Two commonly used acids, phosphoric acid and citric acid, are versatile and play significant roles in manufacturing processes, food preservation, and even cleaning solutions. Despite their similarities, they differ in chemical properties, applications, and health implications. This article delves into the differences between phosphoric acid and citric acid, providing a comprehensive comparison of these two essential acids.
Chemical Structure and Properties
Phosphoric Acid:
A tetrahedral structure is produced when the phosphorus atom and the oxygen atoms create covalent connections. It has a strong acidic nature due to the presence of three replaceable hydrogen ions (protons) that can dissociate in water.
- Molecular Formula: H₃PO₄
- pH (concentration): 1.5 – 2.5
- Acidity: Strong acid
- Solubility: Highly soluble in water
- Uses: Fertilizers, food additives, rust removal, and water treatment.
Citric Acid:
With three carboxyl groups (-COOH) and one hydroxyl group (-OH), citric acid is a weak organic acid. Citrus fruits frequently contain it, although fermentation can also be used to create it artificially. The structure of citric acid allows it to act as a buffer, adjusting the acidity of solutions and making it less harsh than mineral acids like phosphoric acid.
- Molecular Formula: C₆H₈O₇
- pH (concentration): 2.0 – 3.0
- Acidity: Weak acid
- Solubility: Soluble in water
- Uses: Food and beverage industry, cleaning products, cosmetics, pharmaceuticals.
Manufacturing Processes
Phosphoric Acid Production:
The two main processes used to manufacture phosphoric acid are the wet process and the dry process. In the wet method, calcium sulfate (gypsum) and phosphoric acid are produced by combining phosphate rock with sulfuric acid. The dry process, on the other hand, involves heating phosphate rock in a furnace with a carbon material, typically coke, producing phosphoric acid vapors, which are condensed into a liquid.
Citric Acid Production:
Citric acid is produced primarily through fermentation. The process begins with the fermentation of sugars, often derived from corn or molasses, by certain strains of mold (such as Aspergillus niger). Citric acid is created throughout the fermentation process, refined, and crystallized for usage in a variety of ways.
Comparison of Applications
| Feature | Phosphoric Acid | Citric Acid |
| Source | Synthesized from phosphate rocks | Naturally occurring in citrus fruits |
| Acidity | Strong acid (pH 1.5 – 2.5) | Weak acid (pH 2.0 – 3.0) |
| Common Uses | Fertilizers, rust removers, soft drinks, water treatment | Food and beverages, cleaning products, cosmetics |
| Food Industry | Flavoring agent in sodas, pH regulator | Preservative, flavor enhancer, acidulant |
| Environmental Impact | Can cause environmental harm if improperly disposed | Biodegradable, safer for the environment |
| Health Concerns | Excessive intake linked to kidney problems, bone issues | Generally recognized as safe but excessive intake can cause tooth erosion |
| Industrial Use | Used in detergents, paints, and pharmaceuticals | Used in cosmetics, medicines, and cleaning agents |
Food and Beverage Industry
Phosphoric acid is commonly used in soft drinks, especially cola, where it contributes to the beverage’s tangy flavor and helps to regulate the pH. It is also used as a preservative in certain processed foods.
On the other side, citric acid is frequently used in beverages, jams, jellies, candies, and even processed cheeses as an acidulant, taste enhancer, and preservative. It imparts a tart flavor and helps preserve food by lowering the pH, preventing microbial growth.
Cleaning and Descaling
A common component of many cleaning solutions and rust removers, especially those intended for industrial use, is phosphoric acid. Its ability to react with and dissolve mineral deposits, such as calcium and iron oxide, makes it ideal for cleaning and descaling purposes.
Citric acid, being a weaker acid, is commonly used in household cleaning products, especially for removing limescale from kettles, dishwashers, and other appliances. Its mild nature makes it safe for household use and non-toxic, unlike stronger acids.
Health Implications and Safety
Phosphoric Acid:
In food and drink, phosphoric acid is generally accepted to be safe when used in moderation. However, excessive consumption, particularly through cola drinks, has been linked to health concerns such as kidney damage and reduced bone density. If phosphoric acid is regularly ingested in significant quantities, its extreme acidity can also cause erosion of dental enamel.
Phosphoric acid, being a strong acid, can cause burns and irritation to the skin, eyes, and mucous membranes. When handling concentrated phosphoric acid in an industrial context, protective gear such as gloves and goggles is required.
Citric Acid:
Citric acid is considered a safer option in the food industry. According to the U.S. Food and Drug Administration (FDA), it is usually considered harmless and occurs naturally in fruits. In concentrated forms, it can irritate the skin and eyes, but in regular dietary amounts, it is unlikely to be harmful.
One of the main health concerns associated with citric acid is its potential to erode tooth enamel if consumed excessively, especially in the form of acidic beverages or candies. However, its use in food products is typically well-regulated to prevent any harmful effects.
Environmental Impact
Phosphoric Acid:
If phosphoric acid is not handled or disposed of appropriately, it may have detrimental effects on the environment. It can seriously contaminate streams and is poisonous to aquatic life. Aquatic ecosystems may suffer as a result of water bodies becoming acidified by phosphoric acid-containing industrial effluents. For this reason, the use of phosphoric acid must be controlled in industrial processes, especially in large quantities.
Citric Acid:
Citric acid is biodegradable and poses minimal environmental impact. It is a natural substance found in many plants and fruits, and its production through fermentation processes is more sustainable compared to the mining of phosphate rocks for phosphoric acid.
While both phosphoric acid and citric acid are widely used in various industries, their properties, applications, and potential health and environmental impacts differ significantly. Phosphoric acid is a strong mineral acid, commonly used in fertilizers, rust removers, and soft drinks, while citric acid is a weaker organic acid found in citrus fruits, with applications primarily in food preservation, cleaning, and cosmetics.
Phosphoric acid can be harmful if not handled carefully, especially in concentrated forms, and its excessive use in beverages has been associated with health concerns. In contrast, citric acid is generally considered safer and more environmentally friendly due to its natural origins and biodegradable properties.
Ultimately, the choice between phosphoric acid and citric acid depends on the specific application, safety requirements, and environmental considerations. For industrial and heavy-duty uses, phosphoric acid is often preferred, while citric acid remains the acid of choice for food, beverage, and household applications.
Both acids continue to play crucial roles in various sectors, but understanding their differences is key to ensuring their safe and effective use in everyday life and industrial processes.