What are the physical properties of ammonium sulfite？

Question

Accepted Answer

Ammonium sulfite is an inorganic salt with the chemical formula (NH₄)₂SO₃. Under normal temperature and pressure, it typically exists as a monohydrate (NH₄)₂SO₃·H₂O.

Here are the detailed physical properties:

1. Shape and appearance

It is usually a white or colorless crystalline solid. When it exists in the form of monohydrate, it is a monoclinic crystal.

2. Smell

It has a slight ammonia smell. It may smell irritating because it is unstable and tends to decompose and release ammonia and sulfur dioxide.

3. Solubility

It is soluble in water, which is one of the most important physical properties.

The solubility of the compound in water increased significantly with the increase of temperature.

It is slightly soluble in organic solvents such as ethanol.

Specific solubility data (in monohydrate):

0°C: approximately 32 g / 100 mL of water

100°C: approximately 60.4 g per 100 mL of water

4. Density

The density of ammonium sulfite monohydrate is about 1.41 g/cm3.

5. Stability and Decomposition

Thermal instability: This is a key physicochemical property. Ammonium sulfite is unstable when heated and decomposes.

At about 60-70°C, the hydrate loses its water of crystallization and becomes anhydrous ammonium sulfite.

At higher temperatures (typically above 150°C), it rapidly decomposes, releasing sulfur dioxide (SO₂), ammonia (NH₃), nitrogen (N₂), and water.

Air instability: In the air, particularly in humid conditions, it gradually oxidizes into ammonium sulfate ((NH₄)₂SO₄).

6. Melting point

Ammonium sulfite lacks a defined melting point as it decomposes before reaching it, a classic example of a 'decomposes before melting' substance.

7. Tidal dissolving

It has a certain hygroscopicity and can absorb moisture from the air.