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Sodium metabisulfite
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Sodium metabisulfite
Sodium metabisulfite
Sodium metabisulfite
Structure of sodium metabisulfite
Structure of sodium metabisulfite
Names
Other names
  • Sodium pyrosulfite
  • Sodium disulfite
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.028.794 Edit this at Wikidata
EC Number
  • 231-673-0
E number E223 (preservatives)
KEGG
RTECS number
  • UX8225000
UNII
UN number 1759 (SODIUM METABISULFITE)
  • Key: HRZFUMHJMZEROT-UHFFFAOYSA-L
  • InChI=1S/2Na.H2O5S2/c;;1-6(2)7(3,4)5/h;;(H,1,2)(H,3,4,5)/q2*+1;/p-2
  • [O-]S(=O)S(=O)(=O)[O-].[Na+].[Na+]
Properties
Na2S2O5
Molar mass 190.107 g/mol
Appearance White to yellow powder
Odor Faint SO2
Density 1.48 g/cm3
Melting point 170 °C (338 °F; 443 K) decomposition begins at 150 °C
  • 45.1 g/100 mL (0 °C)
  • 65.3 g/100 mL (20 °C)
  • 81.7 g 100 mL (100 °C)
Solubility Very soluble in glycerol
Slightly soluble in ethanol
Hazards
GHS labelling:
GHS05: CorrosiveGHS07: Exclamation mark
Danger
H302, H318
P264, P270, P280, P301+P312, P305+P351+P338, P310, P330, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
2
0
1
NIOSH (US health exposure limits):
PEL (Permissible)
 None[1]
REL (Recommended)
 TWA 5 mg/m3[1]
IDLH (Immediate danger)
 N.D.[1]
Safety data sheet (SDS) Mallinckrodt MSDS
Related compounds
Other anions
 Sodium sulfite
Sodium bisulfite
Other cations
 Potassium metabisulfite
Related compounds
 Sodium dithionite
Sodium thiosulfate
Sodium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Sodium metabisulfite or sodium pyrosulfite (IUPAC spelling; Br. E. sodium metabisulphite or sodium pyrosulphite) is an inorganic compound of chemical formula Na2S2O5. The substance is sometimes referred to as disodium metabisulfite. It is used as a disinfectant, antioxidant, and preservative agent.[2] When dissolved in water it forms sodium bisulfite.

Preparation

[edit]
See also: Wellman–Lord process

Sodium metabisulfite can be prepared by treating a solution of sodium hydroxide with sulfur dioxide.[3] When conducted in warm water, Na2SO3 initially precipitates as a yellow solid. With more SO2, the solid dissolves to give the disulfite, which crystallises upon cooling.[4]

SO2 + 2 NaOH → Na2SO3 + H2O
SO2 + Na2SO3 → Na2S2O5

which yields a residue of colourless solid Na2S2O5.

Chemical structure

[edit]

The anion metabisulfite consists of an SO2 group linked to an SO3 group, with the negative charge more localised on the SO3 end. The S–S bond length is 2.22 Å, and the "thionate" and "thionite" S–O distances are 1.46 and 1.50 Å, respectively.[5]

Reactivity

[edit]
Main article: Bisulfite § Reactions

Upon dissolution in water, bisulfite is generated:

Na2S2O5 + H2O → 2 Na+ + 2 HSO3−

Uses

[edit]

Sodium and potassium metabisulfite have many major and niche uses. It is widely used for preserving food and beverages.

  • Sodium metabisulphite is one of the main ingredients in "Drywhite", a composition used to prevent chipped potatoes oxidising during storage prior to use in many fish and chip shops.[6]
  • Sodium metabisulfite is added as an excipient to medications which contain adrenaline (epinephrine), in order to prevent the oxidation of adrenaline.[7] For example, it is added to combination drug formulations which contain a local anaesthetic and adrenaline,[7] and to the formulation in epinephrine autoinjectors, such as the EpiPen.[8] This lengthens the shelf life of the formulation,[7] although the sodium metabisulfite reacts with adrenaline, causing it to degrade and form epinephrine sulfonate.[9]
  • In combination with sodium hydrosulfite it is used as a rust-stain remover[10]
  • It is used in photography as an antioxidant in photographic film development.[11][12]
  • Concentrated sodium metabisulfite can be used to remove tree stumps. Some brands contain 98% sodium metabisulfite, and cause degradation of lignin in the stumps, facilitating removal.[13]
  • It is also used as an excipient in some tablets, such as paracetamol.
  • A very important health related aspect of this substance is that it can be added to a blood smear in a test for sickle cell anaemia (and other similar forms of haemoglobin mutation). The substance causes defunct cells to sickle (through a complex polymerisation) hence confirming disease.
  • It is used as a bleaching agent in the production of coconut cream.
  • It (or liquid SO2) is commonly used as an antimicrobial and antioxidant in winemaking; bottled wine indicates its use with the label "Contains Sulfites" in the US.
  • It is used as a reducing agent to break sulfide bonds in shrunken items of clothing made of natural fibres, thus allowing the garment to go back to its original shape after washing.
  • It is used as an SO2 source (mixed with air or oxygen) for the destruction of cyanide in commercial gold cyanidation processes.
  • It is used as an SO2 source (mixed with air or oxygen) for the precipitation of elemental gold in chloroauric (aqua regia) solutions.
  • It is used in the water treatment industry to quench residual chlorine.
  • It is used in tint etching iron-based metal samples for microstructural analysis.[14][15]
  • It is used as a fungicide for anti-microbe and mould prevention during shipping of consumer goods such as shoes and clothing. Plastic stickers and packaging (such as Micro-Pak™) containing the anhydrous, sodium metabisulfite solid active ingredient are added prior to shipping. The devices absorb moisture from the atmosphere during shipping and release low levels of sulfur dioxide.[16]
  • It is used for preserving fruit during shipping.[17]
  • It is used as an additive in the extraction of starch from tubers,[18] fruit,[19] and cereal crops.[20][21]
  • It is used as a pickling agent to treat high pressure reverse osmosis and nanofiltration water desalination membranes for extended storage periods between uses.
  • It is used to create a bisulfite adduct from ketones to aid in separation of the ketone product. The usage of metabisulfite versus the sulfite is also more entropically favourable.

Safety

[edit]

Sodium metabisulfite, despite not being flammable, decomposes at 150°C releasing toxic sulfur dioxide. It is corrosive when dissolved in water. Some people who are sulfite sensitive may experience an allergic reaction to sodium metabisulfite, sometimes severe, resulting in labeling requirements for food safety.[22] In 2024, it was named ‘allergen of the year 2024’ by the American Contact Dermatitis Society. [23]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Sodium metabisulfite

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from Grokipedia
Sodium metabisulfite is an with the Na₂S₂O₅, appearing as a white to yellowish crystalline powder with a slight sulfurous . It is the sodium salt of the metabisulfite anion (S₂O₅²⁻), which consists of an SO₂ group bonded to an SO₃ group, and it readily dissolves in to form solutions. With a of 190.11 g/mol and a CAS number of 7681-57-4, it serves primarily as an , , and across multiple industries. In the food and beverage sector, sodium metabisulfite functions as a key additive (E223 in the ) to prevent oxidation, inhibit microbial growth, and stabilize colors in products such as wines, dried fruits, , and baked goods. It is particularly vital in , where it sanitizes equipment, arrests fermentation, and preserves bottled wine by releasing (SO₂), which acts against , yeasts, and molds. Beyond food, it finds applications in pharmaceuticals as an in injectables and syrups, in for dechlorination, and in industries like , textiles, and pulp bleaching due to its reducing properties. From a safety perspective, sodium metabisulfite is classified as (acute toxicity category 4) and can cause serious eye damage, , and respiratory issues upon , primarily through SO₂ release in moist environments. It poses risks to asthmatics and those sensitive to sulfites, potentially triggering allergic reactions, and is harmful to aquatic life with long-term effects. Handling requires protective , and exposure limits include a of 5 mg/m³ for respirable dust. Despite these hazards, it is produced industrially by reacting with and is widely regulated for safe use in approved concentrations.

Properties

Physical properties

Sodium metabisulfite (Na₂S₂O₅) is a white or yellowish-white crystalline powder or free-flowing granules possessing a faint sulfurous odor. Its molecular weight is 190.107 g/mol. The density is 1.48 g/cm³. The compound decomposes at approximately 150–180°C without melting. Sodium metabisulfite exhibits high solubility in water, up to 65 g/100 mL at 20°C, with slight solubility in ethanol (1.35 g/100 mL) and insolubility in acetone and toluene. Aqueous solutions are acidic, with a pH typically ranging from 4.0 to 4.8 for a 10% solution. The substance is stable under dry conditions but decomposes in moist air or upon heating, releasing . It is hygroscopic, readily absorbing from the atmosphere, which necessitates storage in tightly closed containers away from and oxidizing agents to prevent degradation.

Chemical properties

Sodium metabisulfite exhibits acidic properties in aqueous solutions, serving as a source of ions (HSO₃⁻) that lower the . Upon dissolution in , it undergoes partial to form (NaHSO₃), with a typical 1% solution displaying a of around 4.6. The compound's reducing capabilities stem from the mixed valence states of in the metabisulfite anion (S₂O₅²⁻), where one sulfur atom holds an of +3 and the other +5, yielding an average of +4. These intermediate s allow sodium metabisulfite to function as a potent , facilitating in various chemical processes. Additionally, its behavior arises from the ability to scavenge free radicals and oxygen by donating electrons, thereby inhibiting oxidative reactions and stabilizing susceptible compounds. Thermal decomposition occurs when sodium metabisulfite is heated above 150°C, liberating (SO₂) gas and producing (Na₂SO₃). This process highlights its instability at elevated temperatures, contributing to its handling precautions in practical applications.

Production

Laboratory preparation

Sodium metabisulfite can be prepared in the laboratory by reacting with gas under controlled conditions. The reaction proceeds as follows: 2NaOH+2SO2→Na2S2O5+H2O2 \mathrm{NaOH} + 2 \mathrm{SO_2} \rightarrow \mathrm{Na_2S_2O_5} + \mathrm{H_2O} To perform this synthesis, a solution of sodium hydroxide is prepared in boiled distilled water in a suitable reaction vessel equipped with a gas inlet and stirrer, conducted in a fume hood due to the toxic nature of sulfur dioxide. Air is excluded by passing hydrogen gas through the solution. Sulfur dioxide gas, generated from a cylinder or in situ (e.g., by reaction of sodium sulfite with acid), is passed into the solution until saturation. The solution is then cooled to facilitate crystallization of Na₂S₂O₅, which precipitates as colorless crystals. The product is filtered in an inert atmosphere, washed with cold water to remove impurities, and dried under a stream of hydrogen. An alternative method utilizes sodium carbonate instead of sodium hydroxide. The balanced equation is: Na2CO3+2SO2→Na2S2O5+CO2\mathrm{Na_2CO_3} + 2 \mathrm{SO_2} \rightarrow \mathrm{Na_2S_2O_5} + \mathrm{CO_2} In this procedure, sodium carbonate is dissolved in water, and sulfur dioxide gas is bubbled in until acidification occurs, leading to precipitation of sodium metabisulfite. The mixture is filtered to separate the solid product, followed by washing with cold water. Purification of the crude sodium metabisulfite is achieved through recrystallization. The solid is dissolved in hot water to form a saturated solution, which is then filtered hot to remove insoluble impurities. Upon cooling to room temperature or in an ice bath, pure colorless crystals of Na₂S₂O₅ form and are collected by filtration and washed with ice-cold water. This step helps remove byproducts. Laboratory preparations typically achieve yields of approximately 90% of theoretical based on the limiting reactant, conducted at (20-25°C) to minimize . Precautions include performing the reaction in a well-ventilated , using appropriate PPE for handling SO₂ (which is irritating to respiratory and ocular tissues), avoiding air exposure to prevent oxidation, and avoiding excess heat to prevent SO₂ release from the product. These methods trace back to mid-20th-century practices, as detailed in seminal inorganic synthesis literature from 1946.

Industrial production

The industrial production of sodium metabisulfite primarily occurs through the absorption of (SO₂) gas into alkaline solutions of (NaOH) or (Na₂CO₃) within absorption towers, forming the product via the reaction 2 NaOH + 2 SO₂ → Na₂S₂O₅ + H₂O. This wet process generates a saturated solution of intermediate, which is then further reacted with excess SO₂ to yield sodium metabisulfite, followed by cooling, , , and drying to obtain the solid product. Sulfur dioxide for this process is commonly produced by combusting elemental sulfur in sulfur burners or recovered as a from metal and industrial combustion, with post-2020 developments increasingly focusing on captured SO₂ streams to minimize environmental impact and promote practices in chemical manufacturing. An alternative route involves reacting pre-formed with additional SO₂ according to Na₂SO₃ + SO₂ → Na₂S₂O₅, allowing integration with existing sulfite production facilities. Key equipment includes packed columns or spray towers for gas-liquid absorption to maximize contact efficiency, crystallizers for controlled at temperatures around 20–50°C, and rotary dryers to achieve final moisture content below 0.5%. The process yields different purity grades, with food-grade sodium metabisulfite requiring at least 97% purity and low heavy metal content to meet regulatory standards for preservatives, while technical-grade variants (typically 96.5–98% purity) suffice for industrial uses like . Global production is dominated by facilities in and , with major producers including Hunan Yueyang Sanxiang Chemical Co., Ltd. in and INEOS Calabrian in . Recent optimizations emphasize , such as INEOS's SO₂Clean® process, a proprietary method that generates ultra-pure SO₂ without nitrogen dilution, byproducts, or process , thereby reducing emissions and operational costs in downstream metabisulfite synthesis. This method integrates seamlessly with absorption towers, enhancing overall efficiency in large-scale operations.

Structure and reactivity

Molecular structure

Sodium metabisulfite is an ionic compound with the Na₂S₂O₅, comprising two sodium cations (Na⁺) and the metabisulfite anion (S₂O₅²⁻). The solid form adopts a monoclinic lattice, characterized by non-merohedral twinning that has historically complicated structural determination. The metabisulfite anion, [O₃S–SO₂]²⁻, exhibits a distinctive where a pyramidal SO₃ group is connected to a nearly linear SO₂ moiety via a central S–S bond, resulting in an overall asymmetric dimer-like arrangement. reveals an S–S of approximately 2.22 , with S=O bond lengths averaging around 1.43 in the SO₃ unit and slightly longer S–O distances in the SO₂ portion. Bond angles within the anion include O–S–O angles near 106° in the pyramidal SO₃ group and approximately 120° in the SO₂ moiety, contributing to its stability as a discrete in the lattice. In the , sodium ions are coordinated by oxygen atoms from multiple metabisulfite anions, forming a three-dimensional network that stabilizes the overall packing; the is reported as P2₁/c. This coordination environment influences the white, crystalline appearance of the solid. Spectroscopic techniques confirm the anion's structure, with () and Raman spectra showing characteristic S=O stretching vibrations in the 1100–1200 cm⁻¹ region. Unlike the bisulfite ion (HSO₃⁻), which is monomeric with a single sulfur center, the metabisulfite anion features a unique S₂O₅²⁻ structure akin to a condensed dimer, distinguishing its bonding and reactivity prerequisites.

Chemical reactions

Sodium metabisulfite undergoes hydrolysis in water to form sodium bisulfite according to the equilibrium reaction Na₂S₂O₅ + H₂O ⇌ 2 NaHSO₃. This process results in acidic solutions with a pH typically around 4.6 for a 1% aqueous solution at 25°C, reflecting the pH dependence driven by the weak acid nature of bisulfite and the equilibrium shift toward hydrolysis under neutral to basic conditions. The equilibrium favors bisulfite formation, with the position influenced by temperature and concentration, though specific equilibrium constants are not widely reported due to the rapid and complete hydrolysis in dilute solutions. In acidic conditions, sodium metabisulfite releases gas via the reaction Na₂S₂O₅ + 2 HCl → 2 NaCl + H₂O + 2 SO₂, proceeding through a intermediate formed by initial . The mechanism involves of the ion (HSO₃⁻) to generate unstable , which decomposes to SO₂ and water, with the release rate increasing at lower values. Oxidation of sodium metabisulfite by molecular oxygen leads to dithionate formation, as represented by 2 Na₂S₂O₅ + O₂ → 2 Na₂S₂O₆, a process that occurs slowly in aqueous solutions and is utilized in oxygen scavenging applications. This reaction highlights its reducing nature, where the in the metabisulfite is partially oxidized to S(V) in the dithionate . As a , sodium metabisulfite reacts with iodine in analytical titrations, following the Na₂S₂O₅ + 2 I₂ + 3 H₂O → 2 NaI + 2 HI + 2 H₂SO₄, where the intermediate reduces I₂ to while being oxidized to . This process is pH-dependent and commonly employed to quantify content in samples. Thermal decomposition of sodium metabisulfite occurs above 150°C, yielding Na₂S₂O₅ → Na₂SO₃ + SO₂, with further oxidation in air leading to as the ultimate product through multi-step weight loss processes. Recent studies from 2019 to 2021 have explored sodium metabisulfite's interactions in biodegradable films, where it facilitates controlled release by generating SO₂, enhancing microbial inhibition on film surfaces without compromising biodegradability.

Applications

Food and beverage uses

Sodium metabisulfite (E223) is approved as a in the under Regulation (EC) No 1333/2008, where it functions as a and in various edible products. In the United States, it holds (GRAS) status from the (FDA) under 21 CFR 182.3766 for use as a direct food ingredient, with the exception of meats, and is limited to levels. As a preservative, sodium metabisulfite acts by dissolving in water to release (SO₂), which inhibits the growth of , yeasts, and molds while also preventing enzymatic in fruits and through its reducing properties. This and mechanism extends and maintains product quality in processed foods. In the beverage industry, sodium metabisulfite is commonly added to wine for stabilization, with typical dosing of 20–50 ppm to prevent oxidation and microbial spoilage during and aging. It is also used in beer production at levels around 10–20 ppm to control fermentation and protect against off-flavors. For dried fruits such as apricots, it preserves color and prevents mold growth, while in processing, it inhibits bacterial contamination and maintains freshness. In bakery applications, it serves as a by weakening structure to improve handling and texture in products like biscuits and pie crusts. Regulatory maximum permitted levels vary by product and jurisdiction; in the , limits reach up to 350 mg/kg (as SO₂) in wine, 450 mg/kg in certain products, and 2,000 mg/kg in dried fruits like apricots. In the , the FDA enforces GRAS guidelines without fixed maxima for most foods but prohibits its use in fresh and requires adherence to levels that do not exceed technological necessity. Labeling requirements mandate declaration of sulfites when residual SO₂ exceeds 10 ppm in finished products, serving as an warning due to potential sensitivity in susceptible individuals. This applies to both and regulations for pre-packaged foods and beverages. Recent developments from 2020 to 2025 include the formulation of food-grade sodium metabisulfite variants optimized for extended shelf life in beverages through improved stability and controlled release. Additionally, its incorporation into biodegradable films has enabled controlled SO₂ release for active , enhancing preservation in fresh produce without direct addition.

Industrial and other applications

Sodium metabisulfite serves as a key reagent in processes, particularly for dechlorination of municipal and industrial effluents to protect downstream equipment and ecosystems. It reacts stoichiometrically with free (typically using 3 mg SMBS per 1 mg Cl₂ in practice due to excess for complete reaction) to form and , with the reaction proceeding via intermediate in : \ceNaHSO3+HOCl−>NaHSO4+HCl\ce{NaHSO3 + HOCl -> NaHSO4 + HCl} This process is essential in systems and wastewater facilities, where residual from disinfection must be removed to prevent damage or environmental release of toxic compounds. Additionally, sodium metabisulfite functions as an in , reducing dissolved levels to minimize in generation systems by converting to ions. In the and pulp industries, sodium metabisulfite acts as a to eliminate excess after bleaching processes, preventing fabric degradation and ensuring color stability. It breaks down chromophoric groups in dyes via sulfonation, aiding in decolorization and within these sectors. In pulp production, it supports the sulfite pulping process by facilitating extraction, where the resulting are utilized as dispersants in formulations. Sodium metabisulfite is employed in as a component of developing and fixing baths, where it serves as a to convert exposed silver halides to metallic silver while preventing over-oxidation of the image. This helps stabilize the photographic and maintain image quality during processing. In pharmaceuticals, sodium metabisulfite functions as an and stabilizer in injectable formulations, such as those containing epinephrine, where it inhibits oxidation to extend and ensure efficacy. It is also added to preparations, like ascorbic acid injections, to prevent degradation by counteracting oxidative reactions. Beyond these, sodium metabisulfite finds applications in for in , where it oxidizes free to less toxic via addition, complying with environmental discharge standards. In , it acts as a and , safeguarding formulations against microbial growth and oxidation while maintaining product integrity. Market trends in 2025 indicate growing demand for sodium metabisulfite in , driven by stricter environmental regulations on effluent quality, with the high-purity grades segment valued at approximately $92 million globally. This expansion reflects its increasing role in sustainable industrial practices across water-intensive sectors.

Health, safety, and environmental impact

Health effects and toxicity

Sodium metabisulfite exhibits moderate upon , with an LD50 value of approximately 1,540 mg/kg in rats according to Test Guideline 401. It acts as an irritant to the , eyes, and , primarily due to the release of gas upon or contact with moisture and acids. Inhalation of dust or generated gas can cause immediate symptoms such as coughing, , and , while direct skin or may lead to redness, , and potential burns. Allergic reactions to sodium metabisulfite are primarily linked to sulfite sensitivity, affecting up to 5% of individuals with , where it can trigger and exacerbate respiratory symptoms. Common manifestations include (urticaria), flushing, and in severe cases, , particularly in sensitive populations exposed through or pharmaceutical preservatives. These reactions are more pronounced in asthmatics due to the compound's ability to release , which irritates airways and may provoke an IgE-mediated response in susceptible individuals. In 2024, sulfites, including sodium metabisulfite, were designated as by the American due to increasing reports of . Chronic exposure to sodium metabisulfite may pose risks of , as evidenced by a 2022 European Food Safety Authority (EFSA) assessment indicating sulfite-induced effects such as prolonged visual latency in experimental models. However, mutagenicity tests, including the Ames assay, have consistently shown negative results, suggesting no genotoxic potential. The primary exposure route of concern is of dust or gas, which is more hazardous than dermal absorption, where penetration is low with an LD50 exceeding 2,000 mg/kg in rats. Regulatory bodies have established limits to mitigate health risks; the National Institute for Occupational Safety and Health (NIOSH) recommends an exposure limit (REL) of 5 mg/m³ as a for sodium metabisulfite dust. For released from the compound, OSHA enforces a of 5 ppm as an 8-hour . In food applications, the U.S. recognizes sodium metabisulfite as under good manufacturing practices, except in meats, with mandatory labeling required if sulfites exceed 10 ppm total SO₂. In pharmaceuticals, it serves as a but is limited to concentrations that avoid exceeding safe exposure thresholds. Recent studies from 2020 to 2025 have further explored neurotoxic effects in biological models, including and tissue damage in hippocampus following subchronic exposure, though relevance remains under investigation. The International Agency for Research on Cancer (IARC) classifies sodium metabisulfite as Group 3, not classifiable as to its carcinogenicity to , based on inadequate evidence from animal and studies.

Safety handling and environmental considerations

Handling sodium metabisulfite requires strict adherence to (PPE) protocols to minimize exposure risks. Workers should wear chemical-resistant gloves, safety goggles or face shields, , and respiratory protection such as NIOSH-approved dust masks or respirators in areas with potential dust generation or (SO₂) release. Avoid direct contact with , eyes, and , and wash thoroughly after handling; do not eat, drink, or smoke in work areas to prevent accidental ingestion. Storage conditions must prevent decomposition and SO₂ evolution, which can occur upon exposure to moisture or acids. Store in a cool, dry, well-ventilated area in tightly sealed containers made of corrosion-resistant materials like or , away from incompatible substances such as strong acids, oxidizers, and metals that could generate hazardous gases. In case of exposure, immediate measures are essential. For contact, remove contaminated clothing and flush affected areas with plenty of for at least 15 minutes; seek medical attention if irritation persists. Eye exposure demands immediate irrigation with for 15-20 minutes while holding eyelids open, followed by professional medical evaluation. of or SO₂ requires moving the person to fresh air; administer oxygen if breathing is difficult and consult a physician, particularly for those with respiratory conditions. For ingestion, do not induce vomiting; rinse mouth with and seek urgent medical help. Disposal of sodium metabisulfite should follow local, state, and federal regulations to avoid environmental release. Neutralize solutions with a dilute base like before discharge into wastewater systems, and monitor effluent for SO₂ levels to comply with limits. Solid wastes may be classified as hazardous if contaminated; incinerate in approved facilities or dispose via licensed waste handlers, ensuring containment to prevent spills. Environmentally, sodium metabisulfite exhibits low persistence in aquatic systems due to rapid hydrolysis in water, forming bisulfite ions that further degrade without significant bioaccumulation. However, releases can contribute to effluent acidification and temporary harm to aquatic life through SO₂ formation, particularly in oxygen-poor conditions. The SO2Clean® production process, adopted more widely since 2021, minimizes emissions by avoiding byproducts like nitrogen oxides and sulfur trioxides, offering a greener alternative for industrial synthesis. Regulatory frameworks classify sodium metabisulfite under REACH in the as Acute Tox. 4 (H302: ) and Eye Dam. 1 (H318: Causes serious eye damage), with additional Aquatic Acute 3 (H402: Harmful to aquatic life) labeling requiring risk assessments for handlers. In the , the EPA oversees its use via the interim registration review for inorganic sulfites, mandating compliance with National Pollutant Discharge Elimination System (NPDES) permits for industrial effluents to control SO₂ discharges and protect . From 2020 to 2025, concerns have centered on improper disposal leading to localized water contamination in sectors like and , prompting stricter monitoring and incentives for low-emission production methods to reduce the overall . When managed correctly, risks remain minimal, supporting sustainable use in compliant operations.

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