Sodium Hydrosulfite (Sodium Dithionite)

Sodium hydrosulphite is a free-flowing, dry, white material available in a variety of grades, each with a different level of sodium dithionite. Sodium hydrosulfite finds applications as a reducing agent by chemically reducing other components by donating an electron or electrons, sulfonating agent by adding sulfur to another chemical compound, and as a cation source for textile, pulp and paper, leather, foods and beverages, polymers, chemical processing, water treatment, gas purification and environmental improvement.

Sodium hydrosulfite is prepared by the reaction of sodium bisulfite with zinc, or by the action of sulfur dioxide on sodium amalgam. Sodium hydrosulphite is the common commercial name for products containing sodium dithionite, Na2S2O4, as the active ingredient.

Sodium dithionite is a powerful, inexpensive, safe and readily available reducing agent. It has been used for more than 70 years in the reduction of aromatic nitro compounds, diazonium salts, a variety of pyridinium compounds, some complex oximes and other nitrogen-containing functional groups.

Sodium dithionite is primarily used as reducing agent for the reduction of vat dyes and sulfur containing dyes. The purpose of this reduction reaction is to form the leuco components of the mentioned dyes, which are soluble in water and have a great affinity for the textile fabric. Sodium dithionite is also used to optimise the colour fastness, which can be achieved by a reductive decomposition of the excess of dye stuff that is adsorbed at the surface of the textile (especially used for polyester fibres). Other applications of sodium dithionite as reducing agent can be found in the removal of pigments on textile that has been dyed in a wrong way and in the reduction of residual hydrogen peroxide (after bleaching) as a pre-treatment for the dye process with reactive dyes.

Besides its use as reducing agent, sodium dithionite is also used as a bleaching agent in reductive bleaching processes. Examples are bleaching of mechanical paper pulp and the bleaching of cotton and wool. It especially reduces carbonyl and alcohol functional groups, which are responsible for the colour of the textile fabric. Unfortunately these processes are not 100 % effective. Therefore a reductive bleaching process is often preceded by an oxidative one (with hydrogen peroxide, sodium hypochlorite or sodium chlorite).

Sodium dithionite is relatively unstable and tends to react with oxygen. This means that in textile processes where sodium dithionite is involved it is common practice to use a large excess of sodium dithionite to be sure that the concerned treatment is quantitative. Because sodium dithionite itself and its reaction products (sulphite and sulphate) are important pollutants, highly polluted waste water is obtained, containing no dissolved oxygen. A method to measure the concentration of sodium dithionite on line could result in a more efficient use of sodium dithionite and less waste. Measurement and control of the concentration of sodium dithionite also would have a positive influence on the quality of the textile fabric.

Sodium dithionite is a powerful reducing agent, and when applied to an iron-fouled resin bed, will reduce any ferric iron present to the soluble ferrous form. The bed can be freed from iron during a normal aqueous cycle. Sodium dithionite will decompose under the influence of heat or moisture. For this reason, the chemical should be kept in sealed watertight containers and stored in a cool, dry place. This way the material can be stored over a prolonged period with negligible loss in activity. Care should be exercised in handling sodium dithionite since, on contact with water, it decomposes quite rapidly, forming gases that can ignite spontaneously.

For this reason, sodium hydrosulfite or sodium dithionite is classified as a flammable solid and is shipped under the appropriate caution label. Any material that is spilled should be promptly cleaned up and the site washed with plenty of water. Partially used containers represent a fire hazard.

Synonyms: sodium dithionite, sodium hydrosulphite, sodium sulphoxylate
Molecular formula: Na2S2O4
CAS No: 7775-14-6