Iron(II) Sulfate
Properties
| State | Solid at room temperature |
| Color | White (anhydrous), blue-green crystals (heptahydrate) |
| Solubility | Soluble in water (25.6 g/100 mL at 20 °C) |
| Melting Point | 680 °C (anhydrous, decomposes) |
About Iron(II) Sulfate
Iron(II) sulfate is one of the oldest characterized inorganic compounds — the alchemists called it green vitriol or copperas, sold it for centuries as a pigment and ink mordant, and obtained it from the natural weathering of pyrite (FeS2 + O2 + H2O → FeSO4 + H2SO4) in mine drainage. The most common form on a chemical-supply shelf today is the heptahydrate FeSO4·7H2O, blue-green monoclinic crystals that effloresce on warm air to lose water and turn dusty white. Dissolve it and you get [Fe(H2O)6]^2+ again — the same pale green high-spin d6 aqua ion you'd get from FeCl2. What sets the sulfate apart in practice is its market: it's the cheapest, most widely available oral iron supplement on the planet, the WHO Essential Medicines List entry for iron-deficiency anemia, and the form prescribed by the billion-dose annually. A standard 325 mg ferrous sulfate tablet delivers about 65 mg of elemental iron, of which 10–15% is absorbed via the DMT1 transporter in the duodenum. Outside medicine, FeSO4 is used to treat hexavalent chromium in cement (the EU mandates ≤2 ppm Cr(VI) in cement to prevent contact dermatitis), as a flocculant in wastewater (similar role to FeCl2), as a moss killer and lawn iron supplement, and analytically in the brown-ring test for nitrate, where Fe(II) reduces NO3- to NO and forms the deep-brown [Fe(H2O)5(NO)]^2+ nitrosyl complex at the H2SO4 layer interface.
Where you'll encounter it
If you've ever opened a brown amber pill bottle labeled 'ferrous sulfate 325 mg' for an anemic friend, watched a cement bag disclose its FeSO4 content for chromate reduction, or made an iron-gall ink the medieval way (FeSO4 + tannic acid + a little gum arabic), it's the same blue-green crystal in all three jobs. In a wastewater plant, FeSO4 is dosed alongside lime to precipitate phosphate as iron phosphate sludge that settles cleanly in the secondary clarifier. In a qualitative-analysis teaching lab, a student making the brown-ring test for nitrate dissolves fresh FeSO4·7H2O, layers concentrated H2SO4 down the side of the tube, and watches the interface darken to deep brown as the [Fe(H2O)5(NO)]^2+ nitrosyl complex forms within seconds — a 100-year-old test that still works because nothing else gives such an unambiguous visual cue at the bench.
Common Uses
- Oral iron-deficiency anemia treatment (325 mg tablet = 65 mg elemental Fe)
- Hexavalent chromium reduction in cement to meet EU 2 ppm Cr(VI) limit
- Wastewater coagulant and phosphate-removal precipitant
- Lawn iron supplement and moss killer (dissolved as 1–2% spray)
- Brown-ring test reagent for nitrate ion in qualitative analysis
- Fenton reaction Fe^2+ source for hydroxyl-radical advanced oxidation processes
Safety Information
GHS: H302 (harmful if swallowed at high doses), H315/H319 (skin and eye irritation). OSHA has no specific PEL; ACGIH TLV for soluble iron salts is 1 mg/m³ as Fe. Iron-overdose toxicity is a leading cause of accidental pediatric poisoning fatalities — as little as 60 mg/kg of elemental iron causes severe toxicity; a single bottle of adult prenatal vitamins can be lethal to a toddler. Symptoms progress from acute GI hemorrhage in the first hours to multi-organ failure 24–48 hours later. Keep iron supplements in child-resistant packaging and store out of reach. Stains skin and clothing; the hydrated salt is mildly hygroscopic and oxidizes slowly in moist air, picking up a yellow surface coat of basic iron(III) sulfate.
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.