Sodium hydrosulphite

Sodium dithionite

Names
Other names D-Ox Hydrolin Reductone Sodium hydrosulfite Sodium sulfoxylate Sulfoxylate Vatrolite Virtex L
Identifiers
CAS Number	7775-14-6 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:66870 N
ECHA InfoCard	100.028.991
EC Number	231-890-0
PubChem CID	24489
RTECS number	JP2100000
UN number	1384
InChI InChI=1S/2Na.H2O4S2/c;;1-5(2)6(3)4/h;;(H,1,2)(H,3,4)/q2*+1;/p-2
SMILES [O-]S(=O)S(=O)[O-].[Na+].[Na+]
Properties
Chemical formula	Na2S2O4
Molar mass	174.107 g/mol (anhydrous) 210.146 g/mol (dihydrate)
Appearance	white to grayish crystalline powder light-lemon colored flakes
Odor	faint sulfur odor
Density	2.38 g/cm3 (anhydrous) 1.58 g/cm3 (dihydrate)
Melting point	52 °C (126 °F; 325 K)
Boiling point	Decomposes
Solubility in water	18.2 g/100 mL (anhydrous, 20 °C) 21.9 g/100 mL (Dihydrate, 20 °C)
Solubility	slightly soluble in alcohol
Hazards
EU classification (DSD) (outdated)	Harmful (Xn)
R-phrases (outdated)	R7, R22, R31
S-phrases (outdated)	(S2), S7/8, S26, S28, S43
NFPA 704	3 2 1
Flash point	100 °C (212 °F; 373 K)
Autoignition temperature	200 °C (392 °F; 473 K)
Related compounds
Other anions	Sodium sulfite Sodium sulfate
Related compounds	Sodium thiosulfate Sodium bisulfite Sodium metabisulfite Sodium bisulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N  (what is YN ?)
Infobox references

Sodium dithionite (also known as sodium hydrosulfite) is a white crystalline powder with a weak sulfurous odor. It is the sodium salt of dithionous acid. 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:

Raman spectroscopy and single-crystal X-ray diffraction studies of sodium dithionite in the solid state reveal that sodium dithionite exists in different forms. In one anhydrous form, the dithionite ion has C
2 geometry, almost eclipsed with a 16° O-S-S-O torsional angle. In the dihydrated form (Na
2S
2O
4.2H
2O), the dithionite anion has a shorter S-S bond length and a gauche 56° O-S-S-O torsional angle.

The weak S-S bond causes the dithionite anion to dissociate into the [SO₂]⁻ radical anion in aqueous solution, which has been confirmed by ESR spectroscopy. 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.

Sodium dithionite is stable when dry, but is slowly oxidized by air when in solution. Even with the absence of air, solutions of sodium dithionite deteriorate due to the following reaction: