POTASSIUM CYANIDE (KCN): PROPERTIES, USES, STRUCTURES, RISKS, - CHEMISTRY - 2025

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.

Solid KCN potassium cyanide. morienus (uploaded by de: Benutzer: BXXXD from de: wiki). Source: Wikimedia Commons.

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 composed 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 that use common algae to remove potassium cyanide from waters contaminated with low concentrations of it are being studied.

Structure

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.

Chemical structure of KCN potassium cyanide. Capaccio. Source: Wikimedia Commons.

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.

KCN crystal structure. Benjah-bmm27. Source: Wikimedia Commons.

Nomenclature

- Potassium cyanide

- Potassium cyanide

- Cyanopotassium

Properties

Physical state

White crystalline solid. Cubic crystals.

Molecular weight

65.116 g / mol.

Melting point

634.5 ° C

Boiling point

1625 ° C.

Density

1.55 g / cm ³ at 20 ° C.

Solubility

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.

pH

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 water to form hydrocyanic acid HCN and liberates an OH ^- ion remains:

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 forming 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 have 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.

Obtaining

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 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.

Silver

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.

Some jewelry items are plated with silver using KCN salts. Author: StockSnap. Source: Pixabay.

Gold

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.

Gold-plated electrical connectors possibly using KCN salts. Cjp24. Source: Wikimedia Commons.

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, boats, 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.

Gold mining in South Africa in 1903 using KCN resulting in deadly pollution of the surrounding environment. Argyll, John Douglas Sutherland Campbell, Duke of, 1845-1914; Creswicke, Louis. Source: Wikimedia Commons.

Risks

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.

Potassium cyanide is a poison. Author: Clker-Free-Vector-Images. Source: Pixabay.

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 enter a state of lack of oxygen, because although they try to breathe they cannot use it.

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

Additional risks

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

When in contact with moisture it releases HCN which is highly flammable and very toxic.

KCN is very poisonous to aquatic organisms as well. 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 have 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, since 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 this could be an effective method for the biological treatment of cyanide contamination.

Image of the alga Chlorella vulgaris observed under a microscope. ja: User: NEON / User: NEON_ja. Source: Wikimedia Commons.

References

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.