Ammonium paratungstate hexahydrate’s chemical formula is (NH4)10H2W12O42•6H2O (APT•6H2O).
Stability of Ammonium Paratungstate Hexahydrate
It is key to generate ammonium paratungstate tetrahydrate ((NH4)10H2W12O42•4H2O or APT•4H2O) from aqueous solution of ammonium tungstate in the production of tungsten powders. Studies have shown that APT•6H2O can be formed under the generation of APT•4H20. Based on this data, it is assumed that APT•6H2O is metastable. Test to determine whether hexahydrate is metastable or not?! Under what conditions is it metastable? Find:
The hexahydrate compound is metastable at a solution concentration of approximately 300-230 g/kg WO3 and temperatures ranging from 90°C to 96°C. And APT•6H2O will not be obtained by adding APT•4H2O seed crystals before crystallization.
Draw the experimental data in a three-dimensional ternary phase diagram to found:
During crystallization, APT•6H2O will recrystallize into APT•4H2O by lowering water activity. During isobaric crystallization, the recrystallization process will also be improved by increasing the temperature of solution. And in industrial practice, there are always some residual crystals in the crystallizer, so that ammonium paratungstate tetrahydrate rather than ammonium paratungstate hexahydrate will be obtained in industry production.
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