the pH values varied from 2.0 to 2.7 with an interval
of 0.1, the additive amount of NaCl from 0.0 to 1.0×10−2 mol with an
interval of 2.5×10−3 mol, the concentration of Na2WO4 are 0.025, 0.050,0.10 and 0.15 M, the reaction temperature from 150 to 180 °C with an
interval of 10 °C and the reaction time from 3.5 to 5.0 h with an interval
of 0.5 h.