Potassium Cyanide (kcn): Properties, Uses, Structures, Risks,

The potassium cyanide is an inorganic compound consisting of a potassium ion K + and cyanide ion CN . Its chemical formula is KCN. It is a white crystalline solid, extremely poisonous.

KCN is very soluble in water and when dissolved it hydrolyzes to form hydrocyanic acid or HCN hydrogen cyanide, which is also very poisonous. Potassium cyanide can form compound salts with gold and silver, which is why it was formerly used to extract these precious metals from certain minerals.

KCN is used to coat cheap metals with gold and silver through an electrochemical process, that is, a method in which an electric current is passed through a solution containing a salt made up of the precious metal, cyanide, and potassium.

Potassium cyanide, because it contains cyanide, must be handled with great care, with suitable implements. It should never be disposed of into the environment, because it is also very toxic to most animals and plants.

However, methods using common algae to remove potassium cyanide from waters contaminated with low concentrations of it are being studied.


KCN is an ionic compound made up of a K + potassium cation and a CN cyanide anion . In this the carbon atom is attached to the nitrogen atom by a triple covalent bond.

In solid potassium cyanide, the CN anion can rotate freely so it behaves as a spherical anion, as a consequence the KCN crystal has a cubic structure similar to that of potassium chloride KCl.


– Potassium cyanide

– Potassium cyanide

– Cyanopotassium


Physical state

White crystalline solid. Cubic crystals.

Molecular weight

65.116 g / mol.

Melting point

634.5 ° C

Boiling point

1625 ° C.


1.55 g / cm 3 at 20 ° C.


Very soluble in water: 716 g / L at 25 ° C and 100 g / 100 mL of water at 80 ° C. Slightly soluble in methanol: 4.91 g / 100 g of methanol at 19.5 ° C. Very slightly soluble in ethanol: 0.57 g / 100 g of ethanol at 19.5 ° C.


An aqueous solution of 6.5 g of KCN in 1 L of water has a pH of 11.0.

Hydrolysis constant

KCN is very soluble in water. When it dissolves, the cyanide ion CN which takes a proton H + from the water to form hydrocyanic acid HCN and liberates an OH ion :

CN + H 2 O → HCN + OH

The hydrolysis constant indicates the tendency with which said reaction is carried out.

K h = 2.54 x 10 -5

Aqueous KCN solutions release HCN hydrogen cyanide into the environment when heated above 80 ° C.

Chemical properties

It is not flammable, but when solid KCN is heated to decomposition it emits very toxic gases of hydrogen cyanide HCN, nitrogen oxides NO x , potassium oxide K 2 O and carbon monoxide CO.

KCN reacts with gold salts to form potassium aurocyanide KAu (CN) 2 and potassium aurocyanide KAu (CN) 4 . These are colorless complex salts. With the silver metal Ag, KCN forms the potassium argentocyanide KAg (CN) 2 .

The cyanide ion of KCN reacts with certain organic compounds that contain halogens (such as chlorine or bromine) and takes their place. For example, it reacts with bromoacetic acid to give cyanoacetic acid.

Other properties

It is hygroscopic, it absorbs humidity from the environment.

It has a mild bitter almond odor, but this is not detected by all people.


KCN is prepared by reacting KOH potassium hydroxide in aqueous solution with HCN hydrogen cyanide. It is also obtained by heating potassium ferrocyanide K 4 Fe (CN) 6 :

K 4 Fe (CN) 6 → 4 KCN + 2 C + N 2 ↑ + Fe

Use in the electroplating of metals

It is used in the process of coating low-value metals with gold and silver. It is an electrolytic process, that is, electricity is passed through an aqueous solution with appropriate salts.


Potassium argentocyanide KAg (CN) 2 is used to coat cheaper metals with silver (Ag).

These are placed in an aqueous solution of potassium argentocyanide KAg (CN) 2 , where the anode or positive pole is a bar of pure silver (Ag) and the cathode or negative pole is the cheap metal that you want to coat with silver.

As an electric current passes through the solution, the silver is deposited on the other metal. When cyanide salts are used, the silver layer is deposited in a finer, more compact and adherent way than in solutions of other compounds.


Similarly in the case of gold (Au), potassium aurocyanide KAu (CN) 2 and potassium aurocyanide KAu (CN) 4 are used to electrolytically gild other metals.

Other uses

Here are some other uses for potassium cyanide.

– For the industrial process of hardening steel by nitriding (addition of nitrogen).

– For cleaning metals.

– In printing and photography processes.

– Formerly it was used for the extraction of gold and silver from the minerals that contain them, but later it was replaced by sodium cyanide NaCN, which is less expensive, although equally toxic.

– As an insecticide for the fumigation of trees, ships, railroad cars and warehouses.

– As a reagent in analytical chemistry, that is, to do chemical analysis.

– To prepare other chemical compounds, such as colorants and dyes.


KCN is a very poisonous compound for animals and most plants and microorganisms. It is classified as super toxic. It is lethal even in very small quantities.

Its harmful effect can occur by inhalation, contact with the skin or eyes, or ingestion. It inhibits many metabolic processes, especially blood proteins that are involved in oxygen transport such as hemoglobin.

It affects the organs or systems that are most sensitive to oxygen deprivation, such as the central nervous system (brain), the cardiovascular system (heart and blood vessels), and the lungs.

Mechanism of action

KCN interferes with the body’s ability to use oxygen.

The cyanide ion CN  of KCN has a high affinity for the ferric ion Fe 3+ , which means that when cyanide is absorbed it reacts rapidly with the Fe 3+ in blood and tissues.

In this way, it prevents the cells from breathing, which go into a state of lack of oxygen, because although they try to breathe, they cannot use it.

Then there is a transitory state of hyperapnea (suspension of breathing) and headache, and finally death from respiratory arrest.

Additional risks

When heated, it produces highly toxic gases such as HCN, nitrogen oxides NO x , potassium oxide K 2 O and carbon monoxide CO.

When it comes into contact with moisture, it releases HCN which is highly flammable and very toxic.

KCN is very poisonous also to aquatic organisms. It should never be disposed of into the environment, as contamination of waters where animals drink and fish inhabit can occur.

However, there are bacteria that produce cyanide such as Chromobacterium violaceum and some species of Pseudomonas .

Recent studies

Researchers found that the green algae Chlorella vulgaris can be used to treat water contaminated with KCN potassium cyanide in low concentrations.

The alga was able to efficiently remove the KCN, as this in low amounts stimulated the growth of the algae since it activated an internal mechanism to resist the toxicity of the KCN.

This means that the algae Chlorella vulgaris has the potential to remove cyanide and that an effective method for the biological treatment of cyanide contamination could be designed with this.


  1. US National Library of Medicine. (2019). Potassium cyanide. National Center for Biotechnology Information. Recovered from pubchem.ncbi.nlm.nih.gov.
  2. Coppock, RW (2009). Threats to Wildlife by Chemical Warfare Agents. In Handbook of Toxicology of Chemical Warfare Agents. Recovered from sciencedirect.com.
  3. Liu, Q. (2017). Evaluation of the Removal of Potassium Cyanide and its Toxicity in Green Algae ( Chlorella vulgaris ). Bull Environ Contam Toxicol. 2018; 100 (2): 228-233. Recovered from ncbi.nlm.nih.gov.
  4. The National Institute for Occupational Safety and Health (NIOSH). (2011). Potassium Cyanide: Systemic Agent. Recovered from cdc.gov.
  5. Alvarado, LJ et al. (2014). Riboswitch Discovery, Structure and Function. Synthesis of Uracil. In Methods in Enzymology. Recovered from sciencedirect.com.

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