Phosphoric acid is one of the most widely used inorganic acids in modern industry. It is used in the production of fertiliser, the processing of food, the treatment of metals, industrial cleaning, water treatment and the manufacture of chemicals.
But what exactly is phosphoric acid, and why is it used in so many different industries?
This guide explains its chemical properties, how it is produced, and its most common industrial uses.
What Is Phosphoric Acid?
Phosphoric acid is an inorganic mineral acid with the chemical formula H₃PO₄. It is also commonly known as orthophosphoric acid.
In its pure form, phosphoric acid is a transparent, crystalline solid. For commercial use, it is usually supplied as a clear, colourless liquid solution in water.
Below are some of the key chemical properties of phosphoric acid:
| Property | Value |
| Chemical name | Phosphoric Acid / Orthophosphoric Acid |
| Linear formula | H₃PO₄ |
| CAS Number | 7664-38-2 |
| Molecular Weight | 97.994 g/mol |
| Appearance | Clear, colorless liquid or transparent crystalline solid |
| Odor | Odorless |
| Density | 1.6845 g/cm³ at 25°C for 85% solution |
| Density of solid | 1.834 g/cm³ |
| Melting point | 40–42.4°C |
| Solubility in water | Miscible |
Commercial phosphoric acid is available in different concentrations and grades depending on the intended application. 85% phosphoric acid, for example, is widely used in industrial applications.
How Is Phosphoric Acid Made?
There are two primary methods for producing phosphoric acid: the wet process and the thermal process.
Wet Process
The wet process is widely used for large-scale industrial production.
In this process, phosphate rock reacts with sulfuric acid to produce phosphoric acid. Calcium sulfate and other by-products are also formed during the reaction. The resulting phosphoric acid can then be filtered, concentrated, and further purified depending on the required specification.
Because of its suitability for large-scale production, the wet process is commonly associated with fertilizer and industrial phosphoric acid production.
Thermal Process
The thermal process is generally used when higher-purity phosphoric acid is required.
Elemental phosphorus is first burned to produce phosphorus oxides. These oxides are then reacted with water to form phosphoric acid.
Compared with the wet process, the thermal process can produce phosphoric acid with a higher level of purity, making it suitable for applications with stricter quality requirements.
What Is Phosphoric Acid Used For?
Phosphoric acid has a wide range of industrial applications. Its chemical properties make it useful as an acidulant, pH regulator, cleaning agent, and raw material for phosphate compounds.
Some of its major applications include:
- Fertilizer production
- Food and beverage processing
- Rust removal
- Metal surface treatment
- Industrial cleaning
- Water treatment
- Chemical manufacturing
- Dental applications
- Detergent production
Let’s take a closer look at some of the most common uses.
1. Phosphoric Acid for Fertilizer Production
One of the largest applications of phosphoric acid is fertilizer production.
Phosphorus is an essential nutrient for plant growth, and phosphoric acid is an important raw material for producing phosphate-based fertilizers.
It can be used to manufacture various phosphate salts and fertilizer products that provide phosphorus for agricultural applications.
For fertilizer manufacturers, factors such as concentration, purity, phosphate content, impurity levels, and supply consistency can be important when selecting a phosphoric acid supplier.
2. Phosphoric Acid in Food and Beverage Processing
Phosphoric acid is also used in the food and beverage industry.
It can function as an acidulant and acidity regulator, helping control the pH and provide a characteristic acidic taste in certain beverages and food products.
Phosphoric acid is particularly well known for its use in cola-type soft drinks.
Different grades of phosphoric acid are available for different applications, so food and beverage manufacturers should ensure that the product meets the required food-grade specifications and applicable regulations.
3. Phosphoric Acid for Rust Removal
Phosphoric acid is widely used for rust removal and metal surface treatment.
Rust is mainly composed of iron oxides and hydroxides. Phosphoric acid reacts with these corrosion products and converts them into phosphate compounds, helping remove or transform rust on metal surfaces.
This makes phosphoric acid useful for:
- Rust removal
- Rust conversion
- Metal cleaning
- Surface preparation
- Pretreatment before painting
- Metal finishing
For industrial applications, the effectiveness of phosphoric acid depends on factors such as acid concentration, temperature, contact time, and the condition of the metal surface.
4. Phosphoric Acid for Industrial Cleaning
Another important application is industrial cleaning.
Phosphoric acid can react with mineral deposits and metal oxides, making it useful for removing:
- Rust
- Limescale
- Calcium deposits
- Mineral scale
- Oxide deposits
- Hard-water residues
Because of these properties, phosphoric acid can be used in certain industrial cleaning formulations for equipment, metal surfaces, pipes, and other applications where mineral deposits need to be removed.
5. Phosphoric Acid in Chemical Manufacturing
Phosphoric acid is also an important raw material in chemical manufacturing.
It can be used to produce various phosphate salts, phosphates, polyphosphates, and other phosphorus-containing chemicals.
These downstream products can be used in:
- Water treatment
- Detergents
- Agriculture
- Food processing
- Industrial chemicals
- Metal treatment
For many chemical manufacturers, phosphoric acid is therefore not the final product but an important raw material for downstream production.
6. Phosphoric Acid in Dental Applications
Phosphoric acid also has specialized applications in the dental industry.
Phosphoric acid etching solutions can be used to prepare tooth surfaces before certain restorative procedures. The treatment modifies the surface of enamel or dentin, helping dental materials achieve better adhesion.
This application requires controlled concentrations and professional handling.
7. Phosphoric Acid in Water Treatment and Other Industries
Beyond the applications mentioned above, phosphoric acid can also be used in:
- Water treatment
- Electro-polishing
- Metal finishing
- Detergent production
- Ceramic processing
- Surface treatment
- Chemical synthesis
Its versatility allows phosphoric acid to serve different functions across a wide range of industrial processes.
This also demonstrates the very broad range of industrial applications of phosphoric acid.