1.Initial concentration of WO3

It was pointed out that increasing the concentration of WO3 lead to the bulk density of ammonium paratungstate decreased. When the concentration of WO3 decreased to a certain degree, the density of ammonium paratungstate was not obvious to saw. It shows that the higher concentration of WO3 in the solution leads to the faster nucleation, and the crystal particles are not easy to grow. When in the lower concentration of WO3, saturation of the solution is changing not obvious, and advantageous to nucleation growth. But when concentration of WO3 is too low, the contact chance of the crystal nucleus and solute molecules is small, and the grain growth is affected. Solution saturation △c decreases resulting in resulting in crystal growth rate (RM) and nucleation rate (rn) decreases. However the slowed down extent of both is different. Supposed that rN slows down larger than rm, in that case nucleation is small, however the growth of the substrate is really large, which is beneficial for preparation of ammonium paratungstate.

 

2. Concentration of NH4Cl 

With NH4Cl added, concentration of NH4 + ion in the solution increases, polymerization and multi-phase reaction balance moves to the right, which will lead to solubility of ammonium paratungstate decreased and △c increased. It is known that the nucleation rate greater than crystal growth rate of APT when increases the super saturation. Ammonium paratungstate crystal size tends to thinner when NH4Cl added. However, ammonium paratungstate generation transformation reaction speed up, low phase velocity, wide area, △c slow to increase, controlled by s for a long time without adding NH4Cl. At the same time, with the increase of NH4+ ion, the same ion effect enhances but the salt effect increase more quickly.

 

An academic literature pointed out that the higher concentration of NH4Cl in solution, the easier to get small and inhomogeneous crystal. Because the presence of NH4Cl and the same ion effect makes the evaporation crystallization reaction moving to the right, that means, saturation of the solution increases, the nucleation rate accelerated, a large number of fine grains generated at the same conditions. The higher concentration of original NH4Cl, the more fine APT crystal obtained. That may due to the presence of the solution, which may be adsorbed by the crystal grains, but not constituting other substances. Such impurities are adsorbed to the crystal surface, covering the active region of it, and slow down the crystal growth rate which is conducive to obtain smaller particles of crystal.