H3PO4 — Phosphoric Acid
Phosphoric acid (H₃PO₄) is a weak mineral acid widely used in fertilizers, food processing, dentistry, and chemical industries for its acidic and rust-removal properties.
Interactive 3D Molecular Structure — H3PO4
Properties
| Chemical Formula | H₃PO₄ |
|---|---|
| Molecular Mass | 97.99 g/mol |
| Physical State | Liquid (syrupy at room temperature) |
| Melting Point | 42.35°C |
| Boiling Point | 213°C (decomposes before boiling) |
| Density | 1.885 g/cm³ (concentrated acid) |
| pH | 1.5 (for 0.1 M solution) |
| Odor | Odorless |
| Color | Colorless to slightly yellow viscous liquid |
| Taste | Sour (acidic) |
| Polarity | Highly polar |
| Type of Bond | Covalent (polar bonds between H, P, and O atoms) |
Introduction to Phosphoric Acid
Phosphoric acid (H₃PO₄) is a triprotic mineral acid that plays a vital role in agriculture, medicine, and industry. It is a non-toxic, colorless, and syrupy liquid that is less corrosive than strong acids like sulfuric or hydrochloric acid. Commonly known as orthophosphoric acid, it is one of the most important compounds of phosphorus and is used extensively in fertilizers, food processing, and metal treatment.
Phosphoric acid is essential in the production of phosphate fertilizers such as monoammonium phosphate (MAP) and diammonium phosphate (DAP), which contribute significantly to global agricultural yield. Due to its mild acidity, it is also used in beverages, dental products, and cleaning agents.
Structure and Bonding
Phosphoric acid has the chemical formula \(H_3PO_4\), consisting of three hydroxyl groups (-OH) and one double-bonded oxygen atom attached to a central phosphorus atom. The phosphorus atom is in a +5 oxidation state. The molecular geometry of phosphoric acid is tetrahedral, with sp³ hybridization around the phosphorus atom.
\(H_3PO_4 \leftrightarrow PO(OH)_3\)
In aqueous solution, H₃PO₄ can ionize in three steps, releasing one proton at each stage:
\(H_3PO_4 \leftrightarrow H^+ + H_2PO_4^-\)
\(H_2PO_4^- \leftrightarrow H^+ + HPO_4^{2-}\)
\(HPO_4^{2-} \leftrightarrow H^+ + PO_4^{3-}\)
This triprotic nature makes phosphoric acid capable of forming three types of salts — dihydrogen phosphate, hydrogen phosphate, and phosphate — depending on the degree of neutralization.
Preparation of Phosphoric Acid
Phosphoric acid can be produced industrially by two main methods: the wet process and the thermal process.
1. Wet Process
This is the most common method for manufacturing phosphoric acid for fertilizers. Phosphate rock (mainly calcium phosphate) is treated with concentrated sulfuric acid to produce phosphoric acid and calcium sulfate (gypsum) as a by-product:
\(Ca_3(PO_4)_2 + 3H_2SO_4 + 6H_2O \rightarrow 2H_3PO_4 + 3CaSO_4 \cdot 2H_2O\)
The acid obtained from this process is called wet-process phosphoric acid and contains impurities like fluorides and sulfates, which are removed for high-purity applications.
2. Thermal Process
In this method, phosphorus is burned in the presence of air to form phosphorus pentoxide, which is then hydrated to form phosphoric acid:
\(P_4 + 5O_2 \rightarrow 2P_2O_5\)
\(P_2O_5 + 3H_2O \rightarrow 2H_3PO_4\)
The acid produced by this process is of very high purity and is mainly used in the food and pharmaceutical industries.
Physical and Chemical Properties
- Appearance: Colorless, odorless, syrupy liquid that solidifies on cooling.
- Acid Strength: A weak triprotic acid that ionizes in three steps to release hydrogen ions.
- Reaction with Bases: Neutralizes bases to form phosphates and water.
- Reaction with Metals: Reacts slowly with metals such as iron, zinc, and aluminum to form hydrogen gas and metal phosphates.
- Dehydration Property: On heating above 200°C, phosphoric acid loses water and converts to pyrophosphoric acid (H₄P₂O₇):
- Oxidation State: Phosphorus remains in the +5 oxidation state in both the acid and its salts.
\(H_3PO_4 + 3NaOH \rightarrow Na_3PO_4 + 3H_2O\)
\(2Al + 2H_3PO_4 \rightarrow 2AlPO_4 + 3H_2\)
\(2H_3PO_4 \xrightarrow{200°C} H_4P_2O_7 + H_2O\)
Uses and Applications
- Fertilizer Industry: The majority of phosphoric acid is used in the production of phosphate fertilizers like DAP (diammonium phosphate), MAP (monoammonium phosphate), and TSP (triple superphosphate).
- Food Industry: Used as a food additive (E338) to acidify beverages, prevent discoloration, and enhance flavor in soft drinks and jams.
- Rust Removal: Phosphoric acid converts rust (iron oxide) into ferric phosphate, which can be easily scrubbed off.
- Dental and Medical Applications: Used in dental cements, tooth etching gels, and as an additive in pharmaceuticals.
- Chemical Industry: Acts as a precursor for phosphate salts, detergents, and cleaning agents.
- Electronics and Metal Treatment: Used in electropolishing, cleaning, and passivation of stainless steel surfaces.
- Water Treatment: Helps control pH and prevent scale formation in boilers and cooling systems.
Health and Safety Considerations
Phosphoric acid is considered a weak acid but can still cause irritation or burns on direct contact with skin or eyes. Inhalation of vapors or aerosols may irritate the respiratory tract. It is classified as moderately corrosive to metals and tissues.
During handling, use gloves, protective goggles, and appropriate ventilation. In case of spills, neutralize with sodium bicarbonate before cleaning. Ingestion of concentrated phosphoric acid can cause gastrointestinal injury and should be treated immediately under medical supervision.
However, at low concentrations, phosphoric acid is safe and is commonly found in soft drinks and dental care products.
Key Reactions of Phosphoric Acid
Reaction with Bases
Phosphoric acid reacts with bases to form corresponding phosphate salts and water:
\(H_3PO_4 + 3NaOH \rightarrow Na_3PO_4 + 3H_2O\)
This property makes it useful for neutralization processes in various industrial applications.
Dehydration Reaction
On heating, phosphoric acid loses water to form pyrophosphoric acid (H₄P₂O₇):
\(2H_3PO_4 \xrightarrow{200°C} H_4P_2O_7 + H_2O\)
Further heating can lead to the formation of metaphosphoric acid (HPO₃).
Reaction with Metals
Phosphoric acid reacts slowly with reactive metals like aluminum and zinc to form metal phosphates and release hydrogen gas:
\(2Al + 2H_3PO_4 \rightarrow 2AlPO_4 + 3H_2\)