What Is Sodium Hyposulfite?
Sodium hyposulfite is the sodium salt of dithionite.
It is also called “sodium dithionite,” “sodium dithionite,” or “sodium hydrosulfite.” When simply called dithionite, it often refers to sodium hyposulfite or the dithionite ion obtained by dissolving it.
When sodium hyposulfite powder comes in contact with a small amount of water in the air, the heat produced by decomposition can cause it to ignite.
Uses of Sodium Hyposulfite
Sodium hyposulfite, like sodium sulfite, is often used as a food additive in food processing. The most common use is as an antioxidant in wine and other beverages.
It is also used as a bleaching agent and an anti-discoloration agent because of its easy solubility in water and strong reducing action. It can also be used to improve the color tone of food products and make undesirable pigmented or colored substances in raw materials colorless.
Half of the world’s sodium hyposulfite is used to dye and bleach textiles, and one-third is used to bleach pulp and paper. Sodium hyposulfite is industrially purified by the reaction of a sodium salt with sulfur dioxide mediated by metallic zinc.
Properties of Sodium Hyposulfite
Sodium hyposulfite is a white crystal at room temperature. It has a pungent odor similar to that of sulfurous acid gas. It has a flash point of 100°C and an ignition point of 200°C.
Sodium hyposulfite is slightly soluble in ethanol and soluble in water. Its melting point is 52°C. It decomposes into sodium sulfate and sulfur dioxide when heated above 90°C in air. In the absence of air, it decomposes violently to sodium sulfite, sodium thiosulfate, sulfur dioxide, and trace amounts of sulfur at temperatures above 150°C.
Structure of Sodium Hyposulfite
The chemical formula of sodium hyposulfite is Na2S2O4, with a molar mass of 174.107 and a density of 2.19 g/cm3. The anhydrous form of sodium hyposulfite is a white monoclinic crystal. The dihydrate is also known and is a yellowish columnar crystal, but is easily dehydrated to an anhydrous form. Furthermore, the dihydrate is unstable because it is easily oxidized by oxygen in the air.
The anhydride of sodium hyposulfite has a C2 symmetric structure with a torsional angle of 16° and an overlapping conformation. The dihydrate, on the other hand, has a Gauche conformation with a torsion angle of 56°.
Other Information on Sodium Hyposulfite
Synthesis of Sodium Hyposulfite
1. Synthesis by Zinc Dust Method
Zinc powder is suspended in water and passed through sulfur dioxide, which dissolves the zinc into zinc dithionite. By adding sodium carbonate or sodium hydroxide, zinc is precipitated as a white precipitate of zinc hydroxide (II). After concentrating under reduced pressure, sodium chloride and methanol are added to precipitate anhydrous sodium hyposulfite.
2. Synthesis by the Soda Formate Method
The sodium formate method was put to practical use by Mitsubishi Gas Chemical Company, Inc. When sodium formate is dissolved in 80% methanol and sodium hydroxide and sulfur dioxide are added, anhydrous sodium hyposulfite precipitates. The advantage of this method over the zinc dust method is that it is less expensive than the zinc dust method, since sodium formate is obtained as a byproduct in the production of polyhydric alcohols.
3. Synthesis by Amalgamation Method
Sodium amalgam prepared in a brine electrolyzer is brought into contact with an aqueous solution of sodium hydrogen sulfite and reduced to yield sodium hyposulfite.
4. Synthesis by the Sodium Borohydride Method
Sodium hyposulfite is produced by adding sulfur dioxide and sodium hydroxide to sodium borohydride, which is a stable reducing agent in strong alkaline solutions.
5. Synthesis by Electrolysis
Sodium hyposulfite is produced by reducing sulfite ions in an electrolytic cell separated by a semipermeable membrane.