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Iron(II) sulfate

Iron(II) sulfate or ferrous sulfate is the chemical compound with the formula (Fe S O₄). It is most commonly encountered as the blue-green heptahydrate. Known since ancient times as copperas, the alchemical symbol for iron (II) sulfate is represented by unicode symbol U+26A8: ⚨.

Hydrates

Iron(II) sulfate can be found in various states of hydration, and several of these forms exist in nature.

FeSO₄·H₂O (mineral: szomolnokite, relatively rare)
FeSO₄·4H₂O (mineral: rozenite, white, relatively common, may be dehydratation product of melanterite)
FeSO₄·5H₂O (mineral: siderotil, relatively rare)
FeSO₄·6H₂O (mineral: ferrohexahydrite, relatively rare)
FeSO₄·7H₂O (mineral: melanterite, blue, relatively common)

At 90°C, the heptahydrate loses water to form the colorless monohydrate, also called green vitriol or copperas. In its anhydrous, crystalline state, its standard enthalpy of formation is Δ_fH°_solid = -928.4 kJ.mol^-1 and its standard molar entropy is S°_solid = 107.5 J.K^-1.mol^-1. All mentioned mineral forms are connected with oxidation zones of Fe-bearing (pyrite, marcasite, chalcopyrite etc.) ore beds and related environments (like coal fire sites). Many undergo rapid dehydratation and sometimes oxidation.

Production and reactions

In the finishing of steel prior to plating or coating, the steel sheet or rod is passed through pickling baths of sulfuric acid. This treatment produces large quantities of iron(II) sulfate as a by-product.

Fe + H₂SO₄ → FeSO₄ + H₂

Another source of large amounts results from the production of titanium dioxide from ilmenite via the sulfate process. Ferrous sulfate is also prepared commercially by oxidation of pyrite:

2 FeS₂ + 7 O₂ + 2 H₂O → 2 FeSO₄ + 2 H₂SO₄

Reactions

On heating, iron(II) sulfate first loses its water of crystallization and the original green crystals are converted into a dirty-yellow anhydrous solid. When further, the anhydrous material releases sulfur dioxide and white fumes of sulfur trioxide, leaving a reddish-brown iron(III) oxide.

4 FeSO₄ → 2 Fe₂O₃ + 2 SO₂ + 2 SO₃

Like all iron(II) salts, iron(II) sulfate is reducing agent. For example, it reduces nitric acid to nitrogen oxide and chlorine:

6 Fe²⁺ + 6 H⁺ + HNO₃ → 6 Fe³⁺ + 4 H₂O + 2 NO

2 Fe²⁺ + Cl₂ → 2 Fe³⁺ + 2 Cl^-

Uses

Industrially, ferrous sulfate is mainly used as a precursor to other iron compounds. It is a reducing agent, mostly for the reduction of chromate in cement.

Nutritional supplement

Together with other iron compounds, ferrous sulfate is used to fortify foods and to treat iron-deficiency anemia. Copperas was given indiscriminately by untrained persons to slaves in the 18th and 19th centuries for various ailments.

Colorant

Ferrous sulfate was used in the manufacture of inks, most notably iron gall ink, which was used from the middle ages until the end of the eighteenth century. It also finds use in wool dyeing as a mordant.

Two different methods for the direct application of indigo dye were developed in England in the eighteenth century and remained in use well into the nineteenth century. One of these, known as china blue, involved iron(II) sulfate. After printing an insoluble form of indigo onto the fabric, the indigo was reduced to leuco-indigo in a sequence of baths of ferrous sulfate (with reoxidation to indigo in air between immersions). The china blue process could make sharp designs, but it could not produce the dark hues of other methods.

Ferrous sulfate can also be used to stain concrete a yellowish rust color.

Woodworkers use ferrous sulfate solutions to color maple wood a silvery hue.

Other uses

In horticulture it is used as a lawn conditioner and moss killer.

In the second half of the 19th century, ferrous sulfate was also used as a photographic developer for Collodion process images.

Ferrous sulfate is sometimes added to the cooling water flowing through the brass tubes of a turbine condenser. It forms a corrosion-resistant, protective coating on the inside of the tube.

It has been applied for the purification of water by flocculation and for phosphate removal in municipal and industrial sewage treatment plants to prevent eutrophication of surface water bodies.