POTASSIUM FERROCYANIDE: PROPERTIES, RISKS AND USES - CHEMISTRY - 2025

The potassium ferrocyanide, also known as hexacyanoferrate (II) potassium or yellow Prussia, is an inorganic compound of formula K ₄. It is a potassium salt of the ferrocyanide coordination complex (Fe (CN) ₆ ^4-) and is generally found in the trihydrate form K ₄ · H ₂ O. Its structure is shown in figure 1 (EMBL-EBI, 2008).

Historically, it was made from organically sourced carbon dioxide sources, iron filings, and potassium carbonate. Common sources of nitrogen and carbon were leather scrap, offal, or dried blood.

Figure 1: structure of potassium ferrocyanide.

Potassium ferrocyanide is currently produced industrially from hydrogen cyanide, ferrous chloride and calcium hydroxide, the combination of which gives Ca ₂ · 11H ₂ O.

This solution is then treated with potassium salts to precipitate the mixed calcium-potassium CaK ₂ salt, which in turn is treated with potassium carbonate to give the tetrapotassium salt.

Physical and chemical properties

Potassium ferrocyanide is a monoclinic crystal of yellow or pale yellow color, depending on the level of hydration, without a characteristic aroma (National Center for Biotechnology Information., 2017). Its appearance is shown in figure 2 (Potassium ferrocyanide, 2017).

The compound has a molecular weight of 368.35 g / mol for the anhydrous form and 422.388 g / mol for the trihydrate form. It has a density of 1.85 g / ml and a melting point of 70 ° C where it begins to decompose (Royal Society of Chemistry, 2015).

The compound is soluble in water, being able to dissolve 28.9 grams of compound in 100 ml of this solvent. It is insoluble in diethyl ether, ethanol, and toluene. The compound reacts to heat to form potassium cyanide according to the reaction:

K ₄ → 4 KCN + FeC ₂ + N ₂

In combination with concentrated acids, it forms hydrogen cyanide (HCN) which is a highly poisonous, flammable gas and can form explosive mixtures in air (Laffort, 2001).

Reactivity and hazards

Potassium ferrocyanide is a stable compound, incompatible with strong acids and oxidizing agents. Potassium ferrocyanide is non-toxic and does not break down into cyanide in the body. Toxicity in rats is low, with a lethal dose (LD50) at 6400 mg / kg.

The substance is toxic to the lungs and mucous membranes. The compound can cause irritation in case of contact with skin and eyes.

When ingested it can cause irritation of the gastrointestinal tract and in case of inhalation it causes irritation of the nasal mucosa and respiratory system.

Contact with the eyes can result in corneal damage or blindness. Contact with the skin can cause inflammation and blisters.

Inhalation of dust will produce irritation of the gastrointestinal or respiratory tract, characterized by burning, sneezing and coughing. Severe overexposure can cause lung damage, suffocation, unconsciousness, or death.

Eye contact

In case of contact with the eyes, the contact lenses should be checked and removed. Eyes should be flushed immediately with plenty of cold water for at least 15 minutes.

Skin contact

The affected area should be rinsed immediately with plenty of water for at least 15 minutes while removing contaminated clothing and shoes. Cover irritated skin with an emollient.

Wash clothing and shoes before reuse. If the contact is severe, wash with a disinfectant soap and cover the contaminated skin with an antibacterial cream.

Inhalation

The victim should be moved to a cool place. If it is not breathing, artificial respiration should be given. If breathing is difficult, give oxygen.

Ingestion

If the compound is ingested, vomiting should not be induced unless directed by medical personnel. Loosen tight clothing such as a shirt collar, belt, or tie.

In all cases, medical attention should be obtained immediately (Material Safety Data Sheet Potassium ferrocyanide trihydrate, 2013).

Applications

1- Food industry

Prussian yellow is also known for its approved European food additive number: E536 and has various uses in the food industry. It is used as an anticaking agent for both road salt and table salt.

It also has an oenological use to remove copper and iron in winemaking. Copper is used as fungicides in grapes (Wageningen University, 2014).

It is also used for the fermentation of citric acid (DS Clark, 1965). Potassium ferrocyanide can also be used as a fertilizer for plants.

2- Mining and metallurgy industry

Potassium ferrocyanide is used to remove copper from molybdenum minerals. It is also used for the cementation of steel (Potassium ferrocyanide, K4Fe (CN) 6, 2012).

Cementation is a thermochemical treatment applied to steel parts. The process brings carbon to the surface through diffusion, which is impregnated by modifying its composition.

3- Laboratory reagent

Potassium ferrocyanide is used to determine the concentration of potassium permanganate, a compound often used in titrations based on redox reactions.

Potassium ferrocyanide is used in a mixture with potassium ferricyanide and phosphate buffered solution to provide a buffer for beta-galactosidase, which is used to cleave X-Gal (5-bromo-4-chloro-3-indolyl-β -D- galactopyranoside, giving a bright blue visualization where an antibody (or other molecule), conjugated to Beta-gal, has bound to its target.

The compound is also used for the production of Prussian blue. When reacting with Fe (III) it gives a Prussian blue color, which is why it is used as an identification reagent for iron in laboratories.

It is also used for the determination of zinc in zinc sulfide samples. The sample is dissolved in 6N HCl and boiled under a hood to remove hydrogen sulfide, neutralized with ammonium hydroxide and 3 drops of HCl are added.

It is heated to boiling and 5 ml of 1 N potassium ferrocyanide solution are added. The presence of a white zinc ferrocyanide precipitate indicates a positive test for this element (Mehlig, 1927).

References

1. S. Clark, KI (1965). Effect of potassium ferrocyanide on the chemical composition of molasses mash used in the citric acid fermentation. Biotechnology and bioengineering Volume 7, Issue 2, 269–278. Recovered from onlinelibrary.wiley.com.
2. EMBL-EBI. (2008, January 16). potassium hexacyanoferrate (4−). Recovered from ebi.ac.uk.
3. (2001, March 5). POTASSIUM FERROCYANURE. Recovered from laffort.com.
4. Material Safety Data Sheet Potassium ferrocyanide trihydrate. (2013, May 21). Recovered from sciencelab.com.
5. Mehlig, JP (1927). Use of potassium ferrocyanide as a confirmatory test for zinc. Chem. Educ. 4 (6), 722. Recovered from pubs.acs.org.
6. National Center for Biotechnology Information.. (2017, April 15). PubChem Compound Database; CID = 161067. Recovered from pubchem.ncbi.nlm.nih.gov.
7. Potassium ferrocyanide. (2017, March 25). Recovered from sciencemadness.org.
8. Potassium ferrocyanide, K4Fe (CN) 6. (2012). Recovered from atomistry.com.
9. Royal Society of Chemistry. (2015). potassium ferrocyanide. Recovered from chemspider.com.
10. Wageningen University. (2014, August 14). E536: Potassium ferrocyanide. Recovered from food-info.net.