Arsenic trisulfide

Arsenic trisulfide
General
Other names	Arsenic(III) sulfide Orpiment
Molecular formula	As₂S₃
Molar mass	246.041 g/mol
Appearance	yellow-orange powder or crystals
CAS number	^*
Properties
Density and phase	3.43 g/cm³, solid
Solubility in water	insoluble
Melting point	310 °C
Boiling point	707 °C
Hazards
EU classification	not listed
NFPA 704
Related compounds
Other anions	As₂O₃
Other cations	P₄S₆
Related compounds	P₄S₁₀, AsCl₃
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Arsenic trisulfide is the chemical compound with the formula As₂ S₃. This bright yellow solid is well known because it occurs as the mineral orpiment, has been used as a pigment, and has played a role in the analysis of arsenic compounds.

The other important As-S phase is realgar, As₄S₄, which is red-orange and also occurs as a mineral.

Structure

As₂S₃ adopts a polymeric structure consisting of ruffled sheets of pyramidal, three-coordinate As(III) centers interconnected by two-coordinate sulfide centers.Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press. Upon heating, the polymer "cracks" to give gaseous As₄S₆ molecules that repolymerize upon condensation. As₄S₆ adopts the adamantane structure, adopted also by P₄O₆ and As₄O₆.

Synthesis and reactions

As₂S₃ forms when aqueous solutions containing As(III) are treated with H₂S. Arsenic was in the past analyzed and assayed by this reaction, which results in the precipitation of As₂S₃, which is then weighed. As₂S₃ can even be precipitated in 6M HCl. As₂S₃ is so insoluble that it is not toxic.

As₂S₃ characteristically dissolves upon treatment with aqueous solutions containing sulfide ions. The dissolved arsenic species is the pyramidal trianion AsS₃^3-:

As₂S₃ + 6 NaSH → 2 AsS₃^3- + 3 H₂S

As₂S₃ is the anhydride of the hypothetical thioarsenous acid, As(SH)₃. Upon treatment with polysulfide ions, As₂S₃ dissolves to give a variety of species containing both S-S and As-S bonds. One of the more noteworthy derivatives is S₇As-S^-, a ring that contains an exocyclic sulfido center attached to the As atom. As₂S₃ also dissolves in strongly alkaline solutions to give a mixture of AsS₃^3- and AsO₃^3-.Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

"Roasting" As₂S₃ in air gives volatile, toxic derivatives, this conversion being one of the hazards associated with the refining of heavy metal ores:

2 As₂S₃ + 9 O₂ → As₄O₆ + 6 SO₂

Role in ancient artistry

The ancient Egyptians reportedly used orpiment, natural or synthetic, as a pigment in artistry and cosmetics.

Contemporary use

Arsenic trisulfide manufactured into amorphous form is used as a chalcogenide glass for infrared optics. It is transparent between 620 nm and 11 µm. The arsenic trisulfide glass is much more resistant to oxidation than crystalline arsenic trisulfide, so there do not have to be toxicity precautions. ^* It can be also used as an acousto-optic material.

Arsenic trisulfide is also used as a tanning agent.

Precipitation of arsenic trisulfide is used as ananalytical test for presence of dissimilatory arsenic-reducing bacteria (DARB). ^*

Safety

As₂S₃ is so insoluble that it is considered non-toxic. Commercial "orpiment" can however contain substantial amounts of As₂O₃, which are soluble and therefore highly toxic. Crystalline As₂S₃ however tends to oxidize on the surface, forming a layer of toxic arsenic trioxide.

References

External links

Sulfides | Arsenic compounds | Optical materials

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