Simple, high yield and reagent-saving synthesis of pure α-K6P2W18O62 · 14H2O

A new, simple, high yield and reagent-saving synthesis of α-K6P2W18O62 · 14H2O was achieved by preforming metatungstate from Na2WO4 · 2H2O before adding slowly the appropriate excess of H3PO4 4M. Direct precipitaton with KCl was followed by redissolution of the crude solid in Millipore water and prolonged heating at 80 °C in a pH < 2 medium. The yield is almost quantitative, based upon the original amount of sodium tungstate. Keys to the maximization of the yield are also the P−W ratio, the initial [H+–WO42−] ratio, the pH of the reaction mixture. To facilitate discussion, an excess parameter R* is defined as the ratio between the actual P−W ratio and the stoichiometric value of this parameter in the complex. It is demonstrated that the value of R* can be substantially diminished compared to its values in the previous literature, provided the pH of the reaction mixture is properly controlled.