Sodium Sulphite (na2so3): Structure, Properties, Uses

The sodium sulfite or sodium sulfite, the chemical formula Na 2 SO 3 , is a soluble sodium salt is obtained as reaction product of sulfurous acid (or sulfur oxide (IV)) with sodium hydroxide .

Between 1650 and 1660, Glauber began to manufacture sodium sulfite from common salt (NaCl) and concentrated sulfuric acid . This process is considered as the beginning of the chemical industry.

The sulfite process produces wood pulp, which is observed as almost pure cellulose fiber by using various salts of sulfurous acid to extract the lignin from wood chips.

Thus, sulfites have a large number of applications of different types, including in the food industry as an additive. Its most relevant functions include its ability to inhibit enzymatic and non-enzymatic browning, the control and inhibition of microbial growth, the prevention of oxidative rancidity and the modification of the rheological properties of food.

Preparation of sodium sulfite

Generally, on a laboratory scale, sodium sulfite is produced from the reaction of sodium hydroxide solution with sulfur dioxide gas (2NaOH + SO 2 → Na 2 SO 3 + H 2 O).

Then the evolution of SO 2 by adding a few drops of concentrated hydrochloric acid will indicate if the sodium hydroxide is almost disappeared, converted into aqueous sodium sulfite (Na 2 SO 3 + 2HCl → 2NaCl + SO 2 + H 2 O).

On the other hand, this chemical compound is obtained industrially by reacting sulfur dioxide with a sodium carbonate solution.

The initial combination generates sodium bisulfite (NaHSO 3 ), which then reacts with sodium hydroxide or sodium carbonate is converted into sodium sulfite. These reactions can be summarized in the global reaction SO 2 + Na 2 CO 3 → Na 2 SO 3 + CO 2 .

Chemical structure

All forms of sodium sulfite are characterized by being white, crystalline and hygroscopic solids, which have the ability to easily attract and retain water molecules from the surrounding environment, which are normally at room temperature.

The type of crystalline structure is linked to the presence of water in the compound. Anhydrous sodium sulfite has an orthorhombic or hexagonal structure and, in the event of the presence of water molecules in the compound, its structure changes (for example, sodium sulfite heptahydrate has a monoclinic structure).


This species has certain physical and chemical properties that differentiate it from other salts, which are described below:

Chemical properties

As a saturated aqueous solution, this substance has a pH of about 9. Furthermore, solutions exposed to air eventually oxidize to sodium sulfate.

On the other hand, if the sodium sulfite is allowed to crystallize from the aqueous solution at room temperature or lower, it does so as a heptahydrate. Heptahydrate crystals blooming in hot, dry air, they also oxidize in air to form sulfate.

In this sense, the anhydrous form is much more stable against oxidation by air. Sulfite is incompatible with acids, strong oxidants, and high temperatures. It is also insoluble in ammonia and chlorine.

Physical properties

Anhydrous sodium sulfite has a molar mass of 126.43 g / mol, a density of 2.633 g / cm 3 , a melting point of 33.4 ° C (92.1 ° F or 306.5 K), a boiling point of 1,429 ° C (2,604 ° F or 1,702 K), and it is not flammable. Also, the solubility (measured at a temperature of 20 ° C) is 13.9 g / 100 ml.


Due to its reactive properties, sodium sulfite is very versatile and is currently and widely used in different types of industries.

-It is widely used in water treatment and elimination of dissolved oxygen in boiler water.

-It also has applications in the paper industry (semi-liquid pulp).

-In photography it is used in the manufacture of developers.

-To an adequate degree it is used in food preservation and antioxidant.

-In the textile industry it is used in bleaching and antichloro processes.

-It is also used as a reducing agent.

-In addition, it is used in the secondary recovery of oil wells.

-It is even used in the manufacture of organic compounds, colorants, inks, viscose rayon and rubbers.

-It is used in the manufacture of numerous chemicals, including potassium sulfate, sodium sulfite, sodium silicate, sodium hyposulfite, and sodium aluminum sulfate.


Effects of exposure to the compound

Prolonged or repeated exposure to this substance can cause dermatitis and sensitivity reactions. Exposure to sulfite-sensitive, asthmatic, and atopic individuals can cause severe bronchoconstriction and reduce levels of forced expiratory volume.

Similarly, the acidic decomposition of sodium sulfite can release toxic and dangerous fumes of sulfur oxides, including sulfur dioxide, which can cause permanent lung deterioration due to chronic and acute exposure.

Similarly, acute sulfur dioxide poisoning is rare because the gas is easily detected. It is so irritating that contact cannot be tolerated.

Symptoms include coughing, hoarseness, sneezing, watery eyes, and shortness of breath. However, employees with unavoidable high exposures can suffer significant and possibly fatal lung damage.


Sodium sulfite is a non-hazardous solution, which is commonly used as a wastewater dechlorination agent. High concentrations contribute to the high demand for chemical oxygen in aquatic environments.

Consumption of foods with preservatives

One of the additives that can cause problems in sensitive people is the group known as sulfiting agents, which include various inorganic sulfite additives (E220-228), including sodium sulfite (SO 2 ).

In hypersensitive or asthmatic people, the consumption of foods with sulfites or the inhalation of sulfur dioxide, can be toxic.

These compounds are responsible for broncho-constriction that results in shortness of breath. The only treatment for this overreaction is to avoid foods and beverages that contain sulfites.


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