Ammonium Sulphate
Ammonium sulphate, chemically represented as (NH₄)₂SO₄, is a white crystalline solid widely used in agriculture, chemical industries, anlaboratory applications. It is one of the most important inorganic nitrogen fertilisers, valued for its stability, high solubility, and ability to supply both nitrogen and sulphur, two essential nutrients for plant growth. Beyond its agricultural significance, ammonium sulphate serves as a versatile reagent in industrial chemistry, food processing, pharmaceuticals, and biotechnology. Its simplicity, low cost, and ease of handling make it a cornerstone compound in the global chemical industry.
Historical Background and Discovery
The discovery of ammonium salts dates back to early chemical explorations of the 17th and 18th centuries. Ammonium sulphate was first recognised during experiments involving the reaction between ammonia and sulphuric acid, both known from early alchemical practices. In the 19th century, it gained industrial prominence as a by-product of coal gas production, where ammonia liberated from coal was neutralised with sulphuric acid to form ammonium sulphate crystals.
The industrial revolution and the rise of the chemical fertiliser industry in the late 1800s accelerated its use. In the early 20th century, the Haber–Bosch process provided a steady supply of ammonia, allowing large-scale, controlled production of ammonium sulphate. Since then, it has become a fundamental component of agricultural development worldwide, playing a crucial role in soil fertility and crop productivity.
Chemical Composition and Structure
Ammonium sulphate consists of two ammonium ions (NH₄⁺) and one sulphate ion (SO₄²⁻). The molecular weight of the compound is 132.14 g mol⁻¹. It crystallises in the orthorhombic system, forming colourless to white granules that are stable under normal conditions.
Key physical properties:
- Chemical formula: (NH₄)₂SO₄
- Molecular mass: 132.14 g mol⁻¹
- Appearance: White, crystalline solid
- Solubility in water: 74.4 g/100 mL (25 °C)
- Melting point: Decomposes above 280 °C
- Density: 1.77 g cm⁻³
- pH (1% solution): ~5.5 (slightly acidic)
- Odour: Odourless or faintly of ammonia
- Hygroscopic nature: Moderate; absorbs moisture slowly from air
Ammonium sulphate is non-volatile and non-combustible, making it safe for storage and transportation. It is slightly acidic in aqueous solution due to partial hydrolysis of the ammonium ion.
Chemical Properties and Reactions
Ammonium sulphate is chemically stable and does not readily react with most materials under normal conditions. However, it displays typical behaviours of both ammonium and sulphate salts.
- Reaction with strong bases: Ammonium sulphate reacts with alkaline substances to release ammonia gas:(NH₄)₂SO₄ + 2NaOH → 2NH₃↑ + Na₂SO₄ + 2H₂O
- Decomposition on heating: When strongly heated, it decomposes into ammonia, nitrogen, sulphur dioxide, and water vapour, leaving behind residue of ammonium bisulphate or sulphur oxides depending on temperature:(NH₄)₂SO₄ → NH₃ + NH₄HSO₄ → N₂ + SO₂ + H₂O (on extreme heating)
- Reaction with calcium hydroxide: Forms ammonia gas and calcium sulphate (gypsum):(NH₄)₂SO₄ + Ca(OH)₂ → 2NH₃ + CaSO₄ + 2H₂O
- Solubility and ionic dissociation: In water, ammonium sulphate dissociates completely into ammonium and sulphate ions:(NH₄)₂SO₄ → 2NH₄⁺ + SO₄²⁻
These ions make it an excellent source of nitrogen and sulphur for plants and a useful electrolyte in biochemical processes.
Methods of Production
Ammonium sulphate can be produced by several industrial methods, depending on the availability of raw materials and by-products.
1. Direct neutralisation process:
This is the most straightforward and widely used industrial method, where ammonia gas reacts directly with sulphuric acid:2NH₃ + H₂SO₄ → (NH₄)₂SO₄The reaction is exothermic and results in the formation of a slurry of crystalline ammonium sulphate, which is later cooled, crystallised, and dried.
2. By-product recovery from industrial gases:
Historically, ammonium sulphate was recovered from coke oven gas during coal processing. The ammonia contained in the gas was absorbed by sulphuric acid to yield ammonium sulphate. Though less common today, this method remains important in integrated steel plants.
3. From gypsum and ammonia:
A less common route involves the reaction of ammonia with gypsum (CaSO₄·2H₂O) in the presence of carbon dioxide and water:CaSO₄·2H₂O + 2NH₃ + CO₂ → (NH₄)₂SO₄ + CaCO₃ + H₂O
4. From caprolactam production:
In the production of nylon-6, ammonium sulphate is obtained as a major by-product from the purification of caprolactam. This source accounts for a significant portion of the world’s supply, especially in East Asia and Europe.
Agricultural Applications
The largest and most significant use of ammonium sulphate is as a fertiliser. It supplies both nitrogen (21%) and sulphur (24%), making it particularly valuable for crops requiring these nutrients.
Advantages in agriculture:
- Dual nutrient supply: Provides essential nitrogen for vegetative growth and sulphur for protein synthesis.
- Acidifying effect: Beneficial in alkaline soils, as it helps lower pH and improve nutrient availability.
- High stability: Less prone to volatilisation losses compared to urea, ensuring efficient nitrogen utilisation.
- Compatibility: Can be blended with most other fertilisers or applied directly to soil.
- Low hygroscopicity: Easy to store and spread using standard fertiliser equipment.
Crops and soils: Ammonium sulphate is particularly useful for rice, maize, potatoes, and tea plantations. It is also beneficial for sulphur-demanding crops such as canola, onions, garlic, and legumes. However, in acidic soils, prolonged use may cause further acidification, necessitating liming treatments.
Industrial and Non-Agricultural Uses
While agriculture accounts for over 90% of its global consumption, ammonium sulphate also finds extensive use in several non-agricultural fields.
1. Water treatment and purification
Used to precipitate heavy metals and to adjust pH in municipal and industrial wastewater treatment. Its high solubility and ionic nature make it suitable for ion exchange and flocculation processes.
2. Food and beverage industry
In the food industry, ammonium sulphate (E517) serves as a food additive, yeast nutrient, and dough conditioner. It is commonly used in bread making, fermentation processes, and beer brewing to provide nitrogen for yeast metabolism.
3. Pharmaceutical and biotechnology applications
In biochemical laboratories, ammonium sulphate is used for protein purification through the process of salting out. Its ability to selectively precipitate proteins based on solubility differences makes it a vital reagent in protein crystallisation, enzyme concentration, and purification studies.
4. Textiles, leather, and dye industries
It is used in dyeing processes as a mordant and acidifier to stabilise dye baths. In the leather industry, it acts as a deliming agent for removing lime from hides before tanning.
5. Flame retardants and fireproofing agents
Ammonium sulphate, when mixed with other salts, enhances the flame-retardant properties of textiles, wood, and paper by releasing non-flammable gases upon heating.
6. Manufacture of other chemicals
It is employed as an intermediate in the production of ammonium persulphate, ammonium alum, and other nitrogen-containing compounds.
Environmental and Health Aspects
Ammonium sulphate is generally regarded as safe and environmentally manageable when used responsibly. However, improper handling and over-application can lead to ecological and health issues.
Environmental concerns:
- Soil acidification: Prolonged use in neutral or acidic soils can lower soil pH, potentially affecting soil microbial activity and nutrient balance.
- Eutrophication: Runoff containing ammonium and sulphate ions can contribute to nutrient pollution in water bodies, promoting algal blooms.
- Air quality: When mixed with alkaline dust or ammonia emissions, it can contribute to the formation of ammonium sulphate aerosols, which are components of fine particulate matter (PM₂.₅).
Health and safety:
- Ammonium sulphate is non-toxic at normal concentrations but may cause irritation to the skin, eyes, and respiratory tract upon direct contact or inhalation of dust.
- It is non-flammable and non-explosive, making it safe to store and transport.
- In food and pharmaceutical uses, it is considered Generally Recognised as Safe (GRAS) by regulatory authorities.
Appropriate protective measures, such as gloves and masks, are recommended during industrial handling to prevent irritation and ensure occupational safety.
Advantages and Limitations
Advantages:
- Provides a balanced supply of nitrogen and sulphur.
- Highly stable and resistant to volatilisation.
- Readily soluble and easy to apply.
- Enhances soil fertility in alkaline conditions.
- Useful as a by-product in various industries, promoting resource efficiency.
Limitations:
- May cause soil acidification in long-term use.
- Lower nitrogen content compared to urea (21% vs 46%), requiring larger quantities for equivalent nitrogen supply.
- Energy-intensive production when not derived as a by-product.
- Dust formation during handling may cause minor respiratory irritation.
Industrial and Economic Significance
Ammonium sulphate remains a major global fertiliser, particularly in Asia, Africa, and South America, where soil sulphur deficiencies are prevalent. It is valued not only for its nutrient content but also for its role in integrated nutrient management systems. The growing biodiesel and nylon industries ensure a constant supply of by-product ammonium sulphate, supporting circular economy practices.
Economically, it is less expensive than other nitrogenous fertilisers, making it a preferred choice for small-scale and developing farmers. Its use also supports industrial ecosystems by converting waste ammonia and sulphuric acid streams into valuable fertiliser products.
Future Prospects and Sustainability
The future of ammonium sulphate lies in its integration into sustainable agriculture and green chemical production. Efforts are being made to improve efficiency through co-granulated fertilisers combining ammonium sulphate with urea or phosphate compounds. In biotechnology, advanced purification methods are enhancing its role in protein isolation and pharmaceutical formulations.
As industries move toward zero-waste production, ammonium sulphate’s by-product recovery from steel, nylon, and biodiesel manufacturing will continue to exemplify industrial symbiosis. Environmentally, precision agriculture techniques will ensure its optimal application, minimising acidification and nutrient losses.