sodium hydrosulfite

Sodium dithionite
Other names	D-Ox Hydrolin Reductone Sodium hydrosulfite Sodium sulfoxylate Sulfoxylate Vatrolite Virtex L
Identifiers
CAS number	[7775-14-6]
PubChem	24489
EC number	231-890-0
RTECS number	JP2100000
Properties
Molecular formula	Na2S2O4
Molar mass	174.107 g/mol
Appearance	white to grayish crystalline powder
Density	2.19 g/cm3, solid
Melting point	52 °C (325 K)
Boiling point	Decomposes
Solubility in water	very soluble
Hazards
EU Index	016-028-00-1
EU classification	Harmful (Xn)
R-phrases	R7, R22, R31
S-phrases	(S2), S7/8, S26, S28, S42
NFPA 704	3 2 1
Flash point	100 °C
Autoignition temperature	200 °C
Related compounds
Other anions	Sodium sulfite Sodium sulfate
Related compounds	Sodium thiosulfate Sodium bisulfite Sodium metabisulfite Sodium bisulfate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Sodium dithionite (also known as sodium hydrosulfite) 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 from sodium bisulfite by the following reaction:^[1]

2 NaHSO₃ + Zn → Na₂S₂O₄ + Zn(OH)₂

Structure

Raman spectroscopy and single-crystal X-ray diffraction studies of sodium dithionite in the solid state reveals that it exists in different forms. In one anhydrous form, the dithionite ion has C₂ geometry, almost eclipsed with a 16° O-S-S-O torsional angle. In the dihydrated form (Na₂S₂O₄.2H₂O), the dithionite anion has a shorter S-S bond length and a gauche 56° O-S-S-O torsional angle.^[2]

Applications

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. Reaction with formaldehyde produces Rongalite, which is used as a bleach, in, for instance, paper pulp, cotton, wool, and kaolin clay.^[3]

Na₂S₂O₄ + 2 CH₂O → 2 HOCH₂SO−2 + 2 Na⁺

Sodium dithionite 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 g/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 (E^o' -0.66 V vs NHE at pH 7^[4]). Potassium ferricyanide is usually used as an oxidizing chemical in such experiments (E^o' ~ 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.

Geosciences

Sodium dithionite has been used in chemical Enhanced Oil Recovery to stabilize polyacrylamide polymers against radical degradation in the presence of iron. It has also been used in environmental applications to propagate a low E_h front in the subsurface in order to reduce pollutants such as chromium.

See also

• Dithionite

References