Description

Sodium sulphite (sodium sulfite) is an inorganic salt with the formula Na₂SO₃.

Key facts

Appearance: White, crystalline powder

Solubility: Highly soluble in water

Chemical formula: Na₂SO₃

Molar mass: ~126.04 g/mol

Common uses:

As an antioxidant in food and industrial processes

In water treatment (oxygen scavenger)

In photography (preservative in developer solutions)

In pulp and paper manufacturing (sulfite pulping process)

Properties

Reduces oxygen and prevents oxidation (good preservative)

Can release sulfur dioxide (SO₂) when heated or acidified

Acts as a mild reducing agent

Safety

Generally low toxicity but can cause:

Irritation to skin/eyes

Respiratory issues in people sensitive to sulfites

Release of SO₂ gas in acid can be hazardous

Sodium sulphite (Na₂SO₃) is an ionic compound consisting of:

2 sodium cations: Na⁺

1 sulfite anion: SO₃²⁻

Structure of the sulfite ion (SO₃²⁻)

Central atom: Sulfur (S)

Bonded to three oxygen atoms

Trigonal pyramidal geometry

Because of one lone pair on sulfur

Resonance: The negative charge is delocalized over the three oxygen atoms

O
||
O–S:
\
O⁻

(Actual structure is a resonance hybrid; S–O bonds are equivalent.)

How this looks in sodium sulphite

Sodium ions (Na⁺) are not covalently bonded to the sulfite ion.

Na⁺ ions are arranged around the SO₃²⁻ anion in the crystal lattice, held by ionic interactions.

Key points

Anion: SO₃²⁻ (trigonal pyramidal)

Cations: Na⁺ (simple spheres in lattice)

Overall formula: Na₂SO₃