Solubility of Sodium Tungstate

The heat of formation of sodium tungstate has been found to be:

Na2O + WO3 = Na2WO4 + 94,700 calories.

The aqueous solution, which is alkaline, when allowed to crystallise at temperatures above 6° C., yields slender nacreous crystals of the dihydrate, Na2WO4.2H2O, in the form of rhombic bipyramidal scales, a:b:c = 0.8002:1:0.6470, of density 3.259 at 17.5° C. and 3.231 at 19° C. This hydrate is stable in the air, and it is in this form that the salt is generally used. When heated, it loses water at 200° C., becomes opaque, and finally melts. It dissolves readily in hot water, but may be precipitated by means of alcohol. The solution yields white tungstic acid on the addition of mineral acids.

If the aqueous solution is allowed to crystallise at temperatures below 6℃, the decahydrate,Na2WO4·10H2O, is obtained.

The solubility of sodium tungstate has been determined by Funk as follows:

Solid Phase Na2WO4.10H2O.
Temperature,° C.	Grams Na2WO4 in 100 Grams Solution.
-5	30.60
-4	31.87
-3.5	32.98
-2	34.52
0	36.54
+3	39.20
+5	41.02

Solid Phase Na2WO4.2H2O.
Temperature,° C.	Grams Na2WO4 in 100 Grams Solution.
-3.5	41.67
+0.5	41.73
+21	42.27
+43.5	43.98
+80.5	47.65
+100	49.31

These results are shown graphically in fig.