Sodium Sulfite

Sodium sulfite is yet another dechlorinating agent widely used by utilities. It is generally available in powder form. In addition, some companies manufacture in tablet form. Sodium sulfite is slightly alkaline in nature. Sodium sulfite is a reducing agent and is reported to scavenge more oxygen than sodium thiosulfate.

Food industry used sodium sulfite to assist 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.

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 (SO32-). A saturated solution of sodium sulfite in water is mildly basic with an approximate pH value of 9. Such a solution can undergo crystallization to yield heptahydrate crystals of Na2SO3.

Sodium Sulfite

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 oxidized by atmospheric oxygen, giving rise to the corresponding sulfate.

Sodium Sulfite Structure

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

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.

Sodium Sulfite Preparation

1. In laboratories, sodium sulfite is generally prepared from the reaction between gaseous sulfur 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 sulfur 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 generalized to:

Na2CO3 + SO2 → Na2SO3 + CO2

Properties of Sodium Sulfite

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 (33.4oC), the heptahydrate undergoes dehydration. The anhydrous form melts at a temperature of 500o
  • 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 sulfur dioxide.
  • Sodium sulfite reacts with aldehydes to yield a bisulfite adduct. However, sulfonic acids are produced from its reaction with ketones.
  • Solutions of sodium sulfite are oxidized by atmospheric oxygen, yielding sodium sulfate.
  • 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 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.

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