrate(III) anion, is known to have an octahedral coordination geometry. Furthermore, the ferricyanide centers in this coordination compound are known to be cross-linked with positively charged potassium cations. Also, the potassium cations are also known to be bound to the cyanide ligands in this coordination compound.

It can also be noted that when potassium ferricyanide is dissolved in water, the bond between the potassium cation and the cyanide ligand is broken. It can also be noted that the cyanide group acts as a ligand in the ferricyanide anion by donating an electron pair to the central metal atom (which is iron in this case).

Properties of Potassium Ferricyanide

Some important physical and chemical properties of potassium ferricyanide are listed below.

• The chemical formula of potassium ferricyanide is K₃[Fe(CN)₆].
• The molar mass of this coordination compound is equal to 329.24 grams per mole.
• Under standard conditions for temperature and pressure (usually abbreviated to STP), potassium ferricyanide is known to exist in the form of deep red crystals.
• However, it is not uncommon for this coordination compound to exist in the form of pellets or a powder which is either dark red or orange.
• The density of this coordination compound is roughly equal to 1.89 grams per cubic centimetre under standard conditions (when it exists in the solid phase).
• The melting point of potassium ferricyanide is equal to 573 Kelvin. Therefore, this compound melts at a temperature of 300 degrees celsius.
• Potassium ferricyanide does not have any boiling point since it undergoes decomposition before reaching any specific boiling point.
• Potassium ferricyanide is known to be highly soluble in water. For example, the solubility of this compound in cold water is roughly 330 grams per litre.
• At a temperature of 20 degrees celsius, the solubility of potassium ferricyanide in water is known to be approximately equal to 464 grams per litre. Finally, the solubility of this compound in hot water can be as high as 775 grams per litre.
• It can also be noted that potassium ferricyanide is slightly soluble in alcohol and is also soluble in certain acids.
• The crystal structure of this coordination compound is known to be monoclinic.

Uses of Potassium Ferricyanide

Some important applications of potassium ferricyanide are listed below.

• One of the most notable applications of potassium ferricyanide is in the Cyanotype process, where it is employed in the printing processes that yield cyan-blue prints.
• This compound is also known to have various applications in the field of photography. For example, many photographic print toning processes are known to employ potassium ferricyanide.
• Potassium ferricyanide can also be employed as an oxidising agent for the removal of silver from certain colour negatives and colour positives.
• This compound can also be of use during the hardening of iron and the hardening of steel.
• The process of electroplating is often done using this compound as one of the raw materials.
• Potassium ferricyanide is also known to be used in the dyeing of wool.
• This compound is also used as a laboratory reagent. Many organic reactions are conducted employing this compound as a mild oxidising agent.

Apart from these applications, it can also be noted that potassium ferricyanide can be used as an electron transfer agent in many types of amperometric biosensors.

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