Aluminum Phosphate (\(\text{AlPO}_4\)) is an inorganic ionic compound composed of the aluminum cation (\(\text{Al}^{3+}\)) and the phosphate anion (\(\text{PO}_4^{3-}\)). In pure, neutral water, \(\text{AlPO}_4\) is considered insoluble, or only very slightly soluble. The solubility product constant (\(\text{K}_{\text{sp}}\)) is extremely low, approximately \(9.84 \times 10^{-20}\), meaning the vast majority remains a solid precipitate.
General Solubility Rules for Ionic Compounds
The behavior of \(\text{AlPO}_4\) can be predicted using general solubility rules for ionic compounds. These guidelines help anticipate whether a salt will dissolve in an aqueous solution.
The general rule for the phosphate ion (\(\text{PO}_4^{3-}\)) is that most phosphate compounds are insoluble in water. Exceptions include phosphates formed with Group 1 alkali metals (lithium, sodium, potassium, rubidium, or cesium).
Another exception is ammonium phosphate, which contains the polyatomic ammonium ion (\(\text{NH}_4^+\)). Since aluminum (\(\text{Al}^{3+}\)) is a Group 13 metal, it is not an alkali metal or ammonium. Therefore, the rules predict that Aluminum Phosphate will not dissolve significantly.
The Energetic Barrier to Dissolution
Dissolution involves an energetic competition between the energy required to break the crystal lattice and the energy released when ions are surrounded by water. The energy holding the ions together is called Lattice Energy.
Lattice energy is proportional to the magnitude of the charges on the ions. Aluminum phosphate features highly charged ions: a triple positive cation (\(\text{Al}^{3+}\)) and a triple negative anion (\(\text{PO}_4^{3-}\)). These high charges create extremely strong electrostatic attraction, resulting in a very high lattice energy that tightly binds the ions.
The energy released when water molecules stabilize the separated ions is called Hydration Energy. Although the highly charged ions generate significant hydration energy, this energy is insufficient to break the strong ionic bonds. Since the Lattice Energy is substantially greater than the Hydration Energy, the dissolution process is energetically unfavorable, causing insolubility.
How External Conditions Influence Solubility
Although \(\text{AlPO}_4\) is nearly insoluble in pure water, its solubility is significantly influenced by external conditions, especially the solution’s pH level.
In highly acidic solutions, such as those created by adding a dilute mineral acid, the solubility increases considerably. This occurs because the free phosphate ion (\(\text{PO}_4^{3-}\)) reacts with excess hydrogen ions (\(\text{H}^+\)). The phosphate ion becomes protonated, forming species like \(\text{H}_2\text{PO}_4^-\) or \(\text{H}_3\text{PO}_4\).
This protonation removes \(\text{PO}_4^{3-}\) from the solubility equilibrium. According to Le Chatelier’s principle, this drives the dissolution of more solid \(\text{AlPO}_4\) to restore balance. Solubility also increases in strongly alkaline solutions, where the aluminum ion (\(\text{Al}^{3+}\)) forms soluble complex ions with hydroxide (\(\text{OH}^-\)), such as \(\text{Al}(\text{OH})_4^-\).
This pH dependence is crucial in natural systems like soil chemistry. Aluminum phosphate formation locks up phosphorus, making it unavailable for plants in acidic soils. Temperature has a less dramatic effect, and the crystalline structure of \(\text{AlPO}_4\) is stable up to high temperatures.