Binary Systems Na2WO4 - Na2SiO3 and Na2WO4 - K2WO4

The anhydrous normal tungstate, Na2WO4, is prepared by the fusion method described for potassium tungstate, or by complete dehydration of the hydrates at 100° C. or over sulphuric acid. It may be obtained from the mineral wolframite by fusion with alkali as already described.

The anhydrous salt exists as white crystals, of density 4.1833 at 18.5° C. and 4.1743 at 20.5° C., which melt at 698° C. On heating it undergoes two transformations, the first with considerable development of heat, and finally boils. The transition temperatures between the polymorphic forms thus indicated have been determined from the cooling and heating curves as follows:

Method	Transition Point ° C		Melting point of β Form
	δ⇔γ	γ⇔β
Cooling curve	570	. . .	698
Cooling curve	564	588	698
Cooling curve	568	585	698
Cooling curve	572	589	700
Heating curve	587	591	694

The binary systems Na2WO4 - Na2SiO3 and Na2WO4 - K2WO4, and the properties of aqueous solutions of the mixtures, have been investigated.