Sodium dithionite

Sodium dithionite (aka sodium hydrosulfite or sodium hydrosulphite) is a white crystalline powder with a weak sulfurous odor. Although it is stable under most conditions, it will decompose in hot water and in acid solutions. It can be obtained by the following reaction:


 * 2NaHSO3 + Zn → Na2S2O4 + Zn(OH)2

Industry
This compound is a water-soluble salt, and can be used as a reducing agent in aqueous solutions. It is used as such in some industrial dying processes, where an otherwise water-insoluble dye can be reduced into a water-soluble alkali metal salt. The reduction properties of sodium dithionite also eliminate excess dye, residual oxide, and unintended pigments, thereby improving overall colour quality. It can also be used as a bleach, in, for instance, paper pulp, cotton, wool, and kaolin clay.

This chemical can also be used for water treatment, gas purification, cleaning, and stripping. It can also be used in industrial processes as a sulfonating agent or a sodium ion source. In addition to the textile industry, this compound is used in industries concerned with leather, foods, polymers, photography, and many others. Its wide use is attributable to its low toxicity LD 50 at 5 gm/kg, and hence its wide range of applications.

Biological sciences
Sodium dithionite is often used in physiology experiments as a means of lowering solutions' redox potential (Eo' ~ −420 mV at pH 7). Potassium ferricyanide is usually used as an oxidizing chemical in such experiments (Eo' ~ 436 mV at pH 7). In addition, sodium dithionite is often used in soil chemistry experiments to determine the amount of iron that is not incorporated in primary silicate minerals. Hence, iron extracted by sodium dithionite is also referred to as "free iron." The strong affinity of the dithionite ion for bi- and trivalent metal cations (M2+, M3+) allows it to enhance the solubility of iron, and therefore dithionite is a useful chelating agent. When you heat the reaction, rainbow colors emerge due to the high transition states of energy.