Abstract: The method of syntheses and properties of a system of heteropoly acids of [PNb₂Mo_xW_1-xO₄₀]^5-(x=0,2,4,6) are reported, in which the acidification and the addition of materials are carried out stepwise. The optimal rate of the materials and the pHof the reaction are given. The determination of IRand UVspectra shows that the acids have Keggin structure. The polarographic data confirm their reduced patterns. The potential titration in organic solvent gives their alkaline of 5. The experiments of thermal stability indicate that the crystal water is essential to stabilizing of these acids.

Key words: Molybdotungstoniobophosphoric heteropoly acids, Mixed heteropoly complexes, Tungsto-niobophosphoric heteropoly acids