Ca₃(PO₄)₂ — Calcium Phosphate

Calcium phosphate (Ca₃(PO₄)₂) is a white, inorganic compound composed of calcium and phosphate ions. It occurs naturally as minerals like apatite and is a key component of bones and teeth in vertebrates. It forms about 70% of the bone structure, giving rigidity and strength to the skeletal system. Due to its biocompatibility, calcium phosphate is used in medical applications such as bone grafts and dental cements.

Industrially, it plays an important role in fertilizers, food fortification, and animal feed. The compound is also used in pharmaceutical formulations as a source of calcium and phosphorus, essential elements for biological growth and maintenance.

Calcium phosphate has the chemical formula \(Ca_3(PO_4)_2\), consisting of three calcium ions (Ca²⁺) and two phosphate ions (PO₄³⁻). Each phosphate ion contains one phosphorus atom surrounded tetrahedrally by four oxygen atoms, forming a covalently bonded group.

The overall structure is a stable ionic lattice. In nature, calcium phosphate often occurs as hydroxyapatite, \(Ca_5(PO_4)_3(OH)\), which forms the mineral basis of bones and teeth. Hydroxyapatite crystals are embedded in a collagen matrix, creating a composite material that combines strength and flexibility.

In solid-state chemistry, calcium phosphate exists in various forms or polymorphs, including tricalcium phosphate (TCP), monocalcium phosphate (MCP), and dicalcium phosphate (DCP), each with distinct solubility and reactivity profiles.

Natural Occurrence: Calcium phosphate occurs in nature as mineral apatite, fluorapatite \(Ca_5(PO_4)_3F\), and hydroxylapatite \(Ca_5(PO_4)_3(OH)\). It forms in sedimentary rocks, bones, and shells, contributing to geological and biological cycles of calcium and phosphorus.

Industrial Preparation: Calcium phosphate is generally synthesized by reacting calcium salts with phosphate salts or phosphoric acid.

1. Reaction of Calcium Hydroxide with Phosphoric Acid:

This reaction produces calcium phosphate as a white precipitate, which is filtered and dried for use.

2. From Calcium Carbonate and Phosphoric Acid:

This method is commonly used in fertilizer production.

3. By Neutralization of Dicalcium Phosphate:

Dicalcium phosphate can be converted to tricalcium phosphate by heating or reacting with calcium hydroxide in an aqueous medium.

• Appearance: White crystalline or powdery solid, insoluble in water but soluble in dilute acids.
• Thermal Stability: Stable at room temperature; decomposes at high temperatures (>1500°C) forming calcium oxide and phosphorus oxides.

\(Ca_3(PO_4)_2 \xrightarrow{heat} 3CaO + P_2O_5\)

• Acid-Base Behavior: Reacts with acids such as hydrochloric acid to form soluble calcium salts and phosphoric acid.

\(Ca_3(PO_4)_2 + 6HCl \rightarrow 3CaCl_2 + 2H_3PO_4\)

• Solubility: Practically insoluble in water; this property is useful in slow-release fertilizers.
• pH Range: Mildly basic when mixed in water due to partial hydrolysis of phosphate ions.
• Conductivity: Poor conductor of electricity in solid state but can conduct ions in solution when partially dissolved.

• 1. Fertilizer Industry: Calcium phosphate is one of the most important phosphate fertilizers. It provides essential phosphorus for plant growth, improving root development and flowering. Superphosphate fertilizers are derived by treating calcium phosphate with sulfuric acid.
• 2. Food Industry: Used as a food additive (E341) in dairy products, baking powder, and nutritional supplements to supply calcium and phosphorus.
• 3. Medical and Dental Applications: Due to its chemical similarity to bone mineral, calcium phosphate is used in bone grafts, dental implants, and tissue engineering. It promotes bone regeneration and osteointegration.
• 4. Pharmaceutical Applications: Acts as an excipient in tablet formulations, serving as a filler or binding agent.
• 5. Animal Feed: Supplemented in livestock feed to promote bone strength and overall health.
• 6. Water Treatment: Used in water purification processes to remove metal ions by precipitation as insoluble phosphates.

Calcium phosphate is biologically essential, making up a significant portion of bones and teeth in vertebrates. It contributes to skeletal strength, nerve transmission, muscle contraction, and enzyme function. The compound also acts as a phosphate reserve in the body, maintaining calcium-phosphorus balance through bone resorption and deposition.

Hydroxyapatite, a naturally occurring form of calcium phosphate, provides hardness and durability to teeth enamel. Calcium phosphate also plays a role in energy metabolism as phosphate groups are vital components of ATP (adenosine triphosphate), DNA, and RNA.

Calcium phosphate is generally considered safe and non-toxic when used in controlled amounts. However, excessive intake through supplements or diet may lead to hypercalcemia or phosphate imbalance. Inhalation of fine powder can cause minor respiratory irritation.

Safety Measures:

• Use protective gloves and masks during handling to avoid dust inhalation.
• Store in dry, airtight containers away from acids and moisture.
• Dispose of waste responsibly as per environmental safety norms.

In medical applications, synthetic calcium phosphate must be highly pure to prevent immune responses or inflammation when implanted in the body.

Calcium phosphate is considered environmentally safe. However, phosphate runoff from fertilizers can cause eutrophication, leading to algal blooms and oxygen depletion in water bodies. Sustainable agricultural practices recommend controlled application to minimize this risk.

Industries are adopting eco-friendly alternatives like bio-based phosphate fertilizers and recycling phosphate from wastewater to maintain environmental balance.