Synthesis of heteropolyacids

A process for the synthesis of a heteropolyacid substantially free of extraneous cations, said process comprising:

• a. reacting (i) an aqueous solution of one or more alkali or alkaline earth metal salts selected from a tungstate and a molybdate under reflux with (ii) an alkali or an alkaline earth metal silicate or an alkali or an alkaline earth metal phosphate to form in an impure aqueous solution comprising the heteropolyacid of the chosen salt and any other salt formed in situ or used as a reactant;
• b. filtering and cooling the impure aqueous solution comprising the heteropolyacid;
• c. purifying the cooled, impure aqueous solution from (b) by at least one liquid/liquid extraction with an organic solvent to enable the heteropolyacid to form a complex in situ with the organic solvent and allowing the extract to separate into two or more phases including inter alia a dense organic phase which comprises the dissolved complex of the heteropolyacid and which dense organic phase is below the aqueous phase;
• d. recovering the dense organic phase from (c) and mixing it thoroughly with water to form a separate layer of a dilute aqueous mixture and
• e. recovering a concentrated 20-80%w/w aqueous solution of the heteropolyacid substantially free of organic impurities from the dilute aqueous mixture,
• wherein the organic solvent used in the liquid/liquid extraction step (c) comprises a dihydrocarbyl ether having at least 5 carbon atoms.