Chemistry of Aqueous Tungsten Solutions

A characteristic feature of the tungstate ion which only occurs monomerically in alkaline or neutral solutions is its tendency to form condensed, complexed isopolytungstate ions in slightly acidic medium. The ability of polycondensation is common for group 5 and 6 elements and is more pronounced for elements of higher atomic mass within the same group. The dimeric dichromate ion is a typical example. 

Normally, polycondensation is defined by the reaction of anionic species and protons, such as 

However, it seems unlikely that two negatively charged particles can react with each other. 

Condensation aquo ligands of the cationic species are substituted by commonly shared hydroxo groups of the anionic entity (aquo ligands are more loosely boimd as hydroxo ligands)  

In principle, the polycondensation may also be regarded as a dehydration process caused by the increase in proton concentration. In the starting species of the polycondensation process, the fully hydrated monotungstate, the W H20 ratio is 1 4 (pH 6.2). In the various polytungstates prevailing between pH 6.2 and 1, the ratio is somewhere between 1 4 and 1 2, and attains exactly 1 2 in the precipitated tungstic acid at pH 1. 

---