SODIUM BISULFITE (NAHSO3): STRUCTURE, PROPERTIES, USES, OBTAINING - CHEMISTRY - 2025

The sodium bisulfite is an inorganic solid consisting of a sodium ion Na ⁺ and bisulfite ion HSO ₃ ^-. Its chemical formula is NaHSO ₃. It is a white crystalline solid and due to its antioxidant properties it is widely used as a food preservative (for example in some jams).

NaHSO ₃ is a reducing chemical compound, which is the opposite of oxidant, and for this reason it acts as an antioxidant in many of its applications, such as in edible products, as it prevents their deterioration.

Some commercial jams contain sodium bisulfite NaHSO ₃. Author: OpenClipart-Vectors. Source: Pixabay.

However, this use has been questioned as cases of asthma have been reported in people after they have eaten foods that contain sodium bisulfite. It is even said to destroy vitamin B1 in these.

International health organizations have restricted its use to micro amounts in food.

However, sodium bisulfite has many other uses, such as in obtaining iodine, as an anti-infective, to whiten fabrics, for the digestion of wood during the preparation of paper pulp, as a disinfectant for beer and wine barrels, etc.

Chemical structure

Sodium bisulfite is made up of a sodium Na ⁺ cation and a HSO ₃ ^- bisulfite anion.

Chemical structure of sodium bisulfite NaHSO ₃. Edgar181. Source: Wikimedia Commons.

In aqueous solutions bisulfite forms 4 species. In dilute solution there is a balance between the structures shown below:

Structures of the bisulfite ion HSO ₃ ^- in dilute aqueous solution. Author: Marilú Stea.

When the concentration increases, two bisulfite molecules interact with each other, forming the pyrosulfite ion S ₂ O ₅ ^2-:

Structures of the bisulfite ion in concentrated aqueous solution. Author: Marilú Stea.

Nomenclature

-Sodium bisulfite

-Sodium hydrogensulfite

-Sodium hydrosulfite.

Properties

Physical state

Crystalline white solid.

Molecular weight

104.06 g / mol

Melting point

It decomposes.

Density

1.48 g / cm ³ at 20 ° C.

Solubility

Soluble in water: 29 g / 100 g of water.

pH

Its solutions are acidic, with a pH between 2.5 and 5.5.

Chemical properties

In aqueous solution, sodium bisulfite NaHSO ₃ separates into its ions: the sodium cation Na ⁺ and the bisulfite anion HSO ₃ ^-.

If sodium bisulfite is exposed to air it loses some SO ₂ and slowly oxidizes to Na ₂ SO ₄ sulfate.

If heated to decomposition, it emits fumes of sulfur oxides and sodium monoxide.

It is a reducing agent, which is the opposite of oxidant. And for this reason it can behave as an antioxidant.

It has a slight smell of sulfur. It is not flammable.

Risks

Sodium bisulfite NaHSO ₃ is a strong skin and tissue irritant. Dust irritates the eyes, nose and throat. Ingestion irritates the stomach. Large doses can cause violent colic, diarrhea, depression, and death.

It is not combustible but if heated it produces irritating and toxic gases.

Obtaining

It is prepared by saturating a solution of sodium carbonate Na ₂ CO ₃ with sulfur dioxide SO ₂ and crystallizing from the solution.

Applications

In the pulp and paper industry

NaHSO ₃ is used in the digestion of wood, so that it can later be converted into paper. It also serves as a pulp bleach.

It is also used to remove chlorine when paper pulp has been bleached with it.

In the food industry

Sodium bisulfite, due to its reducing property, acts as an antioxidant.

It is used as a preservative in a large number of foods and beverages, including wine and beer, to prevent spoilage and improve flavor.

It is used to whiten foods, such as certain edible starches.

It reduces or prevents food spoilage, allows conditioning of the dough used in baked goods and serves to soften corn kernels during wet milling, among other uses.

In addition to controlling the fermentation of wine and beer, it acts as an antiseptic in its production as it serves as a sterilant and fungicide in barrels and vats.

Wine or beer kegs are sometimes disinfected with NaHSO ₃ sodium bisulfite. Author: Clker-Free-Vector-Images. Source: Pixabay.

In dried fruits it is found in concentrations above 100 ppm (ppm means "parts per million"), in many other foods it is found in concentrations between 10 and 100 ppm, such as frozen and dried potatoes, pickles, sauces and jams.

Commercial pickled foods often contain sodium bisulfite NaHSO ₃. Author: Photo Mix. Source: Pixabay.

Controversy over its use in food

There has been some concern about both the immediate and long-term toxic effects of sodium bisulfite in food. There are divergent opinions.

There are reports of adverse reactions in patients with asthma, which is a respiratory disease, after ingestion of food with NaHSO ₃. Other sources indicate that the bisulfite ion can destroy vitamin B1 or thiamine.

Little girl snorting a medicine to relieve asthma. Author: OpenClipart-Vectors. Source: Pixabay.

However, researchers in 1985 found that sodium bisulfite serves to protect against radiation and chemical agents, and inhibits or prevents the transformation of cells towards cancer. Low doses are the best.

This protective effect has been explained based on its reducing or antioxidant properties and that in this way it could attack free radicals.

On the other hand, the World Health Organization, or WHO (for its acronym in English World Health Organization), recommends as an acceptable level of daily intake about 0.7 mg / Kg of the person's weight.

That means it is recommended not to exceed that amount.

The US Food and Drug Administration, or USFDA, has classified sodium bisulfite as "generally recognized as safe."

In agricultural applications

Used at low concentrations, NaHSO ₃ serves as a plant growth regulator, to improve photosynthesis and promote crop yield.

It has been tested for this purpose in various types of plants, such as strawberry and tea plants.

Strawberry plants improve their performance with small amounts of NaHSO ₃ in irrigation water. Author: Alyssapy. Source: Pixabay.

It is the active ingredient in many pesticides and biocides.

In aquatic environments it can also accelerate the transformation of certain herbicides to less toxic products by removing chlorine.

In obtaining iodine

NaHSO ₃ is the reducing compound used to liberate iodine from sodium iodate NaIO ₃. This is one of the ways to obtain iodine from sources such as nitro from Chile or some saline waters.

After crystallizing the sodium nitrate from the crude Chile nitrate solution, a NaIO ₃ solution remains which is treated with sodium bisulfite NaHSO ₃, generating free iodine.

2 NaIO ₃ + 5 NaHSO ₃ → 3 NaHSO ₄ + Na ₂ SO ₄ + I ₂

In the furniture and wood industry

NaHSO ₃ has been tested to modify soy proteins and improve their adhesion properties with wood in order to glue pieces of wood together to prepare for example chipboards, cardboard or papier-mache, plywood, etc. All this for furniture or boards, among various applications.

Agglomerate of wood chips or remains. Author: Titus Tscharntke. Source: Wikimedia Commons.

This is done to replace traditional formaldehyde-based adhesives because both in their manufacturing process and during use they tend to release formaldehyde into the atmosphere, which is a toxic compound.

Sodium bisulfite improves the solids content of the soy protein adhesive, and reduces its viscosity, improving its fluidity so that it better penetrates the pores of the wood, increasing its cohesion with it and between the pieces.

The NaHSO ₃ modified soy protein adhesive has good water resistance and excellent storage stability due to the antioxidant action of sodium bisulfite.

This confirms that NaHSO ₃ modified soy protein adhesives are comparable to formaldehyde-based adhesives and can be used in the furniture and wood industry, being less polluting.

In various applications

It has multiple uses, generally based on its reducing properties (which is the opposite of oxidant). Here are some applications.

-In photography.

-In the leather tanning, to remove the hair from the hides.

-Therapeutic uses: anti-infective. It is used as an antioxidant in some eye drops.

-In the extraction of natural gas by hydraulic fracturing.

-In the coloring of fibers it is used for the preparation of hot or cold baths, to solubilize certain dyes or colorants.

-As a reducer in laundry or bleaching, to whiten wool, silk and vegetable fibers.

-In chemistry laboratories to remove permanganate stains from skin and clothing. In biochemistry laboratories as a preservative for liquids or solutions that can deteriorate over time. As a chemical reagent.

-For the coagulation of rubber latex.

-In the cosmetic industry as an antioxidant.

-Disinfectant and bleach.

-In the treatment of wastewater to eliminate the chlorine used in the final stage of disinfection and to discharge the already treated water into the environment.

Sodium bisulfite tank in a California wastewater treatment plant, where it is used to remove excess chlorine before purified water is discharged to the environment. Grendelkhan. Source: Wikimedia Commons.

References

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3. Borek, C. et al. (1985). Sodium bisulfite protects against radiogenic and chemically induced transformation in hamster embryo and mouse C3H / 10T-1/2 cells. Toxicol Ind Health 1985Sep; 1 (1): 69-74. Recovered from journals.sagepub.com.
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6. Sunnyvale Cleanwater Program. (2019). Water Pollution Control Plant Master Plan. Recovered from sunnyvalecleanwater.com.
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