POTASSIUM NITRITE (KNO2): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The potassium nitrite is an inorganic salt having the chemical formula KNO ₂, which is chemically and pharmacologically related to the potassium nitrate KNO ₃. Its physical appearance consists of yellowish white crystals, highly hygroscopic, and therefore deliquescent; that is, they dissolve quickly in humid environments.

Its formula indicates that the ratio of the K ⁺ and NO ₂ ^- ions is 1: 1, and they remain united by electrostatic forces or by ionic bonds. No pure natural sources have apparently been found for its crystals, although nitrite anions can be found in soils, fertilizers, plants, and animals.

Potassium nitrite crystals. Source: Leiem

The image above shows what KNO ₂ crystals look like, with pronounced yellow hues. If these crystals are left in contact with the air, they will absorb moisture until they become an aqueous solution; solution that has generated controversies regarding whether or not its use for medical purposes is beneficial.

On the other hand, its crystals, in very small amounts (200 ppm), are used to salinize meats and guarantee their preservation against bacterial action. Likewise, KNO ₂ improves the color of meats, making them more reddish; however, it is subject to several restrictions to avoid the toxic effects of this salt in the body.

Structure of potassium nitrite

Ions that make up the KNO2 represented with a model of spheres and bars. Source: MarinaVladivostok.

The ions present in potassium nitrite are shown above. The K ⁺ cation corresponds to the purple sphere, while the NO ₂ ^- anion is represented by the bluish and red spheres.

The anion NO ₂ ^- is shown with one double and one single bond ^-; but in reality, both bonds are equal product of the resonance of the negative charge between them.

The K ⁺ and NO ₂ ^- ions attract each other in space until they organize a structural pattern with the least energy; this is where the repulsions between equal charges are minimal. And so they create KNO ₂ crystals, whose unit cell is susceptible to temperature changes, which are phase transitions.

For example, at low temperatures (less than 25 ° C), KNO ₂ crystals adopt a monoclinic system (phase I). When the temperature exceeds 25 ° C, a phase transition from monoclinic to rhombohedral (phase II) occurs. Finally, above 40 ° C, the KNO ₂ crystals change to be cubic (phase III).

Also, KNO ₂ can exhibit other crystalline phases (phases IV, V and VI) under high pressure. With this, the K ⁺ and NO ₂ ^- ions end up moving and ordering in different ways in their pure crystals.

Properties

Molecular mass

85.1038 g / mol.

Density

1.9150 g / mL.

Melting point

440.02 ° C (but begins to decompose from 350 ° C, emitting toxic fumes).

Boiling point

537 ° C (explodes).

Water solubility

312 g / 100 g water at 25 ° C.

Deliquescence

Its solubility in water is such that it is hygroscopic; so much so that it exhibits deliquescence, absorbing enough moisture to dissolve. This affinity for water may be due to the energetic stability that K ⁺ ions gain when hydrated, as well as a low enthalpy of the crystal lattice for KNO ₂ crystals.

The crystals can absorb water without dissolving to become a hydrate, KNO ₂ · H ₂ O. In the hydrate the water molecule is found accompanying the ions, which modifies the crystalline structure.

This hydrate (or several of them), can be formed below -9 ° C; at higher temperatures, the water dissolves and hydrates the ions, deforming the crystal.

Solubility in other solvents

Slightly soluble in hot alcohols, and very soluble in ammonia.

pH

6-9. Its aqueous solutions are therefore alkaline, since the NO ₂ ^- anion can be hydrolyzed.

Nomenclature

KNO ₂ may also be named in other ways. 'Potassium nitrite' corresponds to the name for this salt according to the stock nomenclature; 'potassium nitrite', according to the systematic nomenclature, in which the only valence of potassium is highlighted, +1; and potassium dioxonitrate (III), according to the systematic nomenclature.

The name 'potassium dioxonitrate (III)' highlights the +3 valence of the nitrogen atom. Although it is the most recommended name by the IUPAC for KNO ₂, 'potassium nitrite' continues to be the most convenient and the easiest to remember.

Obtaining

The most direct way to synthesize it, but with a lower yield, is through the thermal decomposition of potassium nitrate or saltpeter at 400 ° C or more:

2KNO ₃ => KNO ₂ + O ₂

However, part of the KNO ₂ ends up being decomposed by heat, in addition to other products being formed.

Another method to prepare or synthesize it with a higher yield is by reducing the KNO ₃ in the presence of lead, copper or zinc. The equation for this reaction is as follows:

KNO ₃ + Pb => KNO ₂ + PbO

Potassium nitrate and lead are stoichiometrically mixed in an iron skillet, where they are melted with constant stirring and heating for half an hour. Lead (II) oxide is yellow in color, and the resulting mass is pulverized hot and treated with boiling water. Then the hot mixture is filtered.

The hot filtrate is bubbled with carbon dioxide for five minutes, whereupon insoluble lead carbonate, PbCO ₃, will precipitate. In this way, the lead is separated from the filtrate. Dilute nitric acid is added to the filtrate until the pH is neutral, it is allowed to cool, and finally the water is evaporated so that the KNO ₂ crystals are formed.

Applications

Additive and reagent

Potassium nitrite is used as an additive to cure red meat, maintaining its flavor and color for longer during storage, while delaying the action of bacteria and certain toxins, such as botulinum. Therefore, it exhibits antibacterial action.

KNO ₂ is oxidized to NO, which reacts with myoglobin in meat and, consequently, ends up changing its natural red color. Later, when the meat is cooked it acquires its characteristic strong pink color.

However, under non-specific conditions, KNO ₂ reacts with meat proteins to give rise to nitrosamines, which can become carcinogenic.

On the other hand, KNO ₂ (although preferably NaNO ₂) is an analytical reagent that can be used in the synthesis of azo dyes (the reaction of nitrous acid with aromatic amines), and in the analysis of amino acids.

Antidote

Although it has its negative effects, KNO ₂ acts as an antidote in patients poisoned with cyanides and hydrogen sulfide. Its mechanism consists of oxidizing the Fe ²⁺ to Fe ³⁺ centers of the haemoglobin groups, producing methemoglobin, which then reacts with the CN ^- and HS ^- anions.

Doctors

In the gastric juice of the stomach the anion NO ₂ ^- is reduced to NO, which is known to have a vasodilator action, increasing blood flow. In other regions of the body where the pH is not acidic enough, some enzymes, such as xanthine oxidoreductase, are responsible for reducing NO ₂ ^-.

KNO ₂ has been used to treat ailments and diseases such as angina pectoris and epilepsy (with very negative side effects).

References

1. Wikipedia. (2019). Potassium nitrite. Recovered from: en.wikipedia.org
2. PrebChem. (2016). Preparation of potassium nitrite. Recovered from: prepchem.com
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4. PubChem. (2019). Potassium nitrite. Recovered from: pubchem.ncbi.nlm.nih.gov
5. Chemical Formulation. (2018). Potassium nitrite. Recovered from: formulacionquimica.com
6. National Center for Advancing Translational Sciences. (2011). Potassium nitrite. Recovered from: drugs.ncats.io
7. Richard J. Epley, Paul B. Addis, and Joseph J. Warthesen. (1992). Nitrite in Meat. University of Minnesota.
8. NR Rao, B. Prakash, and M. Natarajan. (1975). Crystal Structure Transformations in Inorganic Nitrites, Nitrates, and Carbonates. Department of Chemistry, Indian Institute of Technology, Kanpur, India.