Potassium Ferricyanide: Structure, Properties & Uses

Potassium Ferricyanide is a bright red salt available in laboratories. It has the chemical formula K₃Fe(CN)₆, consisting of three potassium ions and one ferricyanide ion. It is a coordination compound which indicates that the central metallic ion, Potassium (K), is surrounded by an array of ions, ferricyanide ions [Fe(CN)₆]. Though it is usually red, when dissolved in water it gives a mix of yellow-green fluorescence. Potassium Ferricyanide has extensive use in the industrial preparation of ultramarine pigments that are used in paints, lacquers and decorating materials. Its applications extend further to photography, being an oxidizer and electroplating.

Key Terms: Potassium Ferricyanide, Organic Compound, Chemical Formula, Ions, Ferric Chloride, Hydrogen, Carbon

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What is Potassium Ferricyanide?

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Potassium Carbonate is an organic compound as it does not contain a bond between carbon and hydrogen. The credit for the discovery of Potassium Ferricyanide goes to the German scientist Leopold Gmelin in 1822. The positive ions (cation) Potassium (K) combines with Ferricyanide [Fe(CN)₆] to form a neutral compound. The IUPAC name of Potassium Ferricyanide is Potassium Hexacyanoferrate(III). Other common names for Potassium Ferricyanide are Red Prussiate of Potash and Prussian red.

Potassium Ferricyanide

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Preparation of Potassium Ferricyanide

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There is only one common method for the production of Potassium Ferricyanide:

A solution of Potassium Ferrocyanide K₄[Fe(CN)₆] in water is saturated in a ratio of 1:10 with chlorine. The solution is continuously stirred until the solution develops a deep red colour. The process is completed when the sample starts giving a brownish tint instead of blue precipitate in a solution of Ferric Chloride (FeCl₃).

Then, the solution is evaporated to 1/3 of its weight and is left to cool to room temperature and crystallize. The crystals formed are then collected by filtering them. The original liquor, when further evaporated, gives potassium ferricyanide for the second time.

The obtained potassium ferricyanide is crude in nature. It is purified by dissolving it in 3 parts of water, then the solution is filtered if necessary and evaporated to half its volume. After cooling the crystals are collected by filtration and the purity is tested by dissolving them in a solution of ferric chloride. It must produce neither a blue precipitate nor a blue colour in a solution of ferric chloride.

\(2K_4[Fe(CN)_6]+Cl_2 \longrightarrow 2K_3[Fe(CN)_6] +2KCl\)

Read More: Ferric Hydroxide Solution

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Structure of Potassium Ferricyanide

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The structure of the compound is polymeric but the ferricyanide ligand is octahedral in shape. The iron in ferricyanide is surrounded by six cyanide atoms. The ferricyanide ligand as a whole then attaches to the potassium ion, forming potassium ferricyanide. When dissolved in water, the ferricyanide ligand breaks away resulting incomplete dissociation. 

Structure of Potassium Ferricyanide

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Chemical Properties of Potassium Ferricyanide

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Here are some important chemical properties of potassium ferricyanide: 

• Chemical formula: K₃[Fe(CN)₆] or C₆FeK₃N₆
• Molecular weight: 329.24 g mol^-1
• Chemical Name: Potassium Ferricyanide, Potassium Hexacyanoferrate(III)
• Solubility: Potassium Ferricyanide is water-soluble. Gives a greenish-yellow colour when dissolved in water. It is soluble in alcohol and acidic solutions as well.

Read More: Atomic And Molecular Masses

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Physical Properties of Potassium Ferricyanide

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The important physical properties of potassium ferricyanide are:

• Odor: Potassium Ferricyanide is odourless and tastes bitter and salty.
• Appearance: Potassium Ferricyanide appears as a bright red powder or in the form of crystals. The colour can vary from bright red to dark red to orange.
• Melting Point: 300°C (572°F; 573 K)
• Boiling Point: Decompose soon on heating.
• Density: 1.89 g/cm³
• Parent Compound: Potassium Ferrocyanide K₄[Fe(CN)₆]
• Combustibility: Non-flammable

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Important Chemical Reactions

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Various chemical reactions of potassium ferricyanide are enlisted below:

• Reaction with Concentrated Sulphuric Acid

Potassium Ferricyanide reacts with concentrated hot Sulphuric Acid (H₂SO₄) and water to give potassium sulfate, iron(II) sulfate, ammonium sulfate and carbon monoxide. The reaction occurs in a boiling solution.

K₃Fe(CN)₆ + 6H₂SO₄ + 6H₂O → 2K₂SO₄ + FeSO₄ + 3(NH₄)2SO₄ + 6CO

• Reaction with Fe²⁺

Potassium ferricyanide will react with Fe⁺² solution to produce a dark blue precipitate called Prussian blue. Here, potassium ferricyanide reacts with ferrous iron (Fe⁺²) in an acidic solution to produce an insoluble blue pigment, and both the stain and the pigment are commonly referred to as Turnbull's blue.

K⁺(aq) + Fe²⁺(aq) + [Fe(CN)₆]^4-(aq) ⇌ KFe[Fe(CN)₆](s)

• Reaction with Sodium Thiosulphate

2[Fe(CN)₆]^3- + 2I^- → 2[Fe(CN)6]^4- + I₂

2[Fe(CN)₆]^4- + 2K⁺ + 3Zn²⁺ → K₂Zn₃[Fe(CN)₆]₂

2S₂O₃ + I₂ → (S₄O₆)^2- + 2I^-

Read More: Sodium Thiosulfate

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Uses and Benefits of Potassium Ferricyanide

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• During the time of reel cameras, potassium ferricyanide was used to remove silver from color negatives and positives during bleaching.
• It is used in black and white photography to reduce the density of a negative or gelatin silver print. Thus, brightening the photo.
• Potassium Ferricyanide is used to detect rust in metals as it is one of the components of Ferroxyl indicator solution with phenolphthalein. The solution on coming into contact with Fe²⁺ ions turns Prussian blue.
• It is used in the manufacturing of Murakami’s etching which helps give contrast between binder and carbide phases in cemented carbides. Potassium Ferricyanide and Potassium Hydroxide combine to form Murakami’s reagent.
• It is useful in the hardening of iron and steel.

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Safety Measures for Potassium Ferricyanide

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• Potassium Ferricyanide is usually a non-toxic compound so, no need for much protection. But prevent it from coming in contact with the eyes and nose as it can cause itching and inflammation
• But during acidic reactions, it becomes highly toxic due to the release of hydrogen cyanide gas.

Read More: Potassium Acetate

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Things to Remember

• The IUPAC name of Potassium Ferricyanide is Potassium Hexacyanoferrate(III).
• It is a bright red salt that has the chemical formula K₃Fe(CN)₆, consisting of three potassium ions and one ferricyanide ion.
• Potassium Ferricyanide is categorised as a non-toxic compound unless it is reacted with acidic solutions, where it releases an extremely toxic gas, hydrogen cyanide. If the gas is exhaled for some time it can cause dizziness, headache, shortness of breath, fainting and even death.
• Potassium Ferricyanide reacts with Fe²⁺ to give Prussian blue precipitate. With Fe³⁺ no precipitate is formed. This is a chemical reaction done in laboratories to identify the oxidation of metals.

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