Sodium Hydrosulfite, also known by its scientific name sodium dithionite (Na₂S₂O₄), is a crystalline compound ranging in color from white to pale yellow. It dissolves well in water and is also soluble in alcohols and ethers. Due to its unique chemical properties especially its role as a powerful reducing agent it is widely used in industries such as textile, paper production, and chemical manufacturing. This compound remains stable in its dry state without exposure to air, but rapidly decomposes in the presence of hot water or acidic environments. When exposed to moisture, it undergoes an exothermic reaction.

 

Physical and Chemical Properties of Sodium Hydrosulfite

Chemical Formula: Na₂S₂O₄

Molecular Weight: 56.063 g/mol

Density: 1.79 g/cm³

Melting Point: \~17.2°C

Boiling Point: \~100°C

Key Feature: Very strong reducing agent

Stability: Stable in dry conditions, unstable in the presence of air and moisture

 

 

Production Methods of Sodium Hydrosulfite

 

Reduction of Sodium Bisulfite with Zinc

In this method, sodium bisulfite reacts with sulfurous acid and zinc powder to produce sodium dithionite:

2NaHSO₃ + H₂SO₃ + Zn → ZnSO₃ + Na₂S₂O₄ + 2H₂O

To purify the product, limewater is used to neutralize excess acids, causing ZnSO₃ to precipitate. The product is then separated using sodium chloride and dried with alcohol.

 

Reaction of Formaldehyde, Zinc, and Sulfur Dioxide

A suspension of zinc powder and formaldehyde reacts with sulfur dioxide at 80°C. The final product, pure sodium dithionite, is obtained after vacuum separation.

 

Alternative Methods

Reaction of sodium sulfite with sodium amalgam

* Use of sodium formate with SO₂ and NaOH:

HCOONa + 2SO₂ + NaOH → Na₂S₂O₄ + CO₂ + H₂O

 

 

Applications of Sodium Hydrosulfite

Textiles and Printing: A powerful reducing agent used to remove dyes from fabrics and to fix colors during printing.

Paper and Wood Industry: Used as a bleaching agent in the processing of wood pulp.

Food Industry: Acts as a decolorizing agent in processes like sugar and dried fruit production.

Metal Extraction: Especially useful in the extraction of copper from ore.

Cosmetics: Used as a sulfur-containing component in certain formulations.

Leather Processing: Applied in the removal of hair from hides in the tanning industry.

Pharmaceuticals: Used as a reducing agent in the synthesis of certain drugs.

 

 

Conclusion

Sodium hydrosulfite plays a vital role in various industries due to its strong reducing properties, especially in textile and paper manufacturing. However, its production requires high precision, specialized equipment, and significant costs, which can impact overall profitability. Safe and specialized usage of this chemical is essential for efficiency and sustainability in industrial applications.