Simple, high yield and reagent-saving synthesis of pure alpha-K6P2W18O62 center dot 14H(2)O

A new, simple, high yield and reagent-saving synthesis of alpha-K6P2W18O62.14H(2)O was achieved by preforming metatungstate from Na2WO4.2H(2)O 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 80degreesC 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. (C) 2003 Elsevier B.V. All rights reserved.