Sodium sulfite

Chemical properties

Sodium sulfite,Na2S03, is a white,water-soluble, crystalline solid with a sulfurous, salty taste. It decomposes when heated. Sodium sulfite is used as a source of sulfite,as a chemical intermediate and food preservative, in medicine and paper manufacturing, in photographic developing, and as a bleaching agent in the textile industry. Most boiler operators use sodium sulfite for chemicals cavenging of oxygen in the feedwater. Because it decomposes into acidic gases at the high temperatures that accompany high pressures, sodium sulfite should not be used for this purpose at pressures above 122atm (12.4MPa,or 1.8 ksi).

Chemical properties

Sodium sulfite is a white crystalline solid

Chemical properties

Sodium sulfite occurs as an odorless white powder or hexagonal prisms. Note that the commercially available sodium sulfite is often presented as a white to tan- or pink-colored powder that would not conform to the pharmacopeial specification.

Description

Sodium sulfite (sodium sulphite) is a soluble compound of sodium. It is a product of SO2 scrubbing, a part of the flue gas desulfurization process. It is also used as a preservative to prevent dried fruit from discoloring, and for preserving meats, and is used in the same way as sodium thiosulfate to convert elemental halides to their respective acids, in photography and for reducing chlorine levels in pools.

Chemical properties

Sodium sulfite is decomposed by even weak acids, giving up sulfur dioxide gas.

Na2SO3 + 2 H+ → 2 Na+ + H2O + SO2

A saturated aqueous solution has pH of ~9. Solutions exposed to air are eventually oxidized to sodium sulfate.

The Uses of Sodium sulfite

sodium sulfite has anti-septic, preservative, and anti-oxidant properties. Sodium sulfite is also a topical anti-fungal.

The Uses of Sodium sulfite

Paper industry (semichemical pulp), reduc- ing agent (dyes), water treatment, photographic developer, food preservative and antioxidant, tex- tile bleaching (antichlor).

The Uses of Sodium sulfite

These white crystals were prepared by passing sulfurous gas over moist sodium carbonate. Sodium sulfite is soluble in water but less so in alcohol. It was used as a preservative for alkaline developers and as a hypo clearing agent in photographic printing.

Definition

ChEBI: An inorganic sodium salt having sulfite as the counterion.

Production Methods

Sodium bisulfite is prepared by reacting sulfur dioxide gas with sodium hydroxide solution. The solid material is obtained by evaporation of water. Further neutralization with sodium hydroxide while keeping the temperature above 33.6°C leads to crystallization of the anhydrous sodium sulfite (below this temperature the heptahydrate form is obtained).

Definition

sodium sulphite: A white solid,Na₂SO₃, existing in an anhydrousform (r.d. 2.63) and as a heptahydrate(r.d. 1.59). Sodium sulphite is solublein water and because it is readily oxidizedit is widely used as a convenientreducing agent. It is preparedby reacting sulphur dioxide with eithersodium carbonate or sodium hydroxide.Dilute mineral acids reversethis process and release sulphur dioxide.Sodium sulphite is used as ableaching agent in textiles and inpaper manufacture. Its use as an antioxidantin some canned foodstuffsgives rise to a slightly sulphuroussmell immediately on opening, butits use is prohibited in meats or foodsthat contain vitamin B1. Sodium sulphitesolutions are occasionally usedas biological preservatives.

General Description

White odorless powder. Density 2.633 g / cm3. Moderately toxic. Sinks in water and dissolves slowly. Also transported as a heptahydrate Na2SO3.7H2O.

Air & Water Reactions

Soluble in water. Reacts with hot water, steam or acids to produce corrosive material.

Reactivity Profile

Sodium sulfite is a reducing agent, particularly under basic conditions. Reacts with oxidizing agents such as peroxides, epoxides, oxoacids. Emits toxic fumes of sodium oxide and oxides of sulfur if heated to decomposition [Lewis, 3rd ed., 1993, p. 1174].

Hazard

Use prohibited in meats and other sources of Vitamin B 1

Health Hazard

When ingested, solutions cause gastric irritation by the liberation of sulfurous acid. Because of rapid oxidation to sulfate, sulfites are well tolerated until large doses are reached; then violent colic and diarrhea, circulatory disturbances, central nervous depression, and death can occur.

Fire Hazard

Literature sources indicate that Sodium sulfite is noncombustible.

Pharmaceutical Applications

Sodium sulfite is used as an antioxidant in applications similar to those for sodium metabisulfite. It is also an effective antimicrobial preservative, particularly against fungi at low pH (0.1% w/v of sodium sulfite is used). Sodium sulfite is used in cosmetics, food products, and pharmaceutical applications such as parenteral formulations, inhalations, oral formulations, and topical preparations.

Contact allergens

Sodium sulfite is mainly used in photographic developers, for fixing prints, bleaching textile fibers, as a reducer in manufacturing dyes, as a remover of Cl in bleached textiles and paper, and as a preservative in the food industry for meat, egg yolks, and so on.

Safety Profile

Poison by intravenous and subcutaneous routes. Moderately toxic by ingestion and intraperitoneal routes. Human mutation data reported. When heated to decomposition it emits very toxic fumes of Na2O and SOx. A reducing agent. See also SULFITES.

Safety

Sodium sulfite is widely used in food and pharmaceutical applications as an antioxidant. It is generally regarded as relatively nontoxic and nonirritant when used as an excipient. However, contact dermatitis and hypersensitivity reactions have been reported. The acceptable daily intake for sodium sulfite has been set at up to 350 mg/kg body-weight daily.
LD50 (mouse, IP): 0.950 g/kg
LD50 (mouse, IV): 0.130 g/kg
LD50 (mouse, oral): 0.820 g/kg
LD50 (rabbit, IV): 0.065 g/kg
LD50 (rabbit, oral): 1.181 g/kg
LD50 (rat, IV): 0.115 g/kg

Potential Exposure

Sodium sulfite is used as a reducing agent; in boiler water treatment; food applications; in photographic developers and fixers; in bleaching of wool, paper, textiles, straw and silk; manufacture of dyes; dechlorination; preservation of meat, fruit and egg products; silvering of glass.

Environmental Fate

Because sodium sulfite is a solid powder that is generally sold as a ‘food grade’ substance, there is very little information available on the environmental fate of sodium sulfite. It has a molecular weight of 126.04 g mol^-1. At 20°C, the solubility in water is 250 000 mg l^-1, and as it is an inorganic salt the vapor is negligible (USEPA, 2011). The log octanol/water partition coefficient is estimated to be
7.78.
If released into water or soil, sodium sulfite would most likely be oxidized to sulfate, which would then be available for use by bacteria or plants as a nutrient.
The US Environmental Protection Agency (EPA) EPI Suite computer program (USEPA, 2011) estimates both a bioconcentration factor and a bioaccumulation factor of 0.89. These factors are less than one, meaning that bioconcentration and/or bioaccumulation of sodium sulfite would be virtually negligible and therefore would not cause any adverse effects to fish and wildlife.

Health hazards

Exposure to sodium sulfite, especially powder, can cause irritation to the skin, eyes, and respiratory tract. If inhaled, sulfites may cause sensitization (develop an allergic reaction). Breathing sulfite dusts may aggravate asthma or other pulmonary (breathing) diseases and may cause headaches, breathing difficulties, or heart irregularity. Ingestion may cause gastrointestinal irritation, nausea, vomiting and diarrhea. Overexposure may result in death.

storage

Sodium sulfite should be stored in a well-closed container in a cool, dry, place. In solution, sodium sulfite is slowly oxidized to sulfate by dissolved oxygen; strong acids lead to formation of sulfurous acid/ sulfur dioxide. On heating, sodium sulfite decomposes liberating sulfur oxides.

Shipping

UN3260 Corrosive solid, acidic, inorganic, n.o.s., Hazard class: 8; Labels: 8-Corrosive material.

Purification Methods

Crystallise the sulfite from warm water (0.5mL/g) by cooling to 0o. Also purify it by repeated crystallisation from deoxygenated water inside a glove-box, and finally drying it under vacuum. [Rhee & Dasgupta J Phys Chem 89 1799 1985.]

Toxicity evaluation

The exact mechanism of toxicity has not been elucidated, although there is a lot of information on how sulfur-based compounds are detoxified by the liver. Sodium sulfite is a mild reducing agent that would most likely cause burning or irritation at the site of exposure or application by altering oxidation–reduction potential and pH.
Sulfites are used widely as antioxidants to keep foods from prematurely spoiling and to keep them looking ‘fresh’ by preventing oxidation and subsequent ‘browning.’ Many people, however, are ‘sulfite sensitive.’ After ingestion of food or beverages containing sulfite, these people may have allergictype reactions such as asthmatic wheezing, hypotension, tingling sensations, and flushing of the skin. The mechanism is unclear but probably has to do with an individual-specific chemical stimulation of the immune system, which in turn releases small amounts of vasoactive substances.

Incompatibilities

A strong reducing agent. Incompatible with oxidizers (chlorates, nitrates, peroxides, permanganates, perchlorates, chlorine, bromine, fluorine, etc.); contact may cause fires or explosions. Keep away from alkaline materials, strong bases. Reacts with strong acids producing toxic sulfur dioxide.

Incompatibilities

Sodium sulfite is incompatible with acids, oxidizing agents, many proteins, and vitamin B1.

Regulatory Status

GRAS listed. Accepted for use as a food additive in Europe. Included in FDA Inactive Ingredients Database (epidural, IM, IV, and SC injections; inhalation solution; ophthalmic solutions; oral syrups and suspensions; otic solutions; topical creams and emulsions). Included in nonparenteral medicines licensed in the UK.