Calcium phosphate, chemically known as tricalcium phosphate or \(\text{Ca}_3(\text{PO}_4)_2\), is an ionic compound composed of calcium cations (\(\text{Ca}^{2+}\)) and phosphate anions (\(\text{PO}_4^{3-}\)). It is classified as insoluble in neutral water. This resistance to dissolving at a neutral pH is a fundamental property underlying its major functions in nature and the human body. However, this insolubility is not absolute and can be significantly altered by changes in the surrounding environment.
Understanding the Rules of Solubility
The solubility of an ionic compound is determined by a competition between lattice energy and hydration energy. Lattice energy holds the ions together within the crystal structure, while hydration energy is released when water molecules stabilize the individual ions. For a compound to dissolve readily, the hydration energy must be greater than the energy required to break the crystal lattice.
\(\text{Ca}_3(\text{PO}_4)_2\) is insoluble because its high lattice energy far exceeds its hydration energy. The calcium ion has a \(+2\) charge and the phosphate ion carries a \(-3\) charge, resulting in a strong electrostatic attraction within the solid structure. This attraction is difficult for water molecules to overcome.
General solubility rules confirm this, noting that most compounds containing the phosphate ion are insoluble in water. Exceptions include phosphates of alkali metals (like sodium and potassium) and the ammonium ion. Since calcium is an alkaline earth metal, its phosphate compound follows the general rule of insolubility.
The Essential Role of Calcium Phosphate in Biological Systems
The insolubility of calcium phosphate is what makes it functionally important in living organisms, especially vertebrates. In the human body, calcium phosphate forms the mineral component of hard tissues like bone and teeth. The specific crystalline structure is called hydroxyapatite, which has the chemical formula \(\text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\).
This structure provides the rigidity and mechanical strength necessary for the skeleton to support the body. If hydroxyapatite were water-soluble, bones and teeth would rapidly dissolve in the body’s aqueous environment.
The controlled process of mineral turnover, known as bone remodeling, relies on this stable, insoluble structure. Beyond structural support, the reservoir of calcium phosphate in the skeleton plays a role in calcium homeostasis. This system ensures that the concentration of calcium ions in the blood remains within the narrow range necessary for nerve signaling and muscle contraction.
How Environmental Factors Influence Calcium Phosphate Solubility
While calcium phosphate is insoluble in neutral water, its solubility is highly sensitive to changes in the surrounding environment, particularly the acidity (pH). Lowering the pH dramatically increases the compound’s solubility. This effect is why calcium phosphate supplements are often recommended to be taken with food, as stomach acid aids in dissolution and absorption.
The underlying chemistry relates to the phosphate ion, which acts as a weak base. In an acidic environment (low pH), excess hydrogen ions react with the phosphate ion (\(\text{PO}_4^{3-}\)), forming hydrogen phosphate (\(\text{HPO}_4^{2-}\)) and dihydrogen phosphate (\(\text{H}_2\text{PO}_4^{-}\)). This reaction removes free phosphate ions from the solution, disrupting the dissolution equilibrium.
According to Le Chatelier’s principle, the system responds by dissolving more solid \(\text{Ca}_3(\text{PO}_4)_2\) to restore balance. This mechanism is relevant in biological processes like the acid erosion of tooth enamel or the formation of certain kidney stones. Temperature is also a minor factor; calcium phosphate becomes slightly less soluble as the temperature increases (negative thermal coefficient).