The anhydrous normal salt, ZnWO4, is obtained as rhombic crystals by fusing together sodium tungstate (1 part), sodium chloride (2 parts), and zinc chloride (2 parts). The monohydrate, ZnWO4.H2O, is obtained by double decomposition as a white amorphous precipitate, slightly soluble in water.
The ditungstate, ZnO.2WO3.3H2O, is obtained as needle-shaped crystals on adding a zinc salt to a solution of sodium ditungstate and concentrating the mixture. It is soluble in 10 parts of water at 15° C.
Zinc Paratungstate, 5ZnO.12WO3.37H2O, may be prepared by the addition of a zinc salt to a solution of sodium paratungstate. It separates as white needle-shaped crystals, which, when heated, turn yellow but do not melt. According to Rosenheim the salt contains 35 molecular proportions of water, of which it loses 28 on heating at 100° C., and he therefore formulates the compound as Zn5(H5[H2(WO4)6])2.28H2O. According to von Knorre, however, the precipitate obtained on adding a solution of zinc sulphate to a boiling solution of sodium paratungstate (in molecular proportions 2:1) is a double salt of composition Na2Zn2W7O24.15H2O, while if the temperature of precipitation is below 60° C., the salt contains 21H2O. Using ammonium paratungstate, the double salt, (NH4)2O.2ZnO.7WO3.13H2O, has been obtained as white needles.
The tritungstate, ZnO.3WO3.5H2O, and the acid salts, 9ZnO.22WO3.66H2O and 2ZnO.5WO3.9H2O, have been described. |