Regularities of formation of molybdates and tungstates

The optimum conditions for precipitation of low-solubility molybdates and tungstanates, namely pH values and initial concentrations for performing subsequent purposeful synthesis, were determined by calculating ionic equilibrium. The regularities governing the formation in solid phase of rare-earth element molybdates and double molybdates and tungstanates were established. All the Ln2O5 - MoO3 systems, irrespective of the composition of the initial components, were characterized by beginning of interaction at 440-490 deg C, the interaction temperature declining with increase of the rare-earth element serial number. A comparison of the results obtained by X-ray-phase and differential thermal analyses of the systems afforded a consecutive pattern of the chemical transformations occurring in the Ln2O3 - MoO3 systems. Molybdate (tungstanate)-ions were shown to be isomorphic with sulphate (chrmomate)-ions