What Is the Solubility of Ammonium Metatungstate?

Ammonium metatungstate (AMT), with the molecular formula H₂₈N₆O₄₁W₁₂, is a significant tungstate compound. Structurally, AMT consists of ammonium cations and metatungstate anions, with its unique Keggin-type polyoxoanion structure contributing to its excellent solubility—a key distinction from ammonium paratungstate.

The solubility of AMT refers to its physical property of dissolving in a specific solvent, indicating the extent of its dissolving capacity. Solubility is defined as the maximum amount of a substance that can dissolve in a given solvent at chemical equilibrium, beyond which no more solute can be accommodated.

AMT is highly soluble in water, with a solubility exceeding 300 g/100 mL at room temperature (approximately 25°C). This high solubility stems from the Keggin anion's highly symmetric tungsten-oxygen cluster structure, which is rich in polar oxygen atoms on its surface, forming strong hydrogen bonds with water molecules. Additionally, the presence of crystal water (typically 3-4 molecules) in AMT enhances its hydrophilicity, allowing it to rapidly disperse and dissolve in water.

Beyond water, AMT can also dissolve in certain dilute acid and alkaline solutions, though its solubility varies depending on the solvent type. Generally, AMT exhibits high solubility in ammonia water, comparable to or slightly higher than in pure water. In dilute solutions of strong acids like hydrochloric acid or nitric acid, AMT's solubility is significant, while in weak acids like acetic acid, its solubility is lower.

AMT's solubility is also influenced by temperature and the presence of impurity ions. As temperature increases, solubility slightly rises due to enhanced molecular motion, which strengthens interactions between ions and solvent molecules, promoting dissolution—though this increase is relatively modest. Furthermore, AMT shows low sensitivity to impurity ions like Ca²⁺ and Mg²⁺ in water, maintaining high solubility even in hard water.

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