Sodium metabisulfite (Na₂S₂O₅) is commonly used as a bleaching agent in various industrial applications. Here’s a detailed overview of its use and properties:

Properties of Sodium Metabisulfite

Chemical Formula: Na₂S₂O₅

Appearance: White or yellowish crystalline powder

Odor: Slight sulfur odor

Solubility: Soluble in water

Uses as a Bleaching Agent

Textile Industry:

Bleaching Wool: Sodium metabisulfite is used to bleach wool, removing natural pigments and impurities. It helps to achieve a uniform, bright appearance.

Fabric Preparation: In cotton and other textiles, it acts as a reducing agent to bleach fabric without the use of harsh chlorine-based bleaches.

Paper and Pulp Industry:

Pulp Bleaching: It is employed in the bleaching of pulp during paper manufacturing. It helps in reducing residual lignin, thus enhancing the whiteness and brightness of the paper.

Color Removal: Used for decolorizing recycled paper, removing inks and other colored impurities.

Food Industry:

Preservation and Bleaching: It is used to preserve the color and appearance of various foods. For instance, it can bleach the natural pigments in certain fruits and vegetables, helping to maintain a consistent and appealing look.

Sodium Metabisulfite Used As Bleaching Agent

Mechanism of Bleaching

Reduction Reaction: Sodium metabisulfite acts as a reducing agent. It releases sulfur dioxide (SO₂) when dissolved in water. The sulfur dioxide then interacts with the pigments or color-causing agents, reducing them to colorless or less-colored compounds.

Sulfurous Acid Formation: Upon dissolving, it forms sulfurous acid (H₂SO₃), which further aids in the bleaching process through reduction and oxidation reactions.

Safety and Handling

Health Hazards: Sodium metabisulfite can cause irritation to the skin, eyes, and respiratory system. Proper handling and personal protective equipment (PPE) are necessary.

Storage: It should be stored in a cool, dry place, away from moisture and acids, to prevent the release of sulfur dioxide gas.