Sodium Metabisulfite, Disodium Disulfite, Sodium Pyrosulfite, Natrium Pyrosulfite, SMBS, E223, 7681-57-4

SODIUM METABISULFITE (SMBS, E223)

1. Chemical Identity and Material Classification

• Chemical Name: Sodium Metabisulfite, Disodium Disulfite, Sodium Pyrosulfite

• Synonyms: SMBS, Sodium Bisulfite Anhydrous (misnomer – actually metabisulfite), Disodium Pyrosulfite

• CAS Number: 7681-57-4

• EC Number (EINECS): 231-673-0

• Molecular Formula: Na₂S₂O₅

• Molecular Weight: 190.11 g/mol

• E Number: E223 (food additive)

• Chemical Class: Inorganic sulfite salt (reducing agent)

• HS Code: 2832.10

• UN Number: Not regulated (for solid); 1759 (corrosive solid, n.o.s. – for solution, depending on concentration)

2. Physical Properties

2.1 General Physical Properties

2.2 Solubility in Water

2.3 Solubility in Other Solvents

2.4 Aqueous Solution Properties

3. Chemical Properties

3.1 Molecular Structure

Structure: NaO–S(=O)–S(=O)–ONa (sodium salt of disulfurous acid)

• Contains two sulfur atoms in +4 oxidation state

• Dissolves in water to form sodium bisulfite (NaHSO₃): Na₂S₂O₅ + H₂O → 2NaHSO₃

• Strong reducing agent (reduces Fe³⁺, Cr⁶⁺, I₂, O₂)

• Releases sulfur dioxide (SO₂) upon acidification

3.2 Solution Equilibria

Na₂S₂O₅ + H₂O ⇌ 2NaHSO₃ ⇌ 2Na⁺ + 2HSO₃⁻

HSO₃⁻ ⇌ H⁺ + SO₃²⁻ (pKa₂ ≈ 7.2)

HSO₃⁻ + H⁺ ⇌ H₂O + SO₂↑ (gas evolution)

3.3 Chemical Reactivity

3.4 Reducing Agent Properties

3.5 Stability and Decomposition

3.6 Incompatibilities

4. Quality Specifications

4.1 Food Grade (FCC / E223)

4.2 Technical / Industrial Grade

5. Production Methods

5.1 Industrial Process (SO₂ Absorption)

Process:

1. Sulfur is burned to produce sulfur dioxide (SO₂)

2. SO₂ gas is passed through a solution of sodium carbonate (Na₂CO₃) or sodium sulfite (Na₂SO₃)

3. Sodium bisulfite (NaHSO₃) is formed

4. Crystallization yields sodium metabisulfite (Na₂S₂O₅)

Reactions:

• Na₂CO₃ + 2SO₂ + H₂O → 2NaHSO₃ + CO₂

• 2NaHSO₃ → Na₂S₂O₅ + H₂O

5.2 From Sodium Sulfite

Reaction: 2Na₂SO₃ + 2SO₂ + H₂O → Na₂S₂O₅ + 2NaHSO₃ (complex)

6. Mechanism of Action

6.1 Antimicrobial Mechanism (Food Preservation)

• SO₂ (released from metabisulfite) penetrates microbial cell membranes

• Reacts with disulfide bonds in proteins and cofactors

• Disrupts enzyme function (especially in bacteria, yeasts, molds)

• Inhibits microbial growth and reproduction

6.2 Antioxidant Mechanism

• Sodium metabisulfite acts as a reducing agent

• Scavenges oxygen (O₂)

• Prevents oxidation of sensitive compounds (vitamins, flavors, colors)

• Synergistic with other antioxidants

6.3 Antichlor Mechanism (Textile Bleaching)

Na₂S₂O₅ + Cl₂ + H₂O → Na₂SO₄ + 2HCl

• Neutralizes residual chlorine after bleaching

• Prevents fiber damage from chlorine

7. Industrial Applications

7.1 Food Industry (E223) – Largest Application

Food additive status: E223 – permitted as food preservative and antioxidant

Regulatory limits (EU, US, Codex):

• Wine: max 200–400 mg/L total SO₂ (varies by wine type)

• Dried fruits: max 500–2,000 mg/kg (depending on fruit)

• Most foods: max 50–300 mg/kg

7.2 Textile Industry

7.3 Leather Industry

7.4 Paper and Pulp Industry

7.5 Chemical Industry

7.6 Water Treatment

7.7 Cosmetics and Personal Care

7.8 Photography

7.9 Mining and Metallurgy

7.10 Rubber and Plastics

8. Toxicology and Safety

8.1 Acute Toxicity

8.2 Chronic Toxicity

8.3 Special Hazards

8.4 GHS Classification

8.5 NFPA Rating

9. Safety Precautions and Personal Protective Equipment (PPE)

• Hazards:

  • Harmful if swallowed (H302)

  • Causes serious eye damage (H318)

  • May cause allergy or asthma symptoms (H334) – sulfite sensitivity

  • Toxic SO₂ gas released upon contact with acids or heating

  • Hygroscopic – absorbs moisture from air

• PPE (mandatory):

  • Chemical splash goggles (EN 166) – mandatory (eye hazard)

  • Dust mask (FFP2/N95) – for dusty operations

  • Chemical-resistant gloves (nitrile)

  • Protective clothing (dust protection)

  • Respiratory protection: Type E (yellow) filter for SO₂

• Engineering controls:

  • Local exhaust ventilation (LEV) for dust control

  • Avoid acid contact (SO₂ gas)

  • Eyewash stations and safety showers

• Storage conditions:

  • Keep tightly closed in original container (HDPE, PP, lined steel)

  • Store in cool, dry, well-ventilated area (5–30°C)

  • Protect from moisture (hygroscopic – decomposes in moist air)

  • Store away from acids (SO₂ gas hazard) and strong oxidizing agents

  • Do not store near food (decomposition products)

• First aid:

  • Inhalation: Move to fresh air; if breathing difficult, give oxygen; if stopped, give artificial respiration; seek medical attention

  • Eye contact: Rinse with water for at least 15 minutes, lifting eyelids; remove contact lenses; seek immediate medical attention

  • Skin contact: Wash with soap and water

  • Ingestion: Rinse mouth; do NOT induce vomiting; drink water; seek medical attention

10. Environmental Fate

11. Storage and Shelf Life

11.1 Storage Conditions

11.2 Shelf Life

11.3 Decomposition Products (upon moisture/heat)

• SO₂ (sulfur dioxide) – toxic, corrosive

• Na₂SO₃ (sodium sulfite) – less effective

• Na₂SO₄ (sodium sulfate) – inert

12. Transport Information

13. Synonyms and Common Names

• English: Sodium Metabisulfite, SMBS, Sodium Pyrosulfite, Disodium Disulfite, Sodium Bisulfite Anhydrous (misnomer)

• Trade names: Soda SO₂, Sodium Metabi, Sulfite 223

• French: Métabisulfite de sodium

• German: Natriummetabisulfit

• Spanish: Metabisulfito de sodio

• Turkish: Sodyum Metabisülfit, Sodyum Pirosülfit, SMBS, E223

14. Regulatory Status

15. Why Choose Sodium Metabisulfite?

Limitations:

• Allergy risk (sulfite sensitivity) – 0.5–5% of asthmatics may react; requires labeling on food products

• Toxic SO₂ release upon acidification – Do not mix with acids

• Hygroscopic – Absorbs moisture; requires airtight storage

• Decomposes in moist air – Loss of SO₂ content over time

• Corrosive – Causes serious eye damage; requires PPE (goggles)

• Not suitable for thiamine-rich foods (vitamin B1) – Sulfites destroy thiamine

16. Comparison with Other Sulfites

17. Sulfite Sensitivity – Labeling Requirements

Warning: Sulfites may cause allergic reactions (hives, itching, swelling, difficulty breathing, anaphylaxis) in sensitive individuals, particularly asthmatics.

18. Frequently Asked Questions (FAQs)

Q1: Is sodium metabisulfite the same as sodium bisulfite?
A1: No. Sodium metabisulfite (Na₂S₂O₅) dissociates in water to form sodium bisulfite (NaHSO₃). They are chemically different but often used interchangeably in solution (same active species – HSO₃⁻).

Q2: Is sodium metabisulfite safe to eat?
A2: Yes, food-grade sodium metabisulfite (E223) is GRAS for use as a preservative and antioxidant. However, sulfite-sensitive individuals (especially asthmatics) may have allergic reactions.

Q3: How does sodium metabisulfite preserve wine?
A3: It releases SO₂, which: (1) kills unwanted microorganisms (bacteria, wild yeasts), (2) acts as an antioxidant (prevents browning), and (3) stabilizes color and flavor.

Q4: Why does sodium metabisulfite smell like rotten eggs?
A4: The characteristic "sulfur" or "rotten egg" smell is from sulfur dioxide (SO₂) released upon decomposition or acidification. Fresh, dry SMBS has only a faint SO₂ odor.

Q5: How do I dispose of sodium metabisulfite?
A5: Dissolve in water, neutralize with sodium carbonate if needed, and flush to wastewater treatment (check local regulations). Do NOT mix with acids (releases toxic SO₂ gas).

19. Storage Recommendations

20. Summary Table – Key Specifications at a Glance

This TDS is prepared in compliance with ISO 11014-1 format and is intended for food technologists, winemakers, textile engineers, water treatment specialists, chemical engineers, and procurement professionals. Certificates of Analysis (CoA), Safety Data Sheets (SDS), and sample validation reports are available upon request.