What is Sodium Metabisulfite? Properties, Uses & Safety Guide

Sodium metabisulfite (Na₂S₂O₅) is a widely used inorganic compound with strong reducing and preservative properties. It plays a critical role in industries such as food processing, water treatment, mining, and chemical manufacturing.

This guide provides a comprehensive, data-driven overview of sodium metabisulfite, covering its chemical properties, industrial applications, safety protocols, and purchasing considerations.

1. Authoritative Definition of Sodium Metabisulfite

Chemical Formula & Molecular Structure

Sodium metabisulfite consists of two sodium (Na) atoms, two sulfur (S) atoms, and five oxygen (O) atoms, forming Na₂S₂O₅.

Its structure is derived from the dimerization of sodium bisulfite (NaHSO₃):

2 NaHSO₃ → Na₂S₂O₅ + H₂O

(Source: National Center for Biotechnology Information, PubChem CID 656671)

International Naming Standards

IUPAC Name: Sodium metabisulfite
CAS No.: 7681-57-4
Other Names: Sodium pyrosulfite, Disodium disulfite

Historical Background

First synthesized in the early 19th century, sodium metabisulfite gained industrial prominence due to its strong reducing and antimicrobial properties. By the 1930s, it became a key preservative in food and beverage industries, later expanding into water treatment and mining applications.

2. Key Properties of Sodium Metabisulfite

Physical Properties (Comparative Table)

Property	Value
Appearance	White/yellowish crystalline powder
Solubility (20°C)	54 g/100 mL water
Melting Point	Decomposes at 150°C
Density	1.48 g/cm³
pH (1% solution)	4.0–5.5

(Data sourced from Ullmann's Encyclopedia of Industrial Chemistry)

Chemical Properties

Strong Reducing Agent: Reacts with oxygen to prevent oxidation.

Na₂S₂O₅ + O₂ → Na₂SO₄ + SO₂

Acid-Base Reactions: Releases sulfur dioxide (SO₂) in acidic conditions, crucial for food preservation.

Na₂S₂O₅ + 2HCl → 2NaCl + 2SO₂ + H₂O

Comparison with Similar Compounds

Feature	Sodium Metabisulfite	Sodium Sulfite
Formula	Na₂S₂O₅	Na₂SO₃
SO₂ Content	~67%	~50%
Solubility	Higher	Lower
Primary Use	Food preservative, water treatment	Paper bleaching

3. Industrial Applications of Sodium Metabisulfite

Food Additive (E223)

Preservative: Inhibits microbial growth in wine, dried fruits, and seafood.
Bleaching Agent: Used in sugar refining to remove impurities.
Regulatory Compliance: FDA (21 CFR 182.3766) and EU (E223) approved.

Water Treatment

Dechlorination: Neutralizes chlorine in wastewater.

Na₂S₂O₅ + 2Cl₂ + 3H₂O → 2NaHSO₄ + 4HCl

Heavy Metal Removal: Precipitates arsenic and chromium in mining effluents.

Mining & Petroleum Industry

Gold Extraction: Enhances cyanide leaching efficiency.
Oil Refining: Reduces hydrogen sulfide (H₂S) in crude oil.

Other Uses

Textile Industry: Removes excess dyes.
Photography: Acts as a fixing agent in film development.

4. Safety & Handling Guidelines

Storage Conditions

Temperature: Below 25°C (OSHA Standard 1910.1200).
Humidity: <50% to prevent caking.

Personal Protective Equipment (PPE)

Gloves: Nitrile or neoprene.
Respirator: Required if handling powder in confined spaces.

Emergency Procedures

Spill Response: Neutralize with sodium carbonate, then rinse with water.
Eye Contact: Flush with water for 15 minutes (per MSDS guidelines).

5. Buying Guide for Industrial Buyers

Industrial vs. Food Grade (Key Differences)

Parameter	Industrial Grade	Food Grade
Purity	≥96%	≥98%
Heavy Metals	<10 ppm	<3 ppm
SO₂ Content	64–67%	65–67%

Packaging Options

25 kg bags (most common).
1000 kg IBC totes (bulk buyers).

Supplier Verification Checklist

✔ ISO 9001 Certification
✔ Third-party lab test reports
✔ SDS (Safety Data Sheet) availability