Sodium polysulfide

Sodium polysulfide

Sodium pentasulfide, a representative component of sodium polysulfide
Names
Other names Sodium sulfane; Viradon
Identifiers
CAS Number	1344-08-7
ECHA InfoCard	100.014.261
UN number	UN3266
Properties
Chemical formula	Na2Sx
Hazards
Safety data sheet	AGFA
NFPA 704	0 3 1
Flash point	Non-combustible
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Sodium polysulfide is a general term for salts with the formula Na₂S_x, where x = 2 to 5. The species S_x^2-, called polysulfide anions, include disulfide (S₂^2-), trisulfide (S₃^2-), tetrasulfide (S₄^2-), and pentasulfide (S₅^2-). In principle, but not in practice, the chain lengths could be longer.^[1] The salts are dark red solids that dissolve in water to give highly alkaline and corrosive solutions. In air, these salts oxidize, and they evolve hydrogen sulfide by hydrolysis.

Structure

The polysulfide anions form chains with S---S bond distances around 2 Å in length. The chains adopt skewed conformations. In the solid state, these salts are dense solids with strong association of the sodium cations with the anionic termini of the chains.^[2]

S₄^2- from the crystal structure, highlighting its skewed conformation.

S₅^2- from the crystal structure, highlighting its skewed conformation.

Production and occurrence

Sodium polysulfide can produced by dissolving sulfur in a solution of sodium sulfide.^[3] Alternatively they are produced by the redox reaction of aqueous sodium hydroxide with sulfur at elevated temperatures.^[4] Finally they arise by the reduction of elemental sulfur with sodium, a reaction often conducted in anhydrous ammonia.

These salts are used in the production of polysulfide polymers, as a chemical fungicide, as a blackening agent on copper jewellery, as a component in a polysulfide bromide battery, as a toner in a photochemical solution, and in the tanning industry to remove hair from hides.

Reactions

As exploited in the sodium-sulfur battery, the polysulfides absorb and release reducing equivalents by breaking and making S-S bonds, respectively. An idealized reaction for sodium tetrasulfide is shown:

Na₂S₄ + 2 Na ⇌ 2 Na₂S₂

Alkylation gives organic polysulfides according to the following idealized equation:

Na₂S₄ + 2 RX → 2 NaX + R₂S₄

Alkylation with an organic dihalide gives polymers called thiokols.

Protonation of these salts gives hydrogen sulfide and elemental sulfur, as illustrated by the reaction of sodium pentasulfide:

Na₂S₅ + 2 H⁺ → H₂S + 1/2 S₈

References

1. Elemental Sulfur and Sulfur-Rich Compounds volumes I and II, Steudel, R., Ed.; Springer-Verlag: Berlin, 2003.
2. Rosén, E.; Tegman, R. "Preparative and X - ray powder diffraction study of the polysulfides Na₂S₂, Na₂S₄ and Na₂S₅" Acta Chemica Scandinavica 1988, volume 25, pp 3329-3336. doi:10.3891/acta.chem.scand.25-3329
3. F. Fehér" Sodium Disulfide", "Sodium Tetrasulfide" "Sodium Pentasulfide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 361-367.
4. Lee, T.C.P. (1999). Properties and applications of elastomeric polysulfides. Rapra Technology. p. 4. ISBN 978-1859571583.