1. Introduction to Nitrous Acid
Nitrous Acid (HNO₂) is a weak, unstable acid known only in solution or in the form of nitrite salts. It is a key reagent in the dye industry.
Appearance: Pale blue solution (due to dissolved N₂O₃)
Molecular Formula: HNO₂
Molar Mass: 47.01 g/mol
Oxidation State of Nitrogen: +3
2. Preparation
Because it is unstable, it is prepared in situ (on the spot) by reacting a nitrite salt with a cold mineral acid:
NaNO₂ + HCl → NaCl + HNO₂
3. Chemical Properties
a) Instability (Disproportionation)
At room temperature, nitrous acid decomposes into nitric acid and nitric oxide:
3HNO₂ → HNO₃ + 2NO + H₂O
b) Dual Role: Oxidizing & Reducing Agent
Since Nitrogen is in the +3 oxidation state (intermediate), it can be oxidized to +5 or reduced to lower states.
- As an Oxidant: 2HI + 2HNO₂ → I₂ + 2NO + 2H₂O
- As a Reductant: HNO₂ + H₂O₂ → HNO₃ + H₂O
c) Reaction with Organic Compounds (Diazotization)
This is the most industrially significant reaction. At low temperatures (0–5°C), it reacts with aromatic primary amines to form diazonium salts:
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
4. Structure
Nitrous acid exists in two isomeric forms: cis and trans. The trans form is generally more stable at room temperature.
5. Uses
- Preparation of diazonium salts for azo dyes.
- Removal of toxic sodium azide.
- In physiological studies as a vasodilator (via conversion to NO).