Iron(III) Sulfate
Properties
| State | Solid (powder or crystalline) |
| Color | Grayish-white (anhydrous); yellow-brown (hydrated) |
| Solubility | Soluble in water; slightly soluble in ethanol |
| Melting Point | 480°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Iron(III) Sulfate
Iron(III) sulfate is the ferric coagulant of choice for water-treatment plants that need to operate across a wide pH window — typically pH 4 to 10, compared to pH 5.5-7.5 for aluminum sulfate (alum). The anhydrous powder is grayish-white but the technical-grade liquid product (around 40-45% Fe2(SO4)3 by weight) and the common nonahydrate Fe2(SO4)3·9H2O are both yellow to dark amber. Hydrolysis in raw water generates positively charged polynuclear iron-hydroxo species like [Fe2(OH)2]4+ and [Fe3(OH)4]5+ that neutralize the negative surface charge on suspended colloids, then sweep them down as a gelatinous Fe(OH)3 floc. Compared to FeCl3, the sulfate form contributes no chloride to the treated water (good for downstream membrane processes) and produces a slightly denser, faster-settling floc. Outside water treatment, Fe2(SO4)3 is the milder alternative to FeCl3 for circuit-board copper etching (slower, but doesn't fume HCl), the standard mordant for iron-tannin black natural dyes, and a soil acidifier and iron supplement for chlorotic blueberries, azaleas, and rhododendrons. It's made industrially by oxidizing FeSO4 with H2O2 or HNO3 in dilute H2SO4, or by dissolving Fe2O3 directly in hot concentrated sulfuric acid.
Where you'll encounter it
If you've ever drunk municipal tap water in a city with hard surface-water sources, photographed an old iron-gall ink manuscript, or amended garden soil to deepen blueberry blossoms, you've benefited from Fe2(SO4)3. Water-treatment operators love it because it works across pH 4-10 — wider than alum's narrow window — so seasonal swings in raw-water alkalinity don't force them to scramble with caustic soda. Garden centers stock it in 5-pound bags for the ericaceous-plant crowd: a tablespoon worked into the soil around a blueberry bush drops the pH and frees up iron the plant otherwise can't absorb in alkaline conditions, which is why the leaves turn from yellow to deep green within a week. Natural-dye textile artists also use it as the iron mordant that takes tannin-soaked silk from beige to charcoal black.
Common Uses
- Drinking-water and wastewater coagulant at 5-50 mg/L typical dose
- Phosphate removal in tertiary sewage treatment for EPA Pi limits
- Copper etchant for PCB shops avoiding HCl fumes from FeCl3
- Mordant for iron-tannin black and gray natural dyes on wool and silk
- Soil acidifier for ericaceous garden plants like blueberries and azaleas
- Pickling additive in copper and brass metal finishing
- Pigment-grade iron oxide precursor via thermal decomposition above 480°C
Safety Information
Solutions are strongly acidic (pH around 1-2 at 10% concentration) and corrosive. GHS classifications: H302 (harmful if swallowed), H315 (skin irritation), H318 (serious eye damage). OSHA PEL for soluble iron compounds is 1 mg/m3 as Fe, and the sulfate dust additionally requires a respirator due to inhalation hazard. Will stain skin and porous surfaces yellow-brown for days. Use nitrile gloves and splash goggles. Spills neutralize with soda ash; never use ammonia solutions, which generate exothermic precipitate.
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.