Salts of heteropoly acids:

The parent acids of the compounds are 12-heteropoly acids of the general formula H_m XY₁₂ O₄₀. nH₂O (m = 3, 4, 5) where X may be phosphorus, arsenic, silicon, germanium and boron and Y different elements such as molybdenum, tungsten and vanadium. Amongst the exchangers of this category, work has been mainly reported on 12- molybdophosphates. Ammonium molybdophosphate and ammonium tungstophosphate are the two widely investigated compounds for their physicochemical behaviour and practical applications. Salts of heteropoly acids act mainly as cation exchangers. They have been mainly used for concentration and purification of Cs¹³⁷from fission products. The behaviour of heteropoly acid salts with quaternary organic cations like pyridinium, picolinium, collidinium has been studied as ion exchangers.