Sodium bisulfite (CAS 7631-90-5), with the chemical formula NaHSO₃, is a white to yellowish crystalline powder widely used across industries for its reducing, bleaching, and preservative properties. Its reactivity and potential hazards require strict handling protocols. Below is an in-depth overview covering chemical properties, applications, safety data, and regulatory information.
Chemical and Physical Properties
|
Property |
Details |
|
Chemical Formula |
NaHSO₃ (usually exists as a solid in equilibrium with disodium metabisulfite, Na₂S₂O₅) |
|
Molar Mass |
104.06 g/mol |
|
Appearance |
White to yellowish powder; hygroscopic (absorbs moisture from air). |
|
Melting Point |
Decomposes above 150°C (releases sulfur dioxide, SO₂). |
|
Solubility |
Highly soluble in water (60 g/100 mL at 20°C); insoluble in organic solvents. |
|
pH (1% Aqueous) |
Acidic (pH 3–5 due to hydrolysis of HSO₃⁻ to H⁺ and SO₃²⁻). |
|
Density |
1.48 g/cm³ |
|
Stability |
Unstable in the presence of strong oxidizers, acids, or heat; releases toxic SO₂ gas. |
Industrial Applications
a. Water Treatment
● Dechlorination: Reduces residual chlorine in water by reacting with Cl₂:
NaHSO₃ + Cl₂ + H₂O → NaHSO₄ + 2HCl
● Typical Dosage: 3–5 mg/L for drinking water; 5–10 mg/L for industrial wastewater.
● Oxygen Scavenging: Prevents corrosion in boilers by removing dissolved oxygen:
2NaHSO₃ + O₂ → 2NaHSO₄
b. Food and Beverage Industry
● Preservative: Inhibits microbial growth and oxidation in fruits, wines, and juices.
● Allowed Limits: EU: ≤200 mg/kg in dried fruits; FDA (USA): GRAS (Generally Recognized As Safe) with usage limits.
● Bleaching Agent: Whitens food starches and flour (e.g., reducing color from polyphenols).
c. Pulp and Paper Manufacturing
● Wood Pulp Bleaching: Converts lignin into water-soluble compounds, improving paper brightness.
● Process Concentration: 1–3% NaHSO₃ solution at 60–80°C.
d. Chemical Synthesis
● Reducing Agent: Used in pharmaceutical intermediates (e.g., reducing nitro groups to amines).
● Polymerization Inhibitor: Prevents premature polymerization of monomers (e.g., acrylic acid).
Safety and Hazard Parameters
a. Health Hazards
|
Exposure Route |
Effects |
|
Inhalation |
Irritates respiratory tract; high exposure causes coughing, wheezing, and SO₂ poisoning. |
|
Skin Contact |
Causes redness, itching, and chemical burns (corrosive in concentrated solutions). |
|
Ingestion |
Burns the mouth, throat, and stomach; may lead to nausea, vomiting, and electrolyte imbalance. |
|
Carcinogenicity |
Not classified as a carcinogen by IARC, NTP, or OSHA. |
b. Environmental Impact
Aquatic Toxicity: LC50 (fish): ~100 mg/L; can lower water pH and harm aquatic life.
Biodegradability: Partially biodegradable; may release sulfite ions, which can react with water to form sulfurous acid.
Packaging and Storage
|
Aspect |
Details |
|
Packaging Materials |
- Bags: 25-kg or 50-kg multi-layer polyethylene (PE) or polypropylene (PP) with inner liners. |
|
Storage Conditions |
- Temperature: 10–30°C; humidity <60% RH. |
|
Labeling |
- UN Number: UN 1313 (Class 8, Corrosive). |
Regulatory Information
|
Region/Regulation |
Requirements |
|
USA (OSHA) |
- PEL (Permissible Exposure Limit): 5 mg/m³ (as SO₂) for 8-hour TWA. |
|
EU (REACH) |
- Registration required for production/import >1 ton/year. |
|
China (GB 12463) |
- Packaging must meet hazardous goods standards; import/export requires chemical safety permits. |
|
International (IMDG/IATA) |
- Classified as a Class 8 corrosive substance; specific packing instructions for sea/air transport. |
Frequently Asked Questions (FAQ)
Q: How does sodium bisulfite react with acids?
A: It reacts vigorously with acids to release sulfur dioxide gas. For example:
NaHSO₃ + HCl → NaCl + H₂O + SO₂↑
This reaction requires strict ventilation and emergency response plans in case of accidental mixing.
Q: Can sodium bisulfite be used in organic farming?
A: In most regions, its use is restricted in organic agriculture due to its synthetic origin. Organic standards prioritize natural or approved substances.
Q: What is the shelf life of sodium bisulfite?
A: Under proper storage conditions (cool, dry, sealed), it can last 1–2 years. However, exposure to air or moisture may shorten its efficacy.
Conclusion
Sodium bisulfite (CAS 7631-90-5) is a versatile yet hazardous chemical with diverse industrial applications. Its use demands careful management of parameters such as dosage, storage, and safety protocols to maximize benefits while minimizing risks to human health and the environment. For specific applications, always consult relevant regulations and safety data sheets.
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