Hyposulfurous Acid Formulas, Characteristics and Uses

David Holt

The hyposulfurous acid or dithionous acid is unknown, unstable in pure form, has no independent existence, and has not been detected in aqueous solution either.

Theoretically it would be a relatively weak acid, comparable to sulfurous acid, H2SO3. Only its salts are known, the dithionites, which are stable and powerful reducing agents. The sodium salt of dithionic acid is sodium dithionite.

  • Formulas
  dithionous acid dithionite anion sodium dithionite
Formulas H2S2O4 S2O42− Na2S2O4
  • CAS: 20196-46-7 hyposulfurous (or dithionous) acid
  • CAS: 14844-07-6 hyposulfurous acid (or dithionous, ion)
  • CAS: 7775-14-6 sodium dithionite (sodium salt of dithionous acid)

2D structure

Dithionous acid
Sodium dithionite

3D structure

Dithionous acid
Part of the crystal structure of sodium dithionite


Physical and chemical properties

  dithionous acid dithionite anion sodium dithionite
Appearance: . . White to off-white crystalline powder
  . . Light lemon flakes
Odor: . . Faint sulfur odor
Molecular weight: 130.132 g / mol 128.116 g / mol 174.096 g / mol
Boiling point: . . It decomposes
Melting point: . . 52 ° C
Density: . . 2.38 g / cm3 (anhydrous)
Water solubility . . 18.2 g / 100 mL (anhydrous, 20 ° C)

Hyposulfurous acid is a sulfur oxo acid with the chemical formula H2S2O4.

Sulfur oxo acids are chemical compounds that contain sulfur, oxygen, and hydrogen. However, some of them are only known from their salts (such as hyposulfurous acid, dithionic acid, disulfide acid, and sulfurous acid).

Among the structural characteristics of the oxoacids that have been characterized we have:

  • Tetrahedral sulfur when coordinated with oxygen
  • Bridge and terminal oxygen atoms
  • Peroxo terminal groups
  • S = S terminals
  • Strings of (-S-) n

Sulfuric acid is the best known sulfuric oxoacid and the most important industrially.

The dithionite anion ([S2O4] 2-) is an oxoanion (an ion with the generic formula AXOY z-) of sulfur formally derived from dithionic acid.

Dithionite ions undergo both acidic and alkaline hydrolysis to thiosulfate and bisulfite, and sulfite and sulfide, respectively:

The sodium salt of dithionic acid is sodium dithionite (also known as sodium hydrosulfite).

Sodium dithionite is a whitish to light yellow crystalline powder that has an odor similar to sulfur dioxide..

It heats up spontaneously on contact with air and humidity. This heat may be enough to ignite the surrounding combustible materials..

Under prolonged exposure to fire or intense heat, containers of this material can rupture violently..

It is used as a reducing agent and as a bleaching agent. It is used and for bleaching paper pulp and in dyeing. It is also used to reduce the nitro group to an amino group in organic reactions..

Although stable under most conditions, it decomposes in hot water and acid solutions.

It can be obtained from sodium bisulfite by the following reaction:

2 NaHSO3 + Zn → Na2S2O4 + Zn (OH) ²

Air and water reactions

Sodium dithionite is a combustible solid that slowly decomposes when in contact with water or water vapor, forming thiosulfates and bisulfites..

This reaction produces heat, which can further accelerate the reaction or cause surrounding materials to burn. If the mixture is confined, the decomposition reaction can result in pressurization of the container, which can rupture forcefully. Staying in the air, it slowly oxidizes, generating toxic sulfur dioxide gases.

Fire danger

Sodium dithionite is a flammable and combustible material. May ignite on contact with moist air or moisture. It can burn quickly with flare effect. May react vigorously or explosively in contact with water.

May decompose explosively when heated or involved in fire. It can be re-ignited after the fire is extinguished. Runoff can create a fire or explosion hazard. Containers may explode when heated.

Health hazard

Upon contact with fire, Sodium Dithionite will produce irritating, corrosive and / or toxic gases. Inhalation of decomposition products can cause serious injury or death. Contact with the substance can cause severe skin and eye burns. Runoff from fire control can cause pollution.


The dithionite ion is used, often in conjunction with a complexing agent (eg citric acid), to reduce iron (III) oxyhydroxide to soluble iron (II) compounds and remove amorphous iron-containing mineral phases (III) in soil analysis (selective extraction).

Dithionite increases the solubility of iron. Thanks to the strong affinity of the dithionite ion for bivalent and trivalent metal cations, it is used as a chelating agent..

The decomposition of dithionite produces reduced species of sulfur that can be very aggressive for the corrosion of steel and stainless steel..

Among the applications of sodium dithionite we have: 

In the 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 dyeing processes, mainly those involving sulfur dyes and vat dyes, in which a water-insoluble dye can be reduced to a water-soluble alkali metal salt (for example, the indigo dye ).

The reducing properties of sodium dithionite also remove excess colorant, residual oxide, and unwanted pigments, thus improving overall color quality..

Sodium dithionite can also be used for water treatment, gas purification, cleaning and extraction. It can also be used in industrial processes as a sulfonating agent or a source of sodium ion..

In addition to the textile industry, this compound is used in industries related to leather, food, polymers, photography, and many others. Also used as a bleaching agent in organic reactions.

In the biological sciences 

Sodium dithionite is often used in physiology experiments as a means of reducing the redox potential of solutions..

In geological sciences

Sodium dithionite is often used in soil chemistry experiments to determine the amount of iron that is not incorporated into primary silicate minerals..

Safety and Risks 

Hazard statements of the Globally Harmonized System of classification and labeling of chemicals (GHS)

The Globally Harmonized System of Classification and Labeling of Chemicals (GHS) is an internationally agreed system, created by the United Nations and designed to replace the various classification and labeling standards used in different countries by using consistent criteria globally..

The hazard classes (and their corresponding GHS chapter), classification and labeling standards, and recommendations for sodium dithionite are as follows (European Chemicals Agency, 2017; United Nations, 2015; PubChem, 2017):

(United Nations, 2015, p.356).
(United Nations, 2015, p.371).
(United Nations, 2015, p.385).


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