Sodium Sulfite

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Sodium sulfite is defined as a white, crystalline, water-soluble solid with a sulfurous and salty taste. When heated, it decomposes, and it is generally available in crystalline, tablet and powder forms. The chemical formula of sodium sulphite is given as Na2SO3.

Sodium sulfite is yet another dechlorinating agent which can be widely used by utilities. Generally, it is available in powder form. In addition, a few companies manufacture this compound in tablet form. It is slightly alkaline in nature. Sodium sulfite compound is a reducing agent, and it is reported to scavenge more oxygen compared to sodium thiosulfate.

What is Sodium Sulfite?

Sodium sulfite is defined as an inorganic salt having the chemical formula as Na2SO3. It is an ionic compound that contains two sodium cations (Na+) and single sulfite anion (SO32-). A saturated solution of sodium sulfite present in the water is mildly basic with a nearby pH value of 9. Such a solution undergoes crystallization to yield heptahydrate crystals of Na2SO3.

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

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Sodium sulfite, in its anhydrous form, as represented above, is a white solid. The major difference between anhydrous Na2SO3 and its heptahydrate is given as the relative stability of the anhydrous form towards the oxidation process. Na2SO3.7H2O is slowly oxidized by the atmospheric oxygen by giving rise to the corresponding sulfate.

Sodium Sulfite Structure

In a sodium sulfite molecule, there are two ionic bonds between the sulfite anion and sodium cations. The structure of a Na2SO3 molecule can be represented as follows:

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Every sodium ion holds a +1 charge. On the other side, the -2 charge present on the sulfite ion is delocalized because of resonance, resulting in a partial charge of -⅔ on every oxygen atom. The overall charge on the Na2SO3 molecule is given as zero.

Sodium Sulfite Preparation

1. Sodium sulfite is generally prepared in laboratories from the reaction between sodium hydroxide (NaOH), and gaseous sulfur dioxide (SO2). The chemical equation for this reaction can be given as follows:

                                     SO2 + 2NaOH → Na2SO3 + H2O

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

2. Na2SO3 is produced on the industrial basis from the reaction between sodium carbonate and sulfur dioxide solution. Initially, sodium bisulfite (NaHSO3) compound is formed. Now, this resultant compound reacts either with sodium hydroxide or sodium carbonate to yield sodium sulfite product. The reaction can be generalized as follow:

                                      Na2CO3 + SO2 → Na2SO3 + CO2

Properties of Sodium Sulfite

A few essential physical and chemical properties of sodium sulfite are given as follows:

Physical Properties of Sodium Sulfite

  • The molar mass of Sodium Sulfite compound is given as 126.043 grams per mole.

  • Anhydrous sodium sulfite exists as odourless and white solid, which contains a density of 2.633 grams per cubic centimetre.

  • The heptahydrate form contains a relatively lower density of 1.561 g/cm3.

  • When heated, this compounds to 306.5K (33.4℃), the heptahydrate undergoes the process of dehydration. The anhydrous form melts at 500 degrees temperature.

  • Sodium sulfite compound does not contain a specific boiling point because it tends to decompose at higher temperatures.

  • Its solubility corresponds to 27g/100mL, and it is moderately soluble in water.

  • The crystal structure of anhydrous Na2SO3 compound is hexagonal, and the heptahydrate crystals contain a monoclinic structure.

Chemical Properties of Sodium Sulfite

  • Upon contact either with strong or weak acids, Na2SO3 compound undergoes decomposition, by liberating sulfur dioxide in gaseous form.

  • Sodium sulfite also reacts with aldehydes to yield a bisulfite adduct. However, the sulfonic acids are formed from its reaction with the help of ketones.

  • Sodium sulfite solutions are oxidized by the atmospheric oxygen, yielding sodium sulfate.

  • Also, this compound is insoluble in chlorine and ammonia.

Uses of Sodium Sulfite

Although the primary applications of Sodium Sulfite lie in the paper and pulp industry, Na2SO3 compound contains other numerous applications. Some of them are listed below.

  • This compound can be often used to prevent dried fruits discolouration because it acts as a preservative.

  • It can also be used in swimming pools to decrease the levels of chlorine in the water.

  • In boiler systems, the sodium sulfite acts as an oxygen scavenger in protecting the system from pitting corrosion. Thus, the water fed to broilers is often treated with the sodium sulfite.

  • The extensive use of Na2SO3 by the textile industry for dechlorinating, desulfurizing, and bleaching purposes.

  • Sodium sulfite can also be used in purifying the trinitrotoluene (TNT) process, in order to produce it fit for military use.

  • This compound is also an essential component in sodium thiosulfate preparation.

Sodium Sulfite Health Hazards

Sulfites are defined as the compounds containing the sulfite ion, where, most often in combination with potassium (potassium sulfite) or sodium (sodium sulfite). Sulfites release irritant gas sulfur dioxide, which acts as a bleaching and preservative agent. It is also used as a naturally occurring agent in some of the food items and in the human body, sulfites can be added to some foods to act as a preservative because they inhibit microbial growth, to increase shelf life and maintain food colour.

Foods to which sulfites can be commonly added include wines, beer and dried fruit. Food starches, such as potato starch, are also used for bleaching and are used in the manufacture of some food packaging products, such as cellophane.

FAQ (Frequently Asked Questions)

1. Explain the Allergenicity of Sodium Sulfite?

Answer: Still, it is unclear why the sulfites elicit an allergic reaction to a few people but not in everyone. Sulfur dioxide is given as an irritant gas, and so, the reflex contraction of the airways has been proposed as a possible mechanism, because the majority of sulfite allergic individuals exhibit symptoms such as asthma.

2. Is Sodium Sulfite Ionic or Covalent?

Answer: Sodium sulfite is defined as an ionic compound that crystallizes in a hexagonal crystal lattice in its anhydrous form. However, this specific compound crystallizes in a monoclinic lattice in its heptahydrate form.

3. Explain Whether Sodium Sulfite is an Acid or a Base?

Answer: Sodium sulphite, with the chemical formula Na2SO3, is defined as a salt that is made from the neutralization reaction between a weak acid (sulphurous acid) and a strong base (sodium hydroxide). Therefore, its aqueous solution is distinctly basic in nature, having a pH value slightly greater than 7.

4. How to Prepare Sodium Sulfite?

Answer: Sodium sulfite is prepared by the treatment with the sulfur dioxide of a sodium hydroxide solution. Initially, when performed in warm water, the Na2SO3 precipitates as yellow solid. This solid dissolves with more SO2 to produce the disulfite, which crystallizes as it cools down.