Nitrite (NO2–)
Table of Contents
Nitrite is a polyatomic ion composed of one nitrogen atom covalently bonded to two oxygen atoms, and represented by the chemical formula NO2–. It belongs to the family of nitrogen oxyanions, which also includes nitrate (NO3–) and related species. [1-4]
Nitrites occur in both inorganic and organic forms. Inorganic nitrites are salts containing the NO2– ion combined with a metal or other cation (e.g., NaNO2, KNO2). Organic nitrites are esters of nitrous acid (HNO2) with the general formula RONO, where R represents an alkyl or aryl group (e.g., CH3ONO, C5H11ONO).
The nitrite ion plays a vital role in the nitrogen cycle, which sustains ecosystems by recycling nitrogen between the atmosphere, soil, and living organisms. Beyond its natural role, nitrite finds widespread applications in industry, medicine, and food preservation.
Physical Properties [1,2]
Structurally, nitrite consists of a nitrogen atom covalently bonded to two oxygen atoms. Resonance gives rise to two equivalent Lewis structures in which the negative charge is delocalized over both oxygen atoms.
The key physical properties of nitrite are summarized in the table below:
| Molecular Mass of NO2– | ~46.0 g/mol |
| Net ionic charge | –1 |
| Nature of N–O bonding | Resonance; negative charge delocalized across both oxygen atoms |
| Average N–O bond order | 1.5 |
| Molecular geometry | Bent (V-shaped) |
| O–N–O bond angle | 115°; consistent with VSEPR theory |
| Typical appearance of nitrate salts | White crystalline solids (in alkali/alkaline-earth nitrites) |
| Solubility in water | High; forms slightly basic solutions |
| Solubility in organic solvent | Very low |
Preparation
Nitrites can be synthesized in the laboratory by several methods: [1]
1. Thermal Decomposition of Nitrates
2 NaNO3 → 2 NaNO2 + O2
2. Neutralization of Nitrous Acid
Nitrous acid (HNO2) is unstable and exists only in solution. It is typically generated in situ and then neutralized with a base to form nitrite salts:
HNO2 + NaOH → NaNO2 + H2O
Chemical Reactions [1]
1. Base Behavior
Nitrite (NO2–) is the conjugate base of nitrous acid (HNO2), which is weak and unstable (pKₐ ≈ 3.16 at 25 °C). In aqueous solution, nitrite can accept a proton (H+) to form HNO2:
NO2– + H+ ⇌ HNO2
The dissociation of nitrous acid is only partial, making the equilibrium shift towards the left.
2. Redox Reaction
The nitrogen atom in nitrite has an oxidation state of +3, which allows the ion to act as both an oxidizing agent and a reducing agent depending on the reaction conditions.
- As an oxidizing agent: Nitrite can accept electrons and be reduced to lower oxidation states, such as +2 in nitric oxide (NO). For example, it oxidizes iodide ions (I–) to iodine (I2).
- As a reducing agent: Nitrite can also donate electrons and be oxidized to a higher oxidation state, +5 in nitrate (NO3–). A classic example is the reduction of permanganate (MnO4–) to Mn2+ in acidic solution.
3. Thermal Decomposition
Upon strong heating, nitrites decompose to produce nitrogen oxides (NO and NO2) along with the corresponding metal oxide. For example:
NaNO2 → Na2O + NO + NO2
However, under some conditions, sodium nitrate (NaNO3) can also form as an intermediate.
Important Nitrite Compounds [4]
| Compound | Formula | Common Uses |
|---|---|---|
| Sodium nitrite | NaNO2 | Food preservative, medicine, dye production, corrosion inhibitor |
| Potassium nitrite | KNO2 | Heat transfer salts, curing agent, organic synthesis |
| Calcium nitrite | Ca(NO2)2 | Concrete additive, antifreeze component |
| Silver nitrite | AgNO2 | Organic synthesis, preparation of nitro compounds |
| Barium nitrite | Ba(NO2)2 | Pyrotechnics, oxidizer in explosives |
Nitrite compounds serve essential functions in food preservation, medicine, construction, and industrial chemistry. Its dual significance in sustaining life and enabling practical applications makes nitrite a vital subject of study in both environmental and applied chemistry.
