Sodium dithionate

Sodium dithionate

Names
IUPAC name Sodium dithionate
Other names Sodium metabisulfate
Identifiers
CAS Number	7631-94-9
ChemSpider	128833
EC Number	231-550-1
Jmol interactive 3D	Image
PubChem	146045
InChI InChI=1S/2Na.H2O6S2/c;;1-7(2,3)8(4,5)6/h;;(H,1,2,3)(H,4,5,6)/q2*+1;/p-2 Key: CSMWJXBSXGUPGY-UHFFFAOYSA-L InChI=1/2Na.H2O6S2/c;;1-7(2,3)8(4,5)6/h;;(H,1,2,3)(H,4,5,6)/q2*+1;/p-2 Key: CSMWJXBSXGUPGY-NUQVWONBAI
SMILES [Na+].[Na+].[O-]S(=O)(=O)S([O-])(=O)=O
Properties
Chemical formula	Na2S2O6
Molar mass	206.106 g/mol
Appearance	White crystalline powder
Density	2.19 g/cm^3
Melting point	190 °C (374 °F; 463 K) (decomposes) 52 °C (dihydrate)
Boiling point	267 °C (513 °F; 540 K) decomposes
Solubility in water	6.27 g/100 mL (0 °C) 15.12 g/100 mL (20 °C) 64.74 g/100 mL (100 °C)
Hazards
NFPA 704	0 3 0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
verify (what is  ?)
Infobox references

For the sterilizing agent, see sodium metabisulfite. For the reducing agent, see sodium dithionite.

Sodium dithionate Na₂S₂O₆ is an important compound for inorganic chemistry. It is also known under names disodium dithionate, sodium hyposulfate, and sodium metabisulfate. The sulfur can be considered to be in its +5 oxidation state.

It should not be confused with sodium dithionite, Na₂S₂O₄, which is a very different compound, and is a powerful reducing agent with many uses in chemistry and biochemistry. Confusion between dithionate and dithionite is commonly encountered, even in manufacturers' catalogues.

Preparation

Sodium dithionate is produced by the oxidation of sodium bisulfite by manganese dioxide:^[1]

2 NaHSO₃ + MnO₂ → Na₂S₂O₆ + MnO + H₂O

Alternatively, it can be prepared by the oxidation of sodium sulfite by the silver(I) cation:^[1]

Na
2SO
3 + 2 Ag+
+ SO−
3 → Na
2S
2O
6 + 2 Ag

Another method is via oxidation of sodium thiosulfate with chlorine:

3 Cl₂ + Na₂S₂O₃·5H₂O + 6 NaOH → Na₂S₂O₆ + 6 NaCl + 8 H₂O

And another method to produce sodium dithionate is treating sodium thiosulfate with sodium hypochlorite solution.

Structure

The dithionate ion represents sulfur that is oxidized relative to elemental sulfur, but not totally oxidized. Sulfur can be reduced to sulfide or totally oxidized to sulfate, with numerous intermediate oxidation states in inorganic moieties, as well as organosulfur compounds. Example inorganic ions include sulfite and thiosulfate.

Sodium dithionate crystallize as orthorhombic crystals of the dihydrate (Na
2S
2O
6). The water of crystallization is lost when heated to 90 °C, and the structure becomes hexagonal.^[2]

Large beautiful single crystals of (Na
2S
2O
6 .2H
2O) has been grown and studied for pulsed lasing purposes (pico second spectroscopy) with great success by E. Haussühl and cols.^[3]

Properties

Sodium dithionate is a very stable compound which is not oxidized by permanganate, dichromate or bromine. It can be oxidized to sulfate under strongly oxidizing conditions: these include boiling for one hour with 5 M sulfuric acid with an excess of potassium dichromate, or treating with an excess of hydrogen peroxide then boiling with concentrated hydrochloric acid. The Gibbs free energy change for the oxidation is about −300 kJ/mol. In addition, the (S
2O
6) anion is not a good reducing group. Therefore, it has been used to form single crystals of large cation complexes in high oxidation states without reduction of the metallic complex.

References

1. W. G. Palmer (1954). Experimental Inorganic Chemistry. CUP Archive. pp. 361–365. ISBN 0-521-05902-X.
2. D. W. Larson; A. B. VanCleave (February 1963). "X-Ray Diffraction Data for Alkali Dithionates". Canadian Journal of Chemistry (The National Research Council of Canada) 41 (2). doi:10.1139/v63-035.
3. Haussühl, E.; A. A. Kaminskii (2010). Laser Physics 15 (5): 714–727. Missing or empty |title= (help)