Sodium dithionite (also known as sodium hydrosulfite) is a white

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

line powder with a

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

ous odor. Although it is stable in dry air, it decomposes in hot

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

and in

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

Structure

The structure has been examined by

Raman spectroscopy Raman spectroscopy () (named after Indian physicist C. V. Raman) is a spectroscopic technique typically used to determine vibrational modes of molecules, although rotational and other low-frequency modes of systems may also be observed. Raman sp ...

and single-crystal X-ray diffraction. The dithionite dianion has C

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

, with almost eclipsed with a 16° O-S-S-O torsional angle. In the dihydrated form (), the dithionite anion has gauche 56° O-S-S-O torsional angle. A weak S-S bond is indicated by the S-S distance of 239 pm, which is elongated by ca. 30 pm relative to a typical S-S bond. Because this bond is fragile, the dithionite anion dissociates in solution into the O₂sup>− radicals, as has been confirmed by

EPR spectroscopy Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the spin ...

. It is also observed that ³⁵S undergoes rapid exchange between S₂O₄²⁻ and SO₂ in neutral or acidic solution, consistent with the weak S-S bond in the anion.

Preparation

Sodium dithionite is produced industrially by reduction of

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...

. Approximately 300,000 tons were produced in 1990. The route using zinc powder is a two-step process: :2SO₂ + Zn → ZnS₂O₄ :ZnS₂O₄ + 2NaOH → Na₂S₂O₄ + Zn(OH)₂ The

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds appli ...

method obeys the following stoichiometry: :NaBH₄ + 8NaOH + 8SO₂ → 4Na₂S₂O₄ + NaBO₂ + 6H₂O Each equivalent of H⁻ reduces two equivalents of sulfur dioxide.

Formate Formate (IUPAC name: methanoate) is the conjugate base of formic acid. Formate is an anion () or its derivatives such as ester of formic acid. The salts and esters are generally colorless.Werner Reutemann and Heinz Kieczka "Formic Acid" in ''Ull ...

has also been used as the reductant.

Properties and reactions

Hydrolysis

Sodium dithionite is stable when dry, but aqueous solutions deteriorate due to the following reaction: : 2 S₂O₄²⁻ + H₂O → S₂O₃²⁻ + 2 HSO₃⁻ This behavior is consistent with the instability of

dithionous acid Dithionous acid is a sulfur oxoacid with the chemical formula H2S2O4. It is unstable in pure form, but its salts, known as dithionites, are stable. It was initially assumed that the ''C''2 symmetric structure HOS(=O)-S(=O)OH is the most stable ...

. Thus, solutions of sodium dithionite cannot be stored for a long period of time. Anhydrous sodium dithionite decomposes to

sodium sulfate Sodium sulfate (also known as sodium sulphate or sulfate of soda) is the inorganic compound with formula Na2SO4 as well as several related hydrates. All forms are white solids that are highly soluble in water. With an annual production of 6 milli ...

and

above 90 °C in the air. In absence of air, it decomposes quickly above 150 °C to

sodium sulfite Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...

sodium thiosulfate Sodium thiosulfate (sodium thiosulphate) is an inorganic compound with the formula . Typically it is available as the white or colorless pentahydrate, . The solid is an efflorescent (loses water readily) crystalline substance that dissolves well i ...

, sulfur dioxide and trace amount of sulfur.

Redox reactions

Sodium dithionite is a reducing agent. At pH 7, the potential is -0.66 V compared to the

normal hydrogen electrode The standard hydrogen electrode (abbreviated SHE), is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials. Its absolute electrode potential is estimated to be at 25 °C, but to form a basis f ...

. Redox occurs with formation of bisulfite: :S₂O₄^2- + 2 H₂O → 2 HSO₃⁻ + 2 e⁻ + 2 H⁺ Sodium dithionite reacts with oxygen: :Na₂S₂O₄ + O₂ + H₂O → NaHSO₄ + NaHSO₃ These reactions exhibit complex pH-dependent equilibria involving bisulfite, thiosulfate, and sulfur dioxide.

With organic carbonyls

In the presence of aldehydes, sodium dithionite reacts either to form α-hydroxy-sulfinates at room temperature or to reduce the aldehyde to the corresponding alcohol above a temperature of 85 °C. Some ketones are also reduced under similar conditions.

Applications

Industry

Being water-soluble, sodium dithionite is used as a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

in some industrial dyeing processes. In the case of

sulfur dye Sulfur dyes are the most commonly used dyes manufactured for cotton in terms of volume. They are inexpensive, generally have good colour fastness, wash-fastness, and are easy to apply. Sulfur dyes are predominantly black, brown, and dark blue. Red s ...

s and

vat dye Vat dyes are a class of dyes that are classified as such because of the method by which they are applied. Vat dyeing is a process that refers to dyeing that takes place in a bucket or vat. The original vat dye is indigo, once obtained only from p ...

s, an otherwise water-insoluble dye can be reduced into a water-soluble

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

salt (e.g.

indigo dye Indigo dye is an organic compound with a distinctive blue color. Historically, indigo was a natural dye extracted from the leaves of some plants of the ''Indigofera'' genus, in particular ''Indigofera tinctoria''; dye-bearing ''Indigofera'' pla ...

). Sodium dithionite can also be used for water treatment, aquarium water conditioners, gas purification, cleaning, and stripping. It has also been applied as a sulfonating agent. In addition to the textile industry, this compound is used in industries concerned with leather, foods, polymers, photography, and many others, often as a decolourising agent. It is even used domestically as a decoloring agent for white laundry, when it has been accidentally stained by way of a dyed item slipping into the high temperature washing cycle. It is usually available in 5 gram sachets termed ''hydrosulfite'' after the antiquated name of the salt. It is the an active ingredient in "Iron Out Rust Stain Remover", a commercial rust product.

Laboratory

Sodium dithionite is often used in

physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

experiments as a means of lowering solutions'

redox potential Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...

(E^o' -0.66 V vs

SHE She most commonly refers to: *She (pronoun), the third person singular, feminine, nominative case pronoun in modern English. She or S.H.E. may also refer to: Literature and films *'' She: A History of Adventure'', an 1887 novel by H. Rider Hagga ...

at pH 7).

Potassium ferricyanide Potassium ferricyanide is the chemical compound with the formula K3 e(CN)6 This bright red salt contains the octahedrally coordinated 3−.html" ;"title="e(CN)6sup>3−">e(CN)6sup>3− ion. It is soluble in water and its solution shows some g ...

is usually used as an oxidizing chemical in such experiments (E^o' ~ .436 V at pH 7). In addition, sodium dithionite is often used in

soil chemistry Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1850s a consulting chemist to the Royal Agricultural Society in England, ...

experiments to determine the amount of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

that is not incorporated in primary

silicate mineral Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

s. 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 Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...

. Aqueous solutions of sodium dithionite were once used to produce ' Fieser's solution' for the removal of oxygen from a gas stream.

Pyrithione Pyrithione is the common name of an organosulfur compound with molecular formula , chosen as an abbreviation of pyridinethione, and found in the Persian shallot. It exists as a pair of tautomers, the major form being the thione 1-hydroxy-2(1''H ...

can be prepared in a two-step synthesis from 2-bromopyridine by oxidation to the ''N''-oxide with a suitable

peracid A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid which contains an acidic –OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the peroxy ...

followed by

substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord for a related one within a chord progression * Substitution (poetry), a variation in poetic scansion * "Substitution" (song), a 2009 song by Silversun Pi ...

using sodium dithionite to introduce the

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

functional group In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...

Photography

It is used in Kodak fogging developer, FD-70. This is used in the second step in processing black and white positive images, for making slides. It is part of the ''Kodak Direct Positive Film Developing Outfit''.

Safety

The wide use of sodium dithionite is attributable in part to its low toxicity at 2.5 g/kg (rats, oral).

References

External links

*Sodium dithionite - ipcs inchem {{Sodium compounds Bleaches Dithionites Sodium compounds Reducing agents