Sodium Sulfite- Na2SO3

Sodium sulfite (Na₂SO₃) is a white, water-soluble, crystalline solid with a sulphurous, salty taste. It decomposes when heated. It is generally available in powder, crystalline, and tablet forms.

Sodium sulfite is yet another dechlorinating agent widely used by utilities. Sodium sulphite is slightly alkaline in nature. Sodium sulfite is a reducing agent and is reported to scavenge more oxygen than sodium thiosulfate.

The food industry used sodium sulfite to assist in the preservation of the new appearance of food products. In many drugs, it is also an element that helps to preserve their potency and stability. Sodium sulfite has been approved by the FDA as a fresh low regulatory priority animal drug.

Table of Content

• What is Sodium Sulfite?
• Structure of Sodium Sulfite- Na₂SO₃
• Chemical data on Sodium Sulfite
• Preparation of Sodium Sulfite
• Physical Properties of Sodium Sulfite
• Chemical Properties of Sodium Sulfite
• Uses of Sodium Sulfite
• Health Hazards of Sodium Sulfite
• FAQs

What is Sodium Sulfite?

Sodium sulfite is an inorganic salt with the chemical formula Na₂SO₃. It is an ionic compound containing two sodium cations (Na⁺) and one sulfite anion (SO₃^2-). A saturated solution of sodium sulfite in water is mildly basic with an approximate pH value of 9. Such a solution can undergo crystallisation to yield heptahydrate crystals of Na₂SO₃.

Sodium Sulfite Powder

In its anhydrous form (image provided above), sodium sulfite is a white solid. The primary difference between anhydrous Na₂SO₃ and its heptahydrate is the relative stability of the anhydrous form towards oxidation. Na₂SO₃.7H₂O is slowly oxidised by atmospheric oxygen, giving rise to the corresponding sulphate.

Structure of  Sodium Sulfite- Na₂SO₃

In a sodium sulfite molecule, there exist two ionic bonds between the sodium cations and the sulfite anion. The structure of a Na₂SO₃ molecule is illustrated below.

Sodium Sulfite Structure

Each sodium ion holds a charge of +1. On the other hand, the -2 charge on the sulfite ion is delocalized due to resonance, resulting in a partial charge of -⅔ on each oxygen atom. The overall charge on a Na₂SO₃ molecule is zero.

Chemical data on Sodium Sulfite

Preparation of  Sodium Sulfite

1. In laboratories, sodium sulfite is generally prepared from the reaction between gaseous sulphur dioxide (SO₂) and sodium hydroxide (NaOH). The chemical equation for this reaction is given by

SO₂ + 2NaOH → Na₂SO₃ + H₂O

The depletion of the NaOH reactant can be detected via the addition of a few drops of concentrated H₂SO₄, resulting in the liberation of SO₂ gas.

2. Industrially, Na₂SO₃ is produced from the reaction between sulphur dioxide and sodium carbonate solution. Initially, sodium bisulfite (NaHSO₃) is formed. This compound now reacts with sodium carbonate or sodium hydroxide to yield the sodium sulfite product. The reaction can be generalised to:

Na₂CO₃ + SO₂ → Na₂SO₃ + CO₂

Some important physical and chemical properties of sodium sulfite are listed in this subsection.

Physical Properties of Sodium Sulfite

• The molar mass of this compound is 126.043 grams per mole.
• Anhydrous sodium sulfite exists as a white, odourless solid which has a density of 2.633 grams per cubic centimetre.
• The heptahydrate form has a relatively lower density of 1.561 g/cm³.
• When heated to 306.5K (33.4^oC), the heptahydrate undergoes dehydration. The anhydrous form melts at a temperature of 500 ^oC.
• Sodium sulfite does not have a specific boiling point since it tends to decompose at high temperatures.
• It is moderately soluble in water, its solubility corresponds to 27g/100mL.
• The crystal structure of anhydrous Na₂SO₃ is hexagonal whereas the heptahydrate crystals have a monoclinic structure.

Chemical Properties of Sodium Sulfite

• Upon contact with strong or weak acids, Na₂SO₃ undergoes decomposition, liberating gaseous sulphur dioxide.
• Sodium sulfite reacts with aldehydes to yield a bisulfite adduct. However, sulfonic acids are produced from their reaction with ketones.
• Solutions of sodium sulfite are oxidised by atmospheric oxygen, yielding sodium sulphate.
• This compound is insoluble in ammonia and chlorine.

Uses of Sodium Sulfite

Although its primary applications lie in the pulp and paper industry, Na₂SO₃ has numerous other applications. A few such applications are listed below.

• This compound is often used to prevent the discolouration of dried fruits since it can act as a preservative.
• It is also used in swimming pools to decrease the chlorine levels in the water.
• In boiler systems, this compound acts as an oxygen scavenger to protect the system from pitting corrosion. Therefore, the water fed to boilers is often treated with sodium sulfite.
• The textile industry makes extensive use of Na₂SO₃ for bleaching, dechlorinating and desulfurizing purposes.
• Sodium sulfite is also used in the process of purifying trinitrotoluene (TNT) in order to make it fit for military use.
• This compound is also an important component in the preparation of sodium thiosulfate.

Sodium Sulfite Health Hazards

Sulfites are compounds containing the sulfite ion, most often in combination with sodium (sodium sulfite) or potassium (potassium sulfite). Sulfites release the irritant gas sulphur dioxide, which acts as a preservative and bleaching agent. As well as occurring naturally in some foods and in the human body, sulfites are added to certain foods to act as a preservative as they inhibit microbial growth, maintain food colour and increase shelf life.

Foods to which sulfites are commonly added include wines, beer and dried fruit. They are also used to bleach food starches, such as potato starch and are used in the production of some food packaging materials such as cellophane.

Allergenicity of Sodium Sulfite

It is still unclear why sulfites elicit an allergic reaction in some people but not in others. Sulphur dioxide is an irritant gas and so reflex contraction of the airways has been proposed as one possible mechanism, as the majority of sulfite allergic individuals exhibit asthma-like symptoms.

Sodium sulfite also finds use in the froth flotation process of purifying ores, extraction of petroleum, and the production of some dyes. To learn more about this compound and other ionic compounds containing sodium, such as sodium chloride, register with BYJU’S and download the mobile application on your smartphone.