Sodium Tungstate Hydrates Described by Formula

From the solutions so prepared Na2WO4 various hydrates have been obtained and are described under many different formula. There appear, however, to be five distinct salts which show distinctive properties, varying from one another in degrees of solubility, crystalline form, etc.

I.5Na2O.12WO3.28H2O is formed when crystallisation takes place at ordinary or lower temperatures. It yields transparent or milky triclinic pinacoidal crystals with
a:b:c = 0.5341:1:1.1148; α = 93° 56', β = 113° 36', γ = 85° 55',
of density 3.987 at 14° C. and stable in air. On heating, the salt loses, according to Scheibler, 10.42 per cent, of water - 21 of the 28 molecules H2O would correspond to a loss of 10.52 per cent.; according to Rosenheim the loss at 100° C. corresponds to 24H2O, and he therefore suggests the formula Na10H4[H4(WO4)6(W2O7)3].24H2O.
The remaining water is lost at 300° C., and the residue, which has density 5.49, is still completely soluble in water. At a red heat - according to Smith at 705.8° C. - the salt melts to a clear, yellowish, oily liquid and undergoes decomposition, for on cooling it sets to a crystalline mass which is only partly soluble in water, the insoluble residue being the tetratungstate, Na2O.4WO3. According to von Knorre the decomposition may be represented thus:
3(5Na2O.12WO3) → 7(Na2O.4WO3) + 8(Na2O.WO3).
Solubility data for sodium paratungstate have been given as follows:
One part of salt dissolves in 8 or 12 parts of cold water, or 12.6 parts of water at 22° C.
If the salt is boiled for some time with water, a solution is obtained which when cooled to 16° to 20° C. contains 1 part of the salt
after 1 day in 0.68 parts of water
after 12 day in 2.6 parts of water
after 72 day in 6.9 parts of water
after 7 month in 9.7 parts of water
after 14 month in 8.8 parts of water
If the salt is boiled with water, or kept for a considerable time in aqueous solution, it is decomposed into the normal and metatungstates. This accounts for the fact that although the cold fresh solution is neutral in reaction, it gradually becomes acid towards phenolphthalein and alkaline towards tropaeolin, especially after boiling; it also explains the apparent increase in solubility with time indicated above.
The solution has at first a sweetish taste, but it gradually becomes sharp and bitter. Rosenheim has determined the equivalent conductivities of solutions at 25° C. containing 1/10 molecule 5Na2O.12WO3in v litres, as follows: 

v =	32	64	128	256	512
Λ =	68.5	79.8	90.8	100.3	110.0