Sodium Sulfide vs Sodium Sulfide Nonahydrate: What's the Difference?

Buyers often ask whether they should purchase "sodium sulfide" or "sodium sulfide nonahydrate". The two products share the same sulfide anion but behave very differently in storage, transport and process dosing. Choosing the wrong one wastes money on freight, generates safety incidents and can throw off the chemistry of an entire production line.

Chemistry at a glance

• Anhydrous sodium sulfide — Na₂S, CAS 1313-82-2, molar mass 78.04 g/mol, ~60% sulfide (S²⁻) on a mass basis.
• Sodium sulfide nonahydrate — Na₂S·9H₂O, CAS 1313-84-4, molar mass 240.17 g/mol, ~21% sulfide on a mass basis with nine waters of crystallisation.

Both deliver the same reactive species (S²⁻) once dissolved, but the nonahydrate carries ~68–72% bound water that you are also paying to ship.

Physical form & appearance

Anhydrous Na₂S is typically supplied as yellow-to-red fused flakes or fine powder and is highly hygroscopic — it pulls moisture out of the air and can self-heat. The nonahydrate is an off-white to light grey / light green crystalline solid, stable at room temperature and easy to weigh on open scales.

Safety and transport

Both grades are UN 1849, Class 8 (Corrosive), but anhydrous Na₂S has a much lower auto-ignition risk threshold and is restricted by many carriers in bulk form. The nonahydrate ships under ambient conditions and is accepted by most container lines without special handling premiums, which is why exporters from India and China overwhelmingly favour it for sea freight.

Assay, moisture and dosing

Modern COAs for the nonahydrate typically show:

• Assay ≥ 98% (Na₂S·9H₂O basis), typically 99.6%.
• Moisture by KF: 67.5%–72.0%.
• Ammonium (NH₄): ≤ 0.005%.
• Sulfite & thiosulfate (as SO₄): ≤ 0.1%.

Anhydrous grades report assay on a Na₂S basis (60%–62% is the most common industrial grade). When converting recipes, remember 1 kg of anhydrous Na₂S delivers roughly the same active sulfide as 3 kg of the nonahydrate — but the nonahydrate dissolves more smoothly and gives a cleaner, less alkaline-shock solution.

When to choose which

Choose anhydrous Na₂S when…

• Freight cost per kg of active sulfide dominates and you have closed, inert handling.
• The downstream process is moisture-sensitive (some organic syntheses).
• You have on-site dissolving and dosing infrastructure designed for hot, exothermic chemistry.

Choose sodium sulfide nonahydrate when…

• Operators are dosing manually from bags, drums or jumbo bags.
• You want a stable, low-dust, ambient-shipped product with predictable solubility.
• End-use is leather, textile, mining flotation, pulp & paper, or effluent treatment — i.e. the vast majority of commercial sulfide demand.

Commercial price comparison

Per metric tonne of product, the nonahydrate is cheaper (typically $420–$860 / MT FOB) than anhydrous flakes ($900–$1,200 / MT FOB). Per kilogram of active sulfide, however, the anhydrous form is more concentrated — make the comparison on S²⁻ delivered, not on bag weight.

Bottom line

For 90% of industrial buyers, sodium sulfide nonahydrate (Na₂S·9H₂O, CAS 1313-84-4) is the right product: safer to handle, easier to dose, and accepted by every major carrier. Reserve anhydrous Na₂S for closed, capital-intensive processes where the freight savings on active sulfide outweigh the handling overhead.