Arsenic trisulfide

Arsenic trisulfide

Names
Preferred IUPAC name Arsenic trisulfide
Other names Arsenic(III) sulfide Orpiment Sulphuret of arsenic
Identifiers
CAS Number	1303-33-9 Y
3D model ( JSmol )	Interactive image
ChemSpider	21241348 Y
ECHA InfoCard	100.013.744
EC Number	215-117-4
PubChem CID	4093503
RTECS number	CG2638000
UNII	44SIJ800OX Y
CompTox Dashboard (EPA)	DTXSID50274153
InChI InChI=1S/As4S6/c5-1-6-3-8-2(5)9-4(7-1)10-3 Y Key: OUFDYFBZNDIAPD-UHFFFAOYSA-N Y InChI=1/As4S6/c5-1-6-3-8-2(5)9-4(7-1)10-3 Key: OUFDYFBZNDIAPD-UHFFFAOYAM
SMILES S1[As]3S[As]2S[As](S[As]1S2)S3
Properties
Chemical formula	As2S3
Molar mass	246.02 g·mol−1
Appearance	yellow or orange crystals
Density	3.43 g/cm3
Melting point	310 °C (590 °F; 583 K)
Boiling point	707 °C (1,305 °F; 980 K)
Solubility in water	insoluble
Solubility	soluble in ammonia
Magnetic susceptibility (χ)	−70.0·10−6 cm3/mol
Structure[1]
Crystal structure	monoclinic
Space group	P21/n (No. 11)
Lattice constant	a = 1147.5(5) pm, b = 957.7(4) pm, c = 425.6(2) pm α = 90°, β = 90.68(8)°, γ = 90°
Coordination geometry	pyramidal (As)
Hazards
GHS labelling:[3][4]
Pictograms
Signal word	Danger
Hazard statements	H300, H331, H400, H411
NFPA 704 (fire diamond)	3 0 0
NIOSH (US health exposure limits):
PEL (Permissible)	[1910.1018] TWA 0.010 mg/m3[2]
REL (Recommended)	Ca C 0.002 mg/m3 [15-minute][2]
IDLH (Immediate danger)	Ca [5 mg/m3 (as As)][2]
Related compounds
Related compounds	Tetraarsenic tetrasulfide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Arsenic trisulfide is the inorganic compound with the formula As₂S₃. It is a dark yellow solid that is insoluble in water. It also occurs as the mineral orpiment (Latin: auripigmentum), which has been used as a pigment called King's yellow. It is produced in the analysis of arsenic compounds. It is a group V/VI, intrinsic p-type semiconductor and exhibits photo-induced phase-change properties.^{[clarification needed]}

Structure

As₂S₃ occurs both in crystalline and amorphous forms. Both forms feature polymeric structures consisting of

It is a semiconductor, with a direct band gap of 2.7 eV.^[6] The wide band gap makes it transparent to infrared light between 620 nm and 11 μm.

Synthesis

From the elements

Amorphous As₂S₃ is obtained via the fusion of the elements at 390 °C. Rapid cooling of the reaction melt gives a glass. The reaction can be represented with the chemical equation:

2 As + 3 S → As₂S₃

Aqueous precipitation

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 6 M HCl. As₂S₃ is so insoluble that it is not toxic.

Reactions

Upon heating in a vacuum, polymeric As₂S₃ "cracks" to give a mixture of molecular species, including molecular As₄S₆.^[7][8] As₄S₆ adopts the adamantane geometry, like that observed for P₄O₆ and As₄O₆. When a film of this material is exposed to an external energy source such as thermal energy (via thermal annealing ^[9]), electromagnetic radiation (i.e. UV lamps, lasers,^[10] electron beams)^[11]), As₄S₆ polymerizes:

2 (As₂S₃)_n ⇌ n As₄S₆

As₂S₃ characteristically dissolves upon treatment with aqueous solutions containing

As₂S₃ + 6 NaSH → 2 AsS3−3 + 3 H₂S^{[clarification needed]}

As₂S₃ is the anhydride of the hypothetical trithioarsenous acid, As(SH)₃. Upon treatment with

"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

Contemporary uses

As an inorganic photoresist

Due to its high refractive index of 2.45 and its large Knoop hardness compared to organic photoresists, As₂S₃ has been investigated for the fabrication of photonic crystals with a full-photonic band-gap. Advances in laser patterning techniques such as three-dimensional direct laser writing (3-D DLW) and chemical wet-etching chemistry, has allowed this material to be used as a photoresist to fabricate 3-D nanostructures.^[13][14]

As₂S₃ has been investigated for use as a high resolution photoresist material since the early 1970s,^[15][16] using aqueous etchants. Although these aqueous etchants allowed for low-aspect ratio 2-D structures to be fabricated, they do not allow for the etching of high aspect ratio structures with 3-D periodicity. Certain organic reagents, used in organic solvents, permit the high-etch selectivity required to produce high-aspect ratio structures with 3-D periodicity.

Medical applications

As₂S₃ and As₄S₄ have been investigated as treatments for acute promyelocytic leukemia (APL).

For IR-transmitting glasses

Arsenic trisulfide manufactured into

Arsenic trisulfide was used for the distinctive eight-sided conical nose over the infra-red seeker of the de Havilland Firestreak missile.

Role in ancient artistry

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

Miscellaneous

Arsenic trisulfide is also used as a tanning agent. It was formerly used with indigo dye for the production of pencil blue, which allowed dark blue hues to be added to fabric via pencil or brush.

Precipitation of arsenic trisulfide is used as an analytical test for presence of dissimilatory arsenic-reducing bacteria (DARB).^[18]

Safety

As₂S₃ is so insoluble that its toxicity is low. Aged samples can contain substantial amounts of arsenic oxides, which are soluble and therefore highly toxic.

Natural occurrence

Orpiment is found in volcanic environments, often together with other arsenic sulfides, mainly realgar. It is sometimes found in low-temperature hydrothermal veins, together with some other sulfide and sulfosalt minerals.

References