Ammonium metatungstate (AMT) is a significant tungstate compound, typically represented by the chemical formula (NH?)?H?W??O??·nH?O (where n is the number of water molecules of crystallization, commonly 1 or 2). Structurally, it belongs to the polyoxometalate family known as "tungsten clusters." Its core structure consists of 12 tungsten-oxygen octahedra connected through shared oxygen atoms, forming a cyclic or cage-like framework, with ammonium ions and protonated water molecules filling the gaps.
The polytungstate anion [H?W??O??]?? forms through the polymerization of tungstic acid (H?WO?) under acidic conditions. When a sodium tungstate or ammonium tungstate solution is concentrated in an acidic environment, tungstate ions (WO?2?) undergo stepwise dehydration and polymerization, eventually forming the stable dodecatungstate ion, which then combines with ammonium ions to produce AMT. The term "meta" in its name reflects an older chemical naming convention for polymeric oxyacid salts (e.g., metaphosphoric acid, metaboric acid), distinguishing it from simpler orthotungstates like ammonium orthotungstate ((NH?)?WO?).
When dissolved in water, AMT fully dissociates into NH?? and [H?W??O??]?? ions, and its acidity is determined by the behavior of these ions in solution:
NH??, the conjugate acid of a weak base, undergoes hydrolysis in water, releasing H? and making the solution acidic. Given the ionization constant of NH?·H?O as 1.8×10??, the hydrolysis constant of NH?? is approximately 5.6×10?1?. Although small, this hydrolysis still contributes a notable H? concentration.
The polytungstate ion [H?W??O??]?? contains two dissociable protons (H?), behaving as a polyprotic weak acid with stepwise dissociation in water. Since polytungstic acid (the conjugate acid H?[H?W??O??]) readily dissociates in water, the dissociation of [H?W??O??]?? dominates over its hydrolysis, making the latter negligible.
In an AMT aqueous solution, H? is produced both by NH?? hydrolysis and [H?W??O??]?? dissociation, resulting in a significantly higher H? concentration than OH?, confirming the solution’s acidity. This is further supported by the fact that AMT synthesis requires acidic conditions, and the protons (H?) in its crystal structure are products of that acidic environment, readily dissociating in water. While polyoxometalates often exhibit "proton sponge" characteristics, the dissociation ability of the AMT anion framework in water surpasses the hydrolysis of NH??, making acidity the dominant trait.
