Arsenic trisulfide
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| Arsenic trisulfide | |
|---|---|
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| General | |
| Other names | Arsenic(III) sulfide Orpiment |
| Molecular formula | As2S3 |
| Molar mass | 246.041 g/mol |
| Appearance | yellow-orange powder or crystals |
| CAS number | [1303-33-9] |
| 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 |
0
4
2
|
| 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) Infobox disclaimer and references |
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Arsenic trisulfide is the chemical compound with the formula As2S3. 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.
Contents |
[edit] Structure
As2S3 adopts a polymeric structure consisting of ruffled sheets of pyramidal, three-coordinate As(III) centers interconnected by two-coordinate sulfide centers.[1] 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.
[edit] 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 AsS33-:
- As2S3 + 6 NaSH → 2 AsS33- + 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 AsS33- and AsO33-.[2]
"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
[edit] Contemporary uses
[edit] Medical applications
As2S3 and As4S4 have been investigated as treatments for acute promyelocytic leukemia (APL).[3] Presumably the mode of action is similar to that for As2O3.
[edit] For IR-transmitting glasses
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, which minimizes toxicity concerns. [1] It can be also used as an acousto-optic material.
[edit] Role in ancient artistry
The ancient Egyptians reportedly used orpiment, natural or synthetic, as a pigment in artistry and cosmetics.
[edit] Miscellaneous
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). [2]
[edit] 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.
[edit] References
- ^ Wells, A.F. (1984). Structural Inorganic Chemistry, Oxford: Clarendon Press.
- ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ D.-P. Lu, J.-Y. Qiu, B. Jiang, Q. Wang, K.-Y. Liu, Y.-R. Liu, S.-S. Chen "Tetra-arsenic tetra-sulfide for the treatment of acute promyelocytic leukemia: a pilot report" Blood 2002, Volume 99, pp. 3136-3143.
