What Is Ferrous Sulphate Heptahydrate? Chemical Structure, Properties & Identification

Ferrous Sulphate Heptahydrate, systematically designated iron(II) sulfate heptahydrate, is an inorganic coordination compound with the chemical formula FeSO₄·7H₂O. Known historically as green vitriol or copperas, it is the most common and industrially accessible hydrate of iron(II) sulphate. With a CAS Registry Number of 7782-63-0, it represents a well-defined crystalline chemical entity distinct from anhydrous ferrous sulphate (CAS 7720-78-7) and the monohydrate (CAS 17375-41-6).

Molecular Structure and Coordination Sphere

While the formula is commonly written as FeSO₄·7H₂O, X-ray crystallographic studies reveal that its true structural formula is [Fe(H₂O)₆]SO₄·H₂O. Six of the water molecules coordinate directly to the central bivalent iron cation (Fe²⁺) in a slightly distorted octahedral geometry, forming the hexaaquairon(II) complex ion [Fe(H₂O)₆]²⁺. The pale blue-green colour of the solid and its dilute solutions arises from d-d electronic transitions (specifically ²T₂g → ²Eg) within this octahedral ligand field.

The seventh water molecule does not coordinate directly to the iron cation; instead, it is held within the crystal lattice through strong hydrogen bonds bridging the sulphate anion (SO₄²⁻) and the coordinated water molecules. This structural arrangement accounts for the stability of the heptahydrate under ambient humidity conditions.

Crystal Habit and Physical Properties

• Physical Appearance: Pale sea-green to bluish-green translucent monoclinic crystals or crystalline powder.
• Crystal Symmetry: Monoclinic, space group P2₁/c, with lattice parameters a = 14.07 Å, b = 6.50 Å, c = 11.04 Å, and β = 105.6°.
• Density: 1.895 g/cm³ at 20°C.
• Melting / Dehydration Point: Begins losing crystal water at 64°C, progressively dehydrating to the tetrahydrate (rozenite) and monohydrate (szomolnokite) before decomposing above 300°C.
• Aqueous pH: A 5% aqueous solution has an acidic pH typically between 3.0 and 4.0 due to hydrolysis.

Aqueous Solubility and Dissolution Thermodynamics

Ferrous Sulphate Heptahydrate exhibits robust solubility in water that increases markedly with temperature:

• At 10°C: ~20.5 g anhydrous FeSO₄ per 100 g H₂O (~37 g heptahydrate/100 mL)
• At 20°C: ~25.6 g anhydrous FeSO₄ per 100 g H₂O (~47 g heptahydrate/100 mL)
• At 50°C: ~48.6 g anhydrous FeSO₄ per 100 g H₂O (~90 g heptahydrate/100 mL)

Dissolution is slightly endothermic. In industrial chemical makeup tanks, mechanical agitation ensures rapid, complete dissolution into a 5% to 10% dosing solution within 10–15 minutes at standard ambient Kenyan water temperatures (20°C–25°C).

Chemical Reactivity and Redox Behavior

The central chemical feature of ferrous sulphate heptahydrate is the bivalent oxidation state (+2) of the iron ion. In neutral to alkaline conditions, or in the presence of dissolved oxygen, Fe²⁺ oxidizes rapidly to trivalent iron (Fe³⁺):

4Fe²⁺ + O₂ + 4H⁺ → 4Fe³⁺ + 2H₂O

In alkaline wastewater (pH > 8.0), this oxidation produces gelatinous, insoluble ferric hydroxide (Fe(OH)₃) flocs with enormous surface area that sweep colloidal contaminants, dyes, and organic matter from solution. Conversely, under acidic conditions, ferrous sulphate acts as a targeted electron donor, making it indispensable for reducing toxic hexavalent chromium (Cr VI) to trivalent chromium (Cr III) in industrial effluent and Portland cement.

Commercial Identification Standards

Commercial buyers should verify that their product meets verified industrial specifications: 98.0% minimum assay (FeSO₄·7H₂O), corresponding to ≥19.7% elemental iron, with water-insoluble matter ≤0.05% and free acidity ≤0.10%. KIKI'S Industrial Chemicals supplies verified 98% min material in heavy-duty 50 kg woven polypropylene bags with heat-sealed polyethylene moisture liners (Net Wt: 50.000 kg, Gross Wt: 50.200 kg, Kaiser Exports India, Lot VP/KE/26/27/05, marked NBO) from its Enterprise Road depot in Nairobi.