Sodium metabisulphite is the sodium salt of sulphurous acid, a chemical compound that belongs to the sulphite group, sulphur-based components that release sulphur dioxide SO₂, an active preservative compound. 

Sodium Metabisulphite is an inorganic compound formed by the combination of sodium ions and metabisulphite. It is commonly used as a preservative and antimicrobial agent in various foods and beverages, as well as a bleaching agent in some industrial applications.

 "Metabisulphite" refers to the group of metabisulphite anions (SO2), which are reducing agents and preservatives. "Sodium" indicates the presence of sodium ions (Na+), which bind to the metabisulphite anions to form the compound. 

Raw materials used in production and their funcions

Sodium Bisulphate: A sodium salt of sulfurous acid, it acts as the base for the production of sodium metabisulphite.

Sulfur Dioxide (SO₂): A gas that reacts with sodium bisulphate to form sodium metabisulphite.

Industrial Chemical Synthesis of Sodium Metabisulphite

• Preparation: The mixture is prepared by combining sodium bisulphate with water to form a solution.
• Reaction with Sulfur Dioxide: Sulfur dioxide is introduced into the sodium bisulphate solution. The reaction produces sodium metabisulphite and water.
• Cooling and Crystallization: After the reaction, the solution is cooled. This leads to the crystallization of sodium metabisulphite.
• Separation and Drying: The sodium metabisulphite crystals are separated from the solution and dried to remove residual moisture.
• Cleaning and Packaging: The dry crystals are then cleaned and packaged for distribution.

It appears as a stable white crystalline powder with a slight odour of sulphur dioxide. Soluble in water. Incompatible with strong oxidising agents, strong acids. On contact with strong acids it releases a poisonous gas.

The sulphite group includes:

What it is used for and where

Food

It is a preservative, used by the food industry and labelled with the number E223 on the European Food Additives List. 

It is used in foods as a bleaching agent and to prevent browning reactions (1).

Cosmetics

It is a restricted ingredient III/99 and  V/9  a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009

Antioxidant agent. Ingredient that counteracts oxidative stress and prevents cell damage. Free radicals, pathological inflammatory processes, reactive nitrogen species and reactive oxygen species are responsible for the ageing process and many diseases caused by oxidation.

Preservative. Any product containing organic, inorganic compounds, water, needs to be preserved from microbial contamination. Preservatives act against the development of harmful microorganisms and against oxidation of the product.

Reducing agent. Ingredient that facilitates permanent hair waving by adjusting the pH to an optimal level.

Safety

It has been declared a safe component for human health since 1959 (Gunnison and Jacobsen, 1987; Kencebay et al.2013).

Episode of allergic contact dermatitis caused by a catheter system containing sodium metabisulphite (2).

Symptoms attributable to sulphite sensitivity can be of varying nature and importance. The most common are headache and generalised itching or swelling, but cases of nausea, bronchoconstriction, diarrhoea, hypotension and shock have also occurred (3).

EFSA's Scientific Panel on Food Additives and Flavourings assessed the risk for toxic elements in sulphur dioxide (E 220-228), based on data submitted by stakeholders, and concluded that the EU specification maximum limits for arsenic, lead and mercury should be lowered and a maximum limit for cadmium should be introduced (4).

Ophthalmic allergen (5).

Other uses

• In agriculture it is used as a chemical fertiliser, herbicide and pesticide.
• Processing agent for non-ferrous metals, coagulator in rubber processing with functions of bleaching agent, mordant, reducing agent.
• Wastewater treatment agent, water purifier, gas purification agent.
• Oxygen scavenger corrosion inhibitor in the oil and gas industry. 
• Reagent for chromatographic analysis.
• Preservative and reducing agent for dyes.
• Dechlorination agent after cotton bleaching and additive for hot cotton processing.

Sodium metabisulphite studies

Sulphites studies

Caratteristiche tipiche del prodotto commerciale  Sodium metabisulfite

Appearance	White powder
pH	4.0-4.6
Melting Point	150°C
Density	1.48
PSA	189.898254
LogP	0.27220
Water Solubility	540 g/L (20 ºC)
Water insoluble	0.01% max
Fe	0.002% max
As	0.00006% max
Heavy Metals(Pb)	0.0003% max
Safety

• Molecular Formula : Na₂O₅S₂  Na₂S₂O₅ 
• Molecular Weight : 190.095 g/mol
• Exact Mass   189.898254
• CAS : 7681-57-4  7757-74-6
• UNII    4VON5FNS3C
• EC Number   231-673-0
• DSSTox Substance ID  
• IUPAC  Sodium metabisulfite
• InChI=1S/2Na.H2O5S2/c;;1-6(2)7(3,4)5/h;;(H,1,2)(H,3,4,5)/q2*+1;/p-2 
• InChl Key      HRZFUMHJMZEROT-UHFFFAOYSA-L
• SMILES   [O-]S(=O)S(=O)(=O)[O-].[Na+].[Na+]
• MDL number  MFCD00167602
• PubChem Substance ID    329824616
• ChEBI  114786
• NACRES NA.21 
• RXCUI     36696
• RTECS   UX8225000
• UN   1759
• ICSC   1461  

Synonyms

• Sodium disulfite
• Sodium bisulphite
• Sodium pyrosulfite
• Sodium bisulfite anhydrous
• Natrium pyrosulfit
• Disodium disulfite
• Disodium disulphite
• disodium oxido(oxo)-kappa(4)-sulfanesulfonate

References________________________________________________________________

(1) Dalefield RR, Mueller U. Gastric mucosal irritation following oral exposure to sodium metabisulphite: A reproducible effect? Regul Toxicol Pharmacol. 2016 Oct;80:277-82. doi: 10.1016/j.yrtph.2016.07.005.

(2) Grosch E, Mahler V.  Allergic contact dermatitis caused by a catheter system containing sodium metabisulfite.  Contact Dermatitis. 2017 Mar;76(3):186-187. doi: 10.1111/cod.12675.

(3) Gunnison AF, Jacobsen DW. Sulfite hypersensitivity. A critical review. CRC Crit Rev Toxicol. 1987;17(3):185-214. doi: 10.3109/10408448709071208. 

Abstract. Sulfiting agents (sulfur dioxide and the sodium and potassium salts of bisulfite, sulfite, and metabisulfite) are widely used as preservatives in foods, beverages, and pharmaceuticals. Within the past 5 years, there have been numerous reports of adverse reactions to sulfiting agents. This review presents a comprehensive compilation and discussion of reports describing reactions to ingested, inhaled, and parenterally administered sulfite. Sulfite hypersensitivity is usually, but not exclusively, found within the chronic asthmatic population. Although there is some disagreement on its prevalence, a number of studies have indicated that 5 to 10% of all chronic asthmatics are sulfite hypersensitive. This review also describes respiratory sulfur dioxide sensitivity which essentially all asthmatics experience. Possible mechanisms of sulfite hypersensitivity and sulfur dioxide sensitivity are discussed in detail. Sulfite metabolism and the role of sulfite oxidase in the detoxification of exogenous sulfite are reviewed in relationship to the etiology of sulfite hypersensitivity.

(4) EFSA Panel on Food Additives and Flavourings (FAF); Younes M, Aquilina G, Castle L, Engel KH, Fowler PJ, Frutos Fernandez MJ, Fürst P, Gundert-Remy U, Gürtler R, Husøy T, Manco M, Mennes W, Moldeus P, Passamonti S, Shah R, Waalkens-Berendsen I, Boon P, Cheyns K, Crebelli R, FitzGerald R, Lambré C, Mirat M, Ulbrich B, Vleminckx C, Mech A, Rincon AM, Tard A, Horvath Z, Wright M. Follow-up of the re-evaluation of sulfur dioxide (E 220), sodium sulfite (E 221), sodium bisulfite (E 222), sodium metabisulfite (E 223), potassium metabisulfite (E 224), calcium sulfite (E 226), calcium bisulfite (E 227) and potassium bisulfite (E 228). EFSA J. 2022 Nov 24;20(11):e07594. doi: 10.2903/j.efsa.2022.7594. 

Abstract. Sulfur dioxide-sulfites (E 220-228) were re-evaluated in 2016, resulting in the setting of a temporary ADI of 0.7 mg SO2 equivalents/kg bw per day. Following a European Commission call for data, the present follow-up opinion assesses data provided by interested business operators (IBOs) and additional evidence identified in the publicly available literature. No new biological or toxicological data addressing the data gaps described in the re-evaluation were submitted by IBOs. Taking into account data identified from the literature search, the Panel concluded that there was no substantial reduction in the uncertainties previously identified in the re-evaluation. Therefore, the Panel considered that the available toxicity database was inadequate to derive an ADI and withdrew the current temporary group acceptable daily intake (ADI). A margin of exposure (MOE) approach was considered appropriate to assess the risk for these food additives. A lower confidence limit of the benchmark dose of 38 mg SO2 equivalents/kg bw per day, which is lower than the previous reference point of 70 mg SO2 equivalents/kg bw per day, was estimated based on prolonged visual evoked potential latency. An assessment factor of 80 was applied for the assessment of the MoE. At the estimated dietary exposures, when using a refined exposure scenario (Data set D), MOEs at the maximum of 95th percentile ranges were below 80 for all population groups except for adolescents. The dietary exposures estimated using the maximum permitted levels would result in MOEs below 80 in all population groups at the maximum of the ranges of the mean, and for most of the population groups at both minimum and maximum of the ranges at the 95th percentile. The Panel concluded that this raises a safety concern for both dietary exposure scenarios. The Panel also performed a risk assessment for toxic elements present in sulfur dioxide-sulfites (E 220-228), based on data submitted by IBOs, and concluded that the maximum limits in the EU specifications for arsenic, lead and mercury should be lowered and a maximum limit for cadmium should be introduced.

(5) Herbst RA, Uter W, Pirker C, Geier J, Frosch PJ. Allergic and non-allergic periorbital dermatitis: patch test results of the Information Network of the Departments of Dermatology during a 5-year period. Contact Dermatitis. 2004 Jul;51(1):13-9. doi: 10.1111/j.0105-1873.2004.00334.x. 

Abstract. Periorbital dermatitis is common and can be due to the external use of ophthalmic drugs. We evaluated patch test results of the Information Network of the Departments of Dermatology. During a 5-year period (1995-99), of a total 49,256 patch-tested patients, 1053 (2.1%) were eventually diagnosed as allergic periorbital contact dermatitis (APD) and 588 (1.2%) as non-allergic periorbital dermatitis (NAPD). Patient characteristics between APD, NAPD and other cases (OCs) differed with respect to sex (19.7% male in both periorbital groups versus 36.3% in OCs), atopic dermatitis (10.4% in APD versus 60.2% in NAPD versus 16.9% in OCs) and age, APD being substantially more often (68.2%) aged 40 and above than NAPD (52.6%). Several of the top allergens in OCs [such as fragrance mix, Myroxylon pereirae resin (balsam of Peru), lanolin alcohol and potassium dichromate] caused significantly fewer positive test reactions in both periorbital groups. In contrast, thimerosal, phenylmercuric acetate, sodium disulfite, gentamicin sulfate, phenylephrine hydrochloride and benzalkonium chloride tested positively significantly more often in APD but not in NAPD, verifying them as true ophthalmic allergens. Finally, in 42 cases (4%) of APD patients, additional allergens were identified by testing of the patients' own substances (mostly beta-blockers, oxybuprocaine and dexpanthenol), supporting the necessity of testing with ophthalmic drugs as is where individual substances are not readily available.