What Is the Solubility of Ammonium Metatungstate in Ammonia Water?

Ammonium metatungstate (AMT), besides being soluble in dilute acid solutions such as dilute hydrochloric acid, dilute nitric acid, and acetic acid, can also dissolve in alkaline solutions like ammonia water.

Ammonia water is a weakly alkaline solution formed by dissolving ammonia gas in water, with a pH typically ranging from 9 to 12, depending on concentration: low-concentration ammonia water has a pH of 9-10, while high-concentration ammonia water ranges from 11-12. The dissolution behavior of AMT in ammonia water is influenced by pH, NH₄⁺ concentration, and the chemical nature of polytungstate ions.

When dissolving in ammonia water, AMT dissociates into ammonium ions (NH₄⁺) and polytungstate ions ([H₂W₁₂O₄₀]⁶⁻). Under weakly alkaline conditions, polytungstate ions remain stable, resulting in a clear solution with high solubility. In strongly alkaline conditions, OH⁻ may cause partial depolymerization of polytungstate ions, forming mononuclear tungstate ions ([WO₄]²⁻) or other species, slightly reducing solubility. The presence of NH₄⁺ from ammonia water alongside AMT’s NH₄⁺ may increase ionic strength, promoting dissolution.

Generally, AMT's solubility in ammonia water is relatively high, comparable to or slightly higher than in pure water (approximately 300-330 g/100 mL at 25°C), though it may vary slightly due to chemical equilibrium shifts at pH values above 9 or high ammonia concentrations.

In low-concentration ammonia water, AMT's solubility is approximately 300-320 g/100 mL (at 25°C), close to or slightly higher than in pure water, with the solution remaining stable and polytungstate ions retaining a Keggin structure. In high-concentration ammonia water, solubility may slightly decrease to 280-300 g/100 mL, as polytungstate ions may partially convert into other tungstate species.

Weakly alkaline ammonia water is commonly used to prepare AMT solutions for impregnating carriers like SiO₂ or Al₂O₃ to produce tungsten-based catalysts, with high solubility ensuring uniform loading. AMT’s high solubility in ammonia water facilitates the preparation of high-concentration solutions for spray drying to generate WO₃. Solutions of AMT in ammonia water are also used for UV-Vis or ICP-MS analysis, requiring pH control to maintain stability.

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