Green Vitriol

Green vitriol, also known as ferrous sulphate or iron(II) sulphate, is a chemical compound historically significant in industry, medicine, and chemistry. Its chemical formula is FeSO₄·7H₂O in its common heptahydrate form. The compound has been known since ancient times and was one of the most widely used metal sulphates before the development of modern synthetic chemicals. It appears as pale green crystals that readily dissolve in water, giving a slightly acidic solution.

Historical Background

The name vitriol originates from the Latin word vitriolum, meaning “glassy,” owing to the glass-like appearance of the crystalline salts. Green vitriol was one of several compounds known collectively as “vitriols,” which included blue vitriol (copper sulphate) and white vitriol (zinc sulphate). These substances were extensively used in alchemy, where “oil of vitriol” (sulphuric acid) was obtained by heating metal vitriols.
During the medieval and early modern periods, green vitriol was a vital ingredient in the preparation of sulphuric acid, often referred to as “spirit of vitriol.” Alchemists and early chemists such as Jabir ibn Hayyan and later Johann Rudolf Glauber investigated its properties, recognising its capacity to release acidic vapours upon heating. In the eighteenth and nineteenth centuries, large-scale production of sulphuric acid by the vitriol process was based on roasting green vitriol, making it a cornerstone of early industrial chemistry.

Physical and Chemical Properties

Green vitriol typically occurs as iron(II) sulphate heptahydrate (FeSO₄·7H₂O), forming pale green to bluish-green crystals. The compound is moderately soluble in water, forming light green solutions that can oxidise upon exposure to air, gradually turning brown as iron(II) ions convert to iron(III).
Key properties include:

• Molecular weight (FeSO₄·7H₂O): 278 g/mol
• Appearance: Pale green crystals
• Solubility: Soluble in water, insoluble in alcohol
• Stability: Decomposes upon heating, losing water of crystallisation and eventually forming iron(III) oxide and sulphur dioxide.

When heated to around 300°C, it loses its water of crystallisation, forming anhydrous iron(II) sulphate, and at higher temperatures decomposes further to ferric oxide (Fe₂O₃).

Industrial Production

Ferrous sulphate is produced as a by-product in several industrial processes, particularly:

• Pickling of steel with sulphuric acid, where iron reacts with the acid to form ferrous sulphate.
• Manufacture of titanium dioxide, where it arises as a secondary product.
• Oxidation of pyrite (iron disulphide) in mining and metallurgical operations.

The crude product is purified by crystallisation from aqueous solution to yield the familiar green crystalline solid.

Uses and Applications

Green vitriol has a wide range of historical and modern uses across industries, agriculture, and medicine.
1. Industrial Applications

• Precursor for sulphuric acid: Historically, the roasting of green vitriol was one of the earliest methods used for producing sulphuric acid.
• Ink manufacture: It was a key ingredient in the production of traditional iron gall ink, used from the Middle Ages until the nineteenth century for writing and official documents.
• Dyeing and tanning: Ferrous sulphate was employed as a mordant in the dyeing of fabrics and as a preservative in leather tanning processes.
• Water treatment: It is used as a coagulant and reducing agent in water purification to remove impurities and to precipitate phosphates.

2. Agricultural UsesIn agriculture, green vitriol serves as a soil amendment and micronutrient fertiliser. It corrects iron chlorosis in plants, a condition that results in yellowing of leaves due to iron deficiency. It also functions as a moss killer on lawns and golf courses and helps in the control of certain soil-borne pathogens.
3. Medicinal ApplicationsMedicinally, ferrous sulphate has long been used as an iron supplement to treat anaemia caused by iron deficiency. It is valued for its ability to increase haemoglobin production and improve oxygen transport in the blood. Historically, it was also used externally in lotions or preparations for treating skin ulcers, though such applications have declined with modern pharmaceutical advancements.
4. Chemical ReagentIn laboratory chemistry, green vitriol serves as a reducing agent and a source of Fe²⁺ ions in various reactions. It is used in qualitative analysis for detecting nitric acid (the brown ring test) and in quantitative titrations involving oxidation–reduction reactions.

Environmental and Safety Considerations

Ferrous sulphate is generally regarded as a low-toxicity substance but must be handled with care due to its acidic and oxidative properties. It can cause mild irritation to the skin or eyes upon contact and is harmful if ingested in excessive quantities. In environmental contexts, ferrous sulphate plays a positive role in reducing chromium(VI) compounds to the less toxic chromium(III) form in industrial effluents.
However, improper disposal can lead to iron contamination in soil and water bodies, which may affect aquatic ecosystems. Therefore, environmental management protocols recommend controlled application and neutralisation of waste products containing iron(II) sulphate.