Sodium dithionite
A reducing agent used in dyeing, water treatment, and laboratory redox control.
Sodium dithionite, also called sodium hydrosulfite, appears as a white crystalline powder and smells of sulfur. It remains stable in dry air but breaks down when exposed to hot water or acidic solutions.
Its structure has been studied using Raman spectroscopy and X-ray crystallography. The dithionite dianion has C2 symmetry, with a nearly eclipsed O–S–S–O torsional angle of 16°. In its dihydrate form, the anion adopts a gauche conformation with a 56° torsional angle. The S–S bond is notably weak, with a bond length of 239 pm—about 30 pm longer than a typical S–S bond. This fragility causes the dithionite anion to split into [SO₂]⁻ radicals in solution, as confirmed by EPR spectroscopy. Additionally, rapid exchange of ³⁵S between S₂O₄²⁻ and SO₂ occurs in neutral or acidic conditions, further evidence of the weak bond.
Industrially, sodium dithionite is made by reducing sulfur dioxide. One method uses zinc powder in two steps: first, sulfur dioxide reacts with zinc to form ZnS₂O₄, which then reacts with sodium hydroxide to yield Na₂S₂O₄ and zinc hydroxide. Another method uses sodium borohydride, where each hydride equivalent reduces two equivalents of sulfur dioxide. Formate can also serve as the reductant.
When dry, sodium dithionite is stable, but its aqueous solutions degrade over time because dithionite reacts with water to form thiosulfate and bisulfite. This mirrors the instability of dithionous acid, so solutions cannot be stored long. In air above 90 °C, the anhydrous compound decomposes into sodium sulfate and sulfur dioxide. Without air, it breaks down above 150 °C into sodium sulfite, sodium thiosulfate, sulfur dioxide, and trace sulfur.
As a reducing agent, sodium dithionite has a redox potential of –0.66 V at pH 7 relative to the normal hydrogen electrode. Its reduction produces bisulfite. It also reacts with oxygen and water to form sodium bisulfate and sodium bisulfite. These reactions involve complex, pH-dependent equilibria among bisulfite, thiosulfate, and sulfur dioxide.
With aldehydes, sodium dithionite can form α-hydroxy-sulfinates at room temperature or reduce the aldehyde to an alcohol above 85 °C. Some ketones are similarly reduced under those conditions.
In industry, it is used as a water-soluble reducing agent in dyeing processes. For sulfur dyes and vat dyes, it converts water-insoluble dyes into water-soluble alkali metal leuc
- chemical_formula
- Na2S2O4
- appearance
- White crystalline powder with sulfurous odor
- stability
- Stable in dry air; decomposes in hot water and acid solutions
- LD50
- 2.5 g/kg (rats, oral)
- production_1990
- Approximately 300,000 tons
- redox_potential_at_pH_7
- -0.66 V vs SHE
Lore & Background
Sodium dithionite is produced industrially by reduction of sulfur dioxide, with a 1990 production of about 300,000 tons. Two common preparation routes are the zinc powder method and the sodium borohydride method. The dithionite anion has a weak S-S bond (239 pm), causing it to dissociate into [SO2]− radicals in solution, confirmed by EPR spectroscopy. It also undergoes rapid 35S exchange with SO2 in neutral or acidic solution.
Aqueous solutions of sodium dithionite deteriorate via hydrolysis, forming thiosulfate and bisulfite, so they cannot be stored for long. Anhydrous sodium dithionite decomposes above 90 °C in air to sodium sulfate and sulfur dioxide; in the absence of air, it decomposes above 150 °C to sodium sulfite, sodium thiosulfate, sulfur dioxide, and trace sulfur. It is a reducing agent with a redox potential of -0.66 V at pH 7, reacting with oxygen to form bisulfate and bisulfite.
In industry, sodium dithionite is used to reduce water-insoluble sulfur dyes and vat dyes into water-soluble leuco salts, including indigo dye. It is also used domestically as a decolouring agent for white laundry, often sold in 5 gram sachets as 'hydrosulphite.' It is the active ingredient in 'Iron Out Rust Stain Remover.' In the laboratory, it lowers redox potential in physiology experiments and is used in soil chemistry to extract 'free iron.' It was once used to produce Fieser's solution for oxygen removal from gas streams, and in photography as part of Kodak fogging developer FD-70.
Reader's Guide
Sodium dithionite is significant as a versatile reducing agent with applications spanning industry, domestic use, and laboratory science. Its ability to reduce water-insoluble dyes into soluble forms makes it essential in textile dyeing, particularly for indigo and vat dyes. The compound's low toxicity (LD50 2.5 g/kg) contributes to its wide use in water treatment, gas purification, and as a decolourising agent in laundry and leather processing. In research, sodium dithionite is a standard tool for controlling redox potential in physiological experiments and for quantifying free iron in soil chemistry. Its weak S-S bond and radical dissociation behavior provide insight into sulfur chemistry. The compound's legacy is tied to its practical utility across multiple fields, from industrial manufacturing to hobbyist stain removal, and its role in fundamental chemical studies of redox reactions and sulfur exchange.
Did You Know?
- The dithionite anion has a weak S-S bond with a distance of 239 pm, about 30 pm longer than a typical S-S bond.
- Aqueous solutions of sodium dithionite cannot be stored for long because they hydrolyze to thiosulfate and bisulfite.
- Sodium dithionite is the active ingredient in 'Iron Out Rust Stain Remover.'
- It was used to produce 'Fieser's solution' for removing oxygen from a gas stream.
More in Sodium Compounds 1-24
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