Grade 1 ammonium paratungstate is a national standard product of APT with grade of APT-1. The ammonium paratungstate may be obtained by the process of losing ammonia in ammonium tungstate solution. Its chemical equation is as follows:
12(NH4)2WO4→5(NH4)2O•12WO3•nH2O+(7-n)H2O+14NH3↑
Ammonium tungstate solution is not stable, so that water and ammonia escape when it is heated, and therefore ammonium paratungstate is continuously generated in solution. During this process, ammonium paratungstate changes from unsaturation to saturation to reach a stable state. Under the mutual action of homogeneous nucleation and heterogeneous nucleation, crystal nucleus is formed, and the crystal nucleus grows and becomes crystal. The purity of the ammonium tungstate solution should be improved to make homogeneous nucleation dominant, thereby reducing the nucleus formed by impurities and increasing the particle size of APT.
In the process of crystallization, particle size of APT may be affected by a lot factors such as relative supersaturation, temperature, stirring speed, impurity content, evaporation rate, and homoionic effect.
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