Calc Phos: How Dibasic Calcium Phosphate Is Made

At a glance

Calcium-phosphate minerals are abundant in nature, but they occur primarily as members of the apatite family rather than as large deposits of the exact compound used for Calc Phos. The Calc Phos ingredient is dibasic calcium phosphate, CaHPO₄, which occurs naturally as the comparatively uncommon mineral monetite. Its dihydrate, CaHPO₄·2H₂O, occurs as brushite.

Because monetite and brushite are not the usual commercial sources, manufacturers commonly begin with apatite-rich phosphate rock, purify its phosphate content, and then deliberately form CaHPO₄ using a separate purified calcium source.

Making pharmaceutical-grade Calc Phos

1. Producing purified phosphate material

Phosphate rock is mined, crushed, concentrated, and purified to reduce clay, silica, carbonate minerals, metal-containing impurities, and other unwanted material. The phosphate rock begins mainly as fluorapatite, Ca₅(PO₄)₃F.

In the wet-process route, fluorapatite is treated with sulphuric acid and water. A simplified balanced reaction is:

2. Reacting phosphate with a purified calcium source

Purified phosphoric acid is reacted with a pharmaceutical-grade calcium source. Calcium carbonate and calcium hydroxide are two possible examples.

Calcium carbonate and phosphoric acid simplified reaction:

In this reaction, the carbonate portion leaves as carbon dioxide and water, while calcium combines with hydrogen phosphate to form CaHPO₄.

Calcium hydroxide and phosphoric acid simplified reaction:

Here, water is the only additional product.

3. Directing the reaction toward the dibasic compound

Phosphoric acid can form several calcium salts. The calcium-to-phosphate ratio, pH, reaction temperature, mixing, and degree of neutralization must therefore be controlled carefully.

Examples of various calcium phosphate salts:

• Monobasic calcium phosphate: Ca(H₂PO₄)₂
• Dibasic calcium phosphate: CaHPO₄
• Tribasic calcium phosphate: Ca₃(PO₄)₂

The manufacturing conditions are selected specifically to produce the dibasic compound required for Calc Phos.

4. Crystal formation and hydration state

Dibasic calcium phosphate is sparingly soluble and forms as a crystalline solid. Temperature, pH, concentration, mixing rate, addition rate, reaction time, and the calcium-to-phosphate ratio influence crystal size, morphology, and hydration state.

The distinction between anhydrous CaHPO₄ and the dihydrate CaHPO₄·2H₂O is important. The two forms have different molecular weights and water contents, so the required form must be created, preserved, and confirmed during quality testing.

5. Separation, washing, drying, and testing

The crystals are separated from the reaction liquid by filtration or centrifugation and washed with purified water to remove soluble by-products, residual acids or bases, and excess starting materials.

Controlled drying preserves the specified anhydrous form rather than unintentionally producing a hydrate or thermally altered material. The powder may then be milled and classified for consistent handling and trituration.

Testing may include identity, calcium assay, phosphate content, calcium-to-phosphate ratio, moisture, hydration state, particle characteristics, fluoride, heavy metals, elemental impurities, and soluble reaction residues.