Antimony pentasulfide is an inorganic antimony and sulfur combination that is also known as antimony red. It is a variable-composition nonstoichiometric chemical. Its composition is unknown. It is a sulfide with a p-block molecular structure. Sulfur contamination is common in commercial samples, which can be eliminated by washing with carbon disulfide in a Soxhlet extractor.
It is an orange-yellow or reddish amorphous powder; density 4.12 g/cm3; decomposes at 75°C; insoluble in water and alcohol; soluble in hydrochloric acid, caustic alkalies and ammonium hydrosulfide.
- Chemical formula: S5Sb2
- Molar mass: 403.82 g·mol−1
- Appearance: yellow to orange powder
- Density: 4.12 g/cm3
- Melting point: 135 °C (275 °F; 408 K) (decomposes)
- Solubility in water: insoluble
- Solubility: soluble in HCl, alkalis, ammonium hydrosulfide
Antimony pentasulfide can be produced by the reaction of antimony with sulfur at a temperature from 250-400 °C in an inert atmosphere.
The compound is made commercially by converting antimony trisulfide to tetrathioantimonate by boiling with sulfur in caustic soda solution:
4 Sb2S3 + 8 S +18 NaOH → 5 Na3SbS4 + 3 NaSbO3 + 9 H2O
The sparingly soluble sodium antimonate is filtered out of the solution. The yellow-orange antimony pentasulfide precipitates out on treatment with hydrochloric acid.
2 Na3SbS4 + 6 HCl → Sb2S5 + 6 NaCl + 3 H2S
It may also be prepared by the reaction of antimony pentachloride in HCl with hydrogen sulfide and removing any free sulfur by extraction with carbon disulfide:
2 SbCl5 + 5 H2S → Sb2S5 + 10 HCl.
Like many sulfides, this compound liberates hydrogen sulfide upon treatment with strong acids such as hydrochloric acid.
6 HCl + Sb2S5 → 2 SbCl3 + 3 H2S + 2 S
Analysis by Mössbauer spectroscopy indicates that this compound is a derivative antimony(III), explaining the production of antimony(III) chloride, rather than antimony(V) chloride, upon acidification. It is, therefore, not analogous to the phosphorus(V) compound phosphorus pentasulfide.
It may be used as a red pigment and is one possible precursor to Schlippe’s salt, Na3SbS4, which can be prepared according to the equation:
3 Na2S + Sb2S5 + 9 H2O → 2 Na3SbS4·9H2O
It is also used in the vulcanization of rubber to produce red rubber.