It is best purified by conversion to the Ba salt by precipitation with BaCl2, which is recrystallised three times before conversion to the sodium salt. The Ba salt (9.5g) is shaken with 200mL of a 1:1 slurry of Dowex 50 (Na+ form) for 2hours. The mixture is filtered, and the resin is washed with H2O (2 x 25mL). The combined filtrates (150mL) are adjusted to pH 7.0 and concentrated in vacuo to 30-40mL and filtered if not clear. Absolute EtOH is added to make 100mL, and then n-hexane is added whereby a white solid and/or a second phase separates. When set aside at room temperature, complete precipitation of the Na salt as a solid occurs. The salt is removed by centrifugation, washed with Me2CO, dried in air then in an oven at 55o to give a stable powder (4.5g). It did not lose weight when dried further over P2O5 at 78o/8hours. The high rotation in the presence of (NH4)6Mo7O24 is not very sensitive to the concentration of molybdate or pH as it did not alter appreciably in 1/3 volume between 2.5 to 25% (w/v) of molybdate or at pH values ranging between 4 and 7. [Cowgill Biochim Biophys Acta 16 613 1955, Embdan et al. Hoppe Seyler's Z Physiol Chem 230 20 1934, Beilstein 3 IV 1051.]
