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). Inhalation limits for soluble iron salts are comparatively lenient; ingestion is the route that matters here. 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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.