FeSO₄ — Ferrous Sulfate

Ferrous sulfate (FeSO₄) is an important inorganic salt composed of iron(II) ions and sulfate ions. It commonly occurs as the heptahydrate form (FeSO₄·7H₂O), which appears as blue-green crystals known as copperas or green vitriol. This compound is widely used in medicine, agriculture, industry, and water treatment.

In the medical field, ferrous sulfate serves as a key source of iron supplementation for treating iron-deficiency anemia. In industrial and laboratory contexts, it is used as a reducing agent due to its Fe²⁺ ions, which readily oxidize to Fe³⁺. Ferrous sulfate also plays a role in dyeing fabrics, wastewater treatment, and the preparation of other iron compounds.

Its abundance, affordability, and diverse applications make ferrous sulfate one of the most commercially significant iron salts.

The chemical structure of ferrous sulfate consists of one ferrous ion (Fe²⁺) and one sulfate ion (SO₄²⁻). In its most common hydrated form, FeSO₄·7H₂O, each formula unit includes seven water molecules coordinated to the iron ion. The heptahydrate crystals are monoclinic in shape and display a pale blue-green hue due to d–d electronic transitions within the Fe²⁺ ion.

The bonding in ferrous sulfate is primarily ionic between Fe²⁺ and SO₄²⁻, while the internal structure of the sulfate ion involves covalent bonds between sulfur and oxygen atoms. The compound can lose its water of crystallization upon heating, forming the monohydrate and anhydrous forms sequentially:

When exposed to moist air, ferrous sulfate slowly oxidizes to form ferric sulfate (Fe₂(SO₄)₃) or basic ferric sulfate, which appears as a brownish coating on the crystals.

Ferrous sulfate is manufactured through several industrial processes, primarily from iron metal or iron-containing by-products. The major methods include:

• 1. Reaction of Iron with Dilute Sulfuric Acid: Metallic iron reacts with dilute sulfuric acid to produce ferrous sulfate and hydrogen gas.

• 2. As a By-product: Ferrous sulfate is often obtained as a by-product in steel pickling processes, where iron is treated with sulfuric acid to remove oxide coatings.

• 3. Oxidation of Pyrite (FeS₂): In older methods, ferrous sulfate was prepared by oxidizing iron pyrite in air and dissolving the resulting product in water.

Commercial-grade ferrous sulfate is usually purified by recrystallization and drying under controlled conditions to obtain the heptahydrate form.

Physical Properties:

• Bluish-green crystalline solid (heptahydrate form).
• Soluble in water, forming pale green solutions.
• Decomposes upon heating, losing water molecules sequentially.
• Oxidizes in air to form brown ferric sulfate.

Chemical Properties:

• 1. Reaction with Oxygen: Ferrous sulfate oxidizes in moist air to form ferric sulfate.

\( 4FeSO_4 + 2H_2O + O_2 \rightarrow 4Fe(OH)SO_4 \)

• 2. Reaction with Alkalies: Forms ferrous hydroxide, a green precipitate that quickly oxidizes to ferric hydroxide.

\( FeSO_4 + 2NaOH \rightarrow Fe(OH)_2 + Na_2SO_4 \)

• 3. Reaction with Acids: Reacts with concentrated sulfuric acid to form ferric sulfate and sulfur dioxide gas on heating.

\( 2FeSO_4 + H_2SO_4 + ½O_2 \rightarrow Fe_2(SO_4)_3 + H_2O \)

• 4. Reducing Property: Acts as a reducing agent in chemical reactions because Fe²⁺ can easily oxidize to Fe³⁺.

\( 2FeSO_4 + H_2SO_4 + 2KMnO_4 \rightarrow Fe_2(SO_4)_3 + K_2SO_4 + 2MnSO_4 + 2H_2O \)

Ferrous sulfate has a wide variety of applications across different fields due to its reducing and nutritional properties:

• 1. Medical Applications: Used as an iron supplement to treat and prevent iron-deficiency anemia. It provides bioavailable iron for hemoglobin synthesis in the human body.
• 2. Water and Wastewater Treatment: Acts as a coagulant and flocculant to remove impurities, phosphorus, and heavy metals from water. It also helps reduce hexavalent chromium to less toxic trivalent chromium.
• 3. Industrial Uses: Employed in the production of inks, pigments, and dyes. It also serves as a mordant in textile dyeing processes.
• 4. Agriculture: Used as a soil amendment to correct iron deficiencies in plants and as a component in fertilizers. It helps prevent chlorosis in crops and turfgrass.
• 5. Analytical Chemistry: Acts as a reducing agent in redox titrations and chemical analysis, especially in determining oxidizing agents like potassium permanganate.
• 6. Environmental Uses: Helps control odor and hydrogen sulfide in wastewater treatment plants.

Ferrous sulfate is considered safe for human use in controlled doses but can be hazardous in excessive amounts or under improper handling conditions. As a medication, it must be prescribed under medical supervision to avoid iron overload.

Toxicity and Hazards: Ingestion of large amounts can lead to symptoms such as nausea, vomiting, abdominal pain, and organ damage. Chronic exposure to dust can irritate the respiratory tract. Environmental spills can cause aquatic toxicity if not neutralized.

Safety Measures:

• Store in a cool, dry place away from oxidizing agents.
• Use protective gloves, masks, and goggles when handling the compound.
• In laboratories, avoid prolonged exposure to moist air to prevent oxidation.
• In case of spillage, neutralize with lime or sodium bicarbonate solution and rinse with water.

In medicine, overdoses can be treated using chelating agents like deferoxamine to remove excess iron from the body.