He Hyposulfurous acid Or dithionic acid is unknown, unstable in pure form, has no independent existence and has also not been detected in aqueous solution.
Theoretically it would be a relatively weak acid, comparable to sulfurous acid, H2SO3. Only its salts are known, the dithionites, which are stable and are 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)
- CAS : 14844-07-6 hyposulfurous (or dithionous, ion)
- CAS : 7775-14-6 sodium dithionite (sodium salt of dithionic acid)
2D structure
Dethionic Acid
Sodium dithionite
3D structure
Dethionic Acid
Dithitoito
Part of the crystalline structure of sodium dithionite
characteristics
Physical and chemical properties
| Dithionous acid | Dithionite anion | Sodium dithionite | |
| Appearance: | . | . | White to off-white crystalline powder |
| . | . | Lemon colored cups | |
| Odor: | . | . | Weak smell of sulfur |
| 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 / cm 3 (anhydrous) |
| Water solubility | . | . | 18.2 g / 100 mL (anhydrous, 20 ° C) |
Hyposulfurous acid is a sulfur oxoacid with the chemical formula H2S2O4.
Sulfur oxo acids are chemical compounds that contain sulfur, oxygen and hydrogen. However, some of them are only known for their salts (such as hyposulfurous acid, dithionic acid, disulfide acid and sulfurous acid).
Among the structural characteristics of the oxoacids we have characterized:
- Tetrahedral sulfur when coordinated with oxygen
- Oxygen atoms in bridge and terminal
- Groups peroxo terminals
- S = S terminals
- Chains of (-S-) n
Sulfuric acid is the most known and most industrially important sulfur oxoacids.
The dithionite anion ([S2O4] 2-) is an oxoanion (an ion with the generic formula AXOY z-) of sulfur formally derived from dithionic acid.
The 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 crystalline powder whitish to light yellow in color, which has a similar odor to sulfur dioxide.
It spontaneously heats on contact with air and moisture. This heat may be sufficient to ignite the surrounding combustible materials.
Under prolonged exposure to fire or severe heat, containers of this material may rupture violently.
It is used as a reducing agent and as a bleaching agent. It is used and to bleach the pulp of paper and in the dyeing. It is also used to reduce the nitro group to an amino group in organic reactions.
Although stable in most conditions, it is broken down into hot water and acid solutions.
It can be obtained from sodium bisulfite by the following reaction:
2 NaHSO 3 + Zn → Na 2 S 2 O 4 + Zn (OH) ²
Air and Water Reactions
Sodium dithionite is a combustible solid that decomposes slowly when in contact with water or water vapor, forming thiosulphates and bisulphites.
This reaction produces heat, which can further accelerate the reaction or cause the surrounding materials to burn. If the mixture is confined, the decomposition reaction may result in pressurizing the vessel, which can rupture strongly. When it remains in the air, it oxidizes slowly, generating toxic gases of sulfur dioxide.
Fire danger
Sodium dithionite is a flammable and combustible material. It may ignite on contact with damp air or moisture. It can burn quickly with flare effect. May react vigorously or explosively in contact with water.
It may decompose explosively when heated or in a fire. It can be re-lit after the fire is extinguished. Runoff may create a fire or explosion hazard. Containers may explode when heated.
Danger to health
When in contact with fire, sodium dithionite will produce irritating, corrosive and / or toxic gases. Inhalation of decomposition products may cause severe injury or death. Contact with the substance may cause severe burns to the skin and eyes. Runoff from fire control can cause contamination.
Applications
The dithionite ion is often used in conjunction with a complexing agent (eg, citric acid) to reduce iron (III) oxyhydroxide in soluble iron (II) compounds and to remove amorphous iron-containing mineral phases (III) in soil analysis (selective extraction).
The dithionite allows to increase the solubility of the iron. Due to the strong affinity of the dithionite ion for bivalent and trivalent metal cations, it is used as a chelating agent.
Decomposition of the dithionite produces reduced sulfur species which can be very aggressive for 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 may be used as a reducing agent in aqueous solutions. It is used as such in some industrial dyeing processes, especially those involving sulfur dyes and dyes in vats, wherein a water insoluble dye can be reduced to a water soluble alkali metal salt (eg indigo dye ).
The reducing properties of the sodium dithionite also remove excess dye, 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 may also be used in industrial processes such as a sulfonating agent or a sodium ion source.
In addition to the textile industry, this compound is used in industries related to leather, food, polymers, photography, and many others. It is also used as a bleaching agent in organic reactions.
In the biological sciences
Sodium dithionite is often used in physiological experiments as a means to reduce the redox potential of the 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 through the use of globally consistent criteria.
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).
References
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- Drozdova, Y., Steudel, R., Hertwig, R. H., Koch, W., & Steiger, T. (1998). Structures and energies of various isomers of dithionous acid, H2S2O4, and its anion HS2O4-1. The Journal of Physical Chemistry A, 102 (6), 990-996. Recovered from: mycrandall.ca
- European Chemicals Agency (ECHA). (2017). Summary of Classification and Labeling. Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation). Sodium dithionite, sodium hydrosulphite. Recovered on February 2, 2017, from: echa.europa.eu
- Jynto (talk), (2011). Dithionous-acid-3D-balls [image] Retrieved from: https://en.wikipedia.org/wiki/Dithionous_acid#/media/File:Dithionous-acid-3D-balls.png
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