Phosphoric acid, with the chemical formula H3PO4, is a central acidic compound in many industries: food processing, agriculture, etc. Its structure, chemical properties and ability to form bonds with various ions make it an essential compound for applications ranging from fertilizer manufacturing to biochemistry.
In this first part, we will detail the fundamental characteristics of phosphoric acid, its molecular structure and unique properties, as well as the different forms in which it is marketed.
1. Introduction to phosphoric acid (H3PO4)
Chemical formula and molecular structure
Phosphoric acid, or orthophosphoric acid, is a mineral oxoprotic acid with three acidic functional groups. Its chemical formula, H3PO4, represents three hydrogen atoms, one phosphorus atom, and four oxygen atoms. The structure of H3PO4 consists of a central phosphorus atom bonded to three -OH (hydroxyl) groups and a doubly bonded oxygen atom, thus forming a tetrahedral molecule. This configuration gives it distinct acidic properties and the ability to release three protons (H+) in aqueous solution, making it a triprotic acid.
Physical and chemical properties
Phosphoric acid is commonly found as a crystalline solid at room temperature with a density of 1.83 g/cm³. However, this solid liquefies at a relatively low temperature of 42.35 °C and decomposes at higher temperatures to form compounds such as pyrophosphoric acid. In aqueous solution, phosphoric acid becomes colorless and viscous, often available in concentrations of 75% or 85%, suitable for industrial needs.
Chemically, H3PO4 is a triacid capable of successively donating its three protons, leading to the formation of conjugate ions: dihydrogen phosphate (H2PO4−), hydrogen phosphate (HPO42−), and phosphate (PO43−). This behavior in solution is determined by specific equilibrium constants (Ka):
- Ka1 = 7.13 x 10⁻³ (pKa1 = 2.16) for the first ionization;
- Ka2 = 6.38 x 10⁻⁸ (pKa2 = 7.21) for the second ionization;
- Ka3 = 3.79 x 10⁻¹³ (pKa3 = 12.32) for the third ionization.
These pKa values indicate that phosphoric acid is a moderate to strong acid for the first dissociation and weaker for the next two, allowing it to be used in applications requiring fine pH control.
Different forms and business solutions
Phosphoric acid is rarely used in its pure form. In industry, it is most often found in aqueous solutions of 75% to 85%, as these solutions are stable, easy to handle, and suitable for various manufacturing processes. Depending on the concentration, there is also superphosphoric acid, a phosphoric acid solution enriched with phosphorus pentoxide (P₂O₅), resulting in a mixture of H₃PO₄ and phosphoric acid oligomers.
Phosphoric acid comes in various forms, from pure acid to diluted solutions, and is marketed according to the specific needs of each industry, whether it be agriculture, detergent production, or cosmetics. These solutions allow manufacturers to utilize the properties of H3PO4 while ensuring greater safety in product handling.
This first part provides a clear and detailed introduction to phosphoric acid, explaining its fundamental characteristics, chemical structure, and the different forms in which it is commonly used.
2. Processes for manufacturing phosphoric acid
Phosphoric acid is primarily manufactured using two industrial processes: the wet process and the thermal process. Each method offers unique characteristics, costs, and specific applications. This section will detail the production process of H3PO4, describing the major steps and the byproducts generated.
The wet process
The wet process is the most widely used for the industrial production of phosphoric acid due to its relatively low cost. It involves chemically attacking phosphate ore with sulfuric acid (H2SO4), a chemical reaction that generates phosphoric acid in the form of an aqueous solution.
- Grinding stage : The raw phosphate is first ground to increase its reaction surface, thus facilitating attack by sulfuric acid.
- Attack and filtration : The crushed phosphate is mixed with concentrated sulfuric acid (98.5%) and phosphoric acid (18-22% P₂O₅). This mixture produces a slurry containing phosphoric acid (29% P₂O₅) and phosphogypsum, a solid by-product. Filtration separates the phosphoric acid from this slurry, allowing the phosphogypsum to be collected for further processing.
- Concentration : To obtain a higher concentration, phosphoric acid is heated to evaporate some of the water, thus increasing the P2O5 content to 54%.
The wet process produces a by-product, phosphogypsum, an industrial residue that poses environmental challenges because it sometimes contains radioactive elements naturally present in the ore. Research is underway to find uses for this residue, for example in construction materials.
The thermal process
The thermal process is less common due to its higher cost, but it allows obtaining a very high purity phosphoric acid, used in particular in food, pharmaceutical applications and in fuel cells.
- Phosphorus production : This process begins with the calcination of apatite (a phosphate ore) in the presence of silica (SiO2) and carbon (C), producing elemental phosphorus in the form of vapor.
- Phosphorus oxidation : Phosphorus vapor is then oxidized in a controlled environment to form phosphorus pentoxide (P2O5).
- Hydration : P2O5 reacts with water vapor to produce extremely pure phosphoric acid, without the impurities found in the wet process.
The thermal process is particularly suitable when purity is essential, especially in the pharmaceutical industries, where the presence of contaminants must be strictly limited.
Comparison of processes and industrial choices
The choice between the wet process and the thermal process depends largely on the final application of the phosphoric acid as well as economic and environmental requirements:
- Wet process : Primarily used to produce fertilizers, detergents and other industrial products, due to its low cost.
- Thermal process : Preferred for applications requiring high-purity acid, despite a higher cost.
This second part explores in detail the manufacturing processes of phosphoric acid, specifying the chemical reactions, the production stages and the uses of the generated by-products.
3. Uses and applications of phosphoric acid
Phosphoric acid is one of the most versatile acids in the chemical industry. It finds applications in numerous sectors, ranging from fertilizer production to the manufacture of food and cleaning products, as well as medical and specialized industrial applications. This section examines the main uses of phosphoric acid in detail.
Phosphate fertilizer production
The most important application of phosphoric acid is in the manufacture of fertilizers. It is a major source of phosphorus, an essential nutrient for plants.
- Superphosphate fertilizers : Phosphoric acid is a key ingredient in the production of superphosphates, which provide soluble forms of phosphorus for efficient absorption by plants. These fertilizers are widely used to improve soil fertility and increase agricultural yields.
- Compound fertilizers : Phosphoric acid is also used in complex fertilizers containing phosphorus, nitrogen, and potassium (NPK). These fertilizers provide crops with a balanced supply of essential nutrients.
Fertilizer production accounts for approximately 80% of global phosphoric acid consumption, highlighting its importance in modern agriculture.
Food additive
Phosphoric acid is commonly used in the food industry as an additive. It has the code E338 and is primarily used for its acidifying and pH-regulating properties.
- Carbonated drinks : Phosphoric acid is a common ingredient in sodas, especially cola-type drinks. It provides acidity that enhances the flavor and helps preserve the product's freshness.
- Processed products : Phosphoric acid is found in various processed food products to stabilize pH and control acidity, particularly in jams, jellies and processed cheeses.
Although this use is widespread, studies indicate that high levels of phosphoric acid consumption can affect health, including increasing the risk of kidney stones and bone fragility.
Industrial cleaner and rust treatment
Phosphoric acid is commonly used as a cleaning and surface treatment agent, particularly for removing rust and preparing metal surfaces.
- Rust removal treatment : Phosphoric acid reacts with iron oxide (rust) to form a layer of iron phosphate that protects the metal from future oxidation. This application is common in the automotive, construction, and metal equipment maintenance industries.
- Cleaning products : Phosphoric acid is an ingredient in specialized cleaning products for removing limescale and rust deposits from hard surfaces such as ceramic and metal. It is also used for cleaning equipment in the food industry, where it helps remove stubborn residues without affecting surface quality.
Medical and pharmaceutical applications
In the medical field, phosphoric acid plays an important role as an intermediate agent and additive in the formulation of certain drugs and dental products.
- Dental : Phosphoric acid is used for etching teeth during dental procedures, creating a rough surface that facilitates the adhesion of composite materials, such as resins and sealants.
- Pharmaceutical products : Phosphoric acid is used as a reagent in the synthesis of various pharmaceutical compounds. It also serves as a buffer to stabilize drug formulations.
Fuel cell industry and other specialized applications
Phosphoric acid also finds applications in high-tech industrial sectors.
- Fuel cells : Phosphoric acid is used as an electrolyte in acid phosphorus fuel cells (APFCs), which are primarily used in stationary power generation applications. It contributes to clean energy production using fuels such as natural gas.
- Chemical synthesis : In petrochemicals and other industrial sectors, phosphoric acid is used as a catalyst to promote certain chemical reactions, particularly those involving the conversion of hydrocarbons.
This third part details the many applications of phosphoric acid, highlighting its importance in the agricultural, food, industrial and medical sectors.
Are you looking for an analysis?
4. Laboratory analysis of phosphoric acid
Accurate determination of phosphoric acid concentration and purity is essential in many sectors, whether to guarantee the quality of finished products or to ensure compliance with health and safety standards. Various analytical techniques are used in the laboratory to analyze phosphoric acid. This section explores the main analytical methods, their applications, and the importance of regulatory compliance in these analyses.
Phosphoric acid analysis techniques
Several analytical methods exist for quantifying phosphoric acid or detecting its impurities in various matrices. Among the most commonly used techniques are:
- Ion chromatography : This technique allows the separation and measurement of ions present in a solution, including phosphate ions derived from phosphoric acid. Ion chromatography is particularly useful for quality analysis in environmental and food industries.
- Spectrophotometry : By measuring the absorption of light at a specific wavelength, this method allows for the estimation of phosphoric acid concentration in a solution. This technique is rapid and well-suited for routine analyses.
- Acid-base titration : Used to determine the concentration of phosphoric acid solutions, this method involves titrating the acid with a base, such as sodium hydroxide (NaOH), until an identifiable equivalence point is reached. Although less sophisticated than other methods, acid-base titration remains an effective method for common industrial applications.
These methods ensure high analytical accuracy and allow the measurement of phosphoric acid concentrations in complex samples, which is essential to ensure product compliance in various industrial sectors.
Importance of conformity testing and standards
Phosphoric acid analyses must adhere to strict standards to ensure product safety and quality. Two main reference frameworks guide these laboratory analyses:
- ISO 17025 : This international standard sets out the requirements for the competence of calibration and testing laboratories. It ensures that the results provided are reliable and reproducible, which is crucial for industries that use phosphoric acid, such as the food and pharmaceutical industries.
- COFRAC Standards : In France, laboratories accredited by the French Accreditation Committee (COFRAC) meet the requirements of ISO standards and ensure strict compliance with European standards. This accreditation guarantees that the analyses performed in these laboratories comply with current regulations.
Specific applications of phosphoric acid analysis
Phosphoric acid analysis is fundamental to meeting the needs of various sectors:
- Food industry : Regulations require that the levels of phosphoric acid used as an additive (E338) be controlled to ensure consumer safety. Laboratories therefore analyze the concentrations in food products, particularly in soft drinks and other processed foods.
- Environmental sector : Due to its widespread industrial use, phosphoric acid can end up in wastewater and soil. Precise analyses can detect traces of phosphoric acid in environmental samples, thus contributing to the protection of ecosystems and the prevention of pollution.
- Medical applications : In pharmaceuticals and medical devices, it is essential to control phosphoric acid levels to avoid any risk of toxicity. Laboratories perform rigorous analyses to ensure that products meet purity and safety standards.
Sampling methods and accuracy requirements
To ensure reliable results, strict sampling protocols are followed in phosphoric acid analyses. For example, specific filters such as PTFE filters coupled with quartz filters containing sodium carbonate (Na₂CO₃) are used to collect samples. These sampling media allow for optimal collection of phosphate ions, guaranteeing a low limit of quantification (LOQ), often in the range of 2 to 4 µg/media depending on the method used.
This fourth part examines in detail the methods of analysis of phosphoric acid, highlighting the importance of quality standards and rigorous sampling protocols to ensure reliable and compliant results.
5. Effects of phosphoric acid on health and the environment
Phosphoric acid, although widely used in various industrial sectors, presents certain risks to human health and the environment. This section explores the potential effects of this compound, the risks associated with its use, and the recommended precautions to minimize these impacts.
Effects on human health
Phosphoric acid is a corrosive acid, and exposure to it, even at moderate concentrations, can cause irritation and burns. Health effects depend on the route of exposure, whether through skin, inhalation, or ingestion.
- Skin and eye exposure : When in direct contact with the skin, phosphoric acid can cause burns and irritation. Splashes in the eyes can cause severe damage to eye tissues, or even loss of vision if exposure is prolonged without proper treatment.
- Inhalation : Phosphoric acid, in the form of vapors or aerosols, can be inhaled, which may cause respiratory irritation. Prolonged inhalation or inhalation of high concentrations can cause more serious respiratory problems, such as chemical bronchitis and lung damage.
- Ingestion : Phosphoric acid is present in some foods and beverages, but at regulated concentrations. However, ingestion of high amounts can cause electrolyte imbalances, digestive problems, and bone demineralization, due to its ability to interfere with the absorption of calcium and other minerals.
Effects on the environment
The industrial use of phosphoric acid and its release into the environment can cause pollution problems, particularly with regard to waterways and soils.
- Water contamination : Discharges of phosphoric acid into wastewater, when not properly treated, can lead to contamination of waterways. Phosphoric acid contributes to eutrophication, a phenomenon that results in excessive algal blooms and a reduction in the water's oxygen content, negatively impacting aquatic ecosystems.
- Soil accumulation : Phosphoric acid can also accumulate in soils, leading to an imbalance in soil chemical composition and crop contamination. In the long term, this can reduce soil quality and limit microbial biodiversity, which is essential for crop health.
Regulations and safety precautions
To limit the negative effects of phosphoric acid on health and the environment, several regulations have been implemented at the national and international levels. Good industrial practices include compliance with these regulations and the adoption of strict safety measures.
- Current regulations : Regulations such as the European REACH regulation impose restrictions on the use and handling of phosphoric acid, requiring precautionary measures in its storage, transport, and disposal. Furthermore, food safety standards limit the concentrations of phosphoric acid in consumable products, ensuring consumer protection.
- Personal protective equipment (PPE) : In industrial settings, it is essential that workers handle phosphoric acid while wearing appropriate protective equipment, such as chemical-resistant gloves, safety glasses, and protective clothing to avoid the risks of direct contact.
- Wastewater treatment : Industrial facilities are required to treat their wastewater containing phosphoric acid before discharging it into waterways or soil. Wastewater treatment systems may include neutralization and chemical precipitation steps to remove residual acid.
