Sodium Sulfide
Properties
| State | Solid (crystalline) |
| Color | Yellow to brick-red (anhydrous); colorless to yellow (hydrate) |
| Solubility | Soluble in water (186 g/L at 20 °C); strongly alkaline solution |
| Melting Point | 1176 °C |
| Boiling Point | Decomposes |
About Sodium Sulfide
Sodium sulfide (Na2S, 78.045 g/mol) is one of the more pungent reagents on a working chemist's shelf — the anhydrous yellow-to-brick-red solid and the more common nonahydrate Na2S.9H2O both smell distinctly of hydrogen sulfide because the S 2- anion partially hydrolyzes in atmospheric moisture (S 2- + H2O -> HS- + OH-), releasing trace H2S even from a sealed bottle. The sulfide ion's basicity is comparable to hydroxide (Kb1 ~ 1) and the second hydrolysis step is small, so 1% Na2S solutions sit at pH 12 to 13 and contain mostly HS- with smaller amounts of free S 2-. Industrially, the dominant use is the kraft pulping process: sodium sulfide combined with sodium hydroxide (the 'white liquor') is cooked with wood chips at 170 °C and 8 atm in a digester for several hours, dissolving the lignin matrix while leaving the cellulose fibers intact. Without the sulfide, the alkaline-only soda process gives weaker, darker pulp; the role of HS- is to attack lignin's beta-aryl-ether bonds via nucleophilic substitution, fragmenting the polymer into water-soluble pieces that wash out of the fibers. The leather industry's other major use is hide unhairing — Na2S degrades the keratin disulfide bonds in hair shafts during the lime-sulfide depilation step before tanning. Smaller volumes show up in mineral flotation as a depressant for galena and chalcocite, and in heavy-metal precipitation for wastewater treatment (Cd 2+, Hg 2+, Pb 2+ all form essentially insoluble sulfides).
Where you'll encounter it
If you have ever walked past a kraft pulp mill — the smell that locals euphemistically call the 'smell of money' is mostly methyl mercaptan and dimethyl sulfide formed as side products when Na2S attacks lignin's methoxy groups, with hydrogen sulfide layered underneath. The mill itself runs a closed sulfide cycle: black liquor containing dissolved lignin and spent NaHS goes to the recovery boiler, where it is burned to give Na2S and Na2CO3 in the bottom smelt; the smelt is dissolved in water, the carbonate is causticized with lime to regenerate NaOH, and the regenerated white liquor goes back to the digester. In a leather tannery, you encounter sodium sulfide as the gray slurry mixed with hydrated lime in the depilation pit — hides go in with hair attached and come out 24 hours later with the hair degraded into a sludge that is squeegeed off before tanning. The chemistry is a clean example of disulfide-bond cleavage: HS- attacks the disulfide bridges (S-S) in keratin, breaking them to two thiols and dissolving the hair shaft.
Common Uses
- Active component of kraft pulping white liquor that fragments lignin in wood chips at 170 °C
- Depilatory agent in lime-sulfide unhairing of cattle hides before chrome tanning
- Sulfide flotation reagent for selective depression of copper and lead minerals in mineral processing
- Heavy-metal precipitant for Hg, Pb, Cd, and Cu wastewater treatment as insoluble metal sulfides
- Reducing agent for converting nitro compounds to amines in dye-intermediate synthesis
- Sulfur dye precursor for indigosol and sulfur-black dyestuffs in textile dyeing
- Rubber-vulcanization auxiliary and antioxidant additive
- Reagent for production of sodium polysulfide solutions used as oilfield H2S scavengers
Safety Information
Severely corrosive and toxic. GHS: H290 (corrosive to metals), H301+H311 (toxic if swallowed or in skin contact), H314 (causes severe skin burns and eye damage), H400 (very toxic to aquatic life). Critically, contact with any acid liberates H2S gas — OSHA PEL for H2S is 20 ppm ceiling with 50 ppm peak, and concentrations above 100 ppm cause olfactory fatigue (you stop smelling it) followed by respiratory paralysis at 500 to 1000 ppm. Sodium sulfide spills must be contained dry and never acid-neutralized in an open lab; cover with sodium bicarbonate to slow hydrolysis and remove with absorbent under fume-hood ventilation. Handle solid Na2S with nitrile gloves, splash goggles, and a fume hood; store in airtight containers under dry inert atmosphere away from acids and oxidizers.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.