Acid Tungstate

The acid tungstate, 4Na2O.10WO3.23H2O, may be prepared by passing carbon dioxide for several days through an aqueous solution of normal sodium tungstate, or by gradually adding formic acid, until the action is distinctly acid, to a solution containing 100 grams of the normal tungstate in 100 c.c. of water.

The action of glacial acetic acid on a solution of sodium tungstate produces a mixture of the salts 4Na2O.10WO3.23H2O and 5Na2O.12WO3.28H2O. The salt, 4Na2O.10WO3.23H2O, forms monoclinic crystals which effloresce rapidly in dry air and have density 4.3. When heated, the salt loses 17 molecular proportions of its water of crystallisation at 100° C., the remainder only being driven off by strong ignition. It melts at 680.8° C. It is soluble in water - 19 parts of the salt dissolve in 100 parts of water at ordinary temperature - forming an acid solution.

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

Tungstate of Soda

Sodium paratungstate is known commercially as "tungstate of soda" and may be prepared on a large scale by fusing wolframite with soda ash and lixiviating the fused mass. On nearly neutralising the boiling solution with hydrochloric acid and allowing to crystallise, large triclinic crystals of the salt separate.

The salt may be formed in solution by any of the following methods:
1.Saturation of a solution of sodium hydroxide, carbonate, or tungstate, with anhydrous tungstic acid.
2.Treatment of a sodium tungstate solution with hydrochloric acid at boiling-point (as described above) until only faintly alkaline to litmus.
3.Addition of a solution of sodium metatungstate (containing 5.8 grams Na2O.4WO3.10H2O) to one of the normal tungstate (containing 2 grams Na2O.WO3.2H2O).
4.Saturation of a solution of normal sodium tungstate with carbon dioxide.
5.Electrolysis of sodium tungstate solution in a cell in which the electrodes are separated by a diaphragm (see above).

Sodium Tungstate as Mordant

The production of colloidal tungsten hydroxide by the electrolysis of a solution of sodium tungstate has already been described. If precautions are taken to prevent the sodium hydroxide formed at the cathode from reaching the anode, for example, by means of a porous partition, it is possible to prepare the paratungstate, or other complex tungstate, from the anode solution.

The use of sodium tungstate has been recommended as a mordant, and it has been used as a fire-proofing material for flannelette, but owing to its solubility it cannot be considered satisfactory and it is not now used.

Sodium ditungstate, Na2O.2WO3, may be obtained by fusing together tungstic anhydride and sodium hydroxide or sodium carbonate, the mixture containing lNa2O:2WO3. On cooling, long needles separate, which on prolonged heating with water dissolve, yielding an alkaline solution which contains metatungstate. The dihydrate, Na2O.2WO3. 2H2O, is described by Rammelsberg as a crystalline precipitate obtained by addition of hydrochloric acid to a solution of the normal tungstate. The hexahydrate, Na2O.2WO3.6H2O, is stated by Lefort to crystallise from a solution containing the normal tungstate (2 molecules) and acetic acid (1 molecule); von Knorre, however, could only obtain the paratungstate from such a solution. The hydrate,Na2O.2WO3.12H2O, has also been described.

Sodium paratungstate is known commercially as "tungstate of soda" and may be prepared on a large scale by fusing wolframite with soda ash and lixiviating the fused mass. On nearly neutralising the boiling solution with hydrochloric acid and allowing to crystallise, large triclinic crystals of the salt separate.

Na2WO4 Refractive Indices

The densities and refractive indices of solutions of various concentrations have been determined as follows:

Grams Na2WO4 in 100 Grams Solution.	Density, d4°20°	Refractive Index, nD20°
2.21	1.0184	1.33586
10.08	1.0949	1.34516
16.56	1.1667	1.35376
20.59	1.2148	1.35933
25.46	1.2789	1.36648
32.68	1.3854	1.37934
38.43	1.4828	1.38890

The equivalent conductivities of solutions containing ½Na2WO4 in v litres at 25° C. are as follows:

v =	32	64	128	256	512	1024
Λ =	95.9	101.8	105.4	110.3	112.9	116.4

The vapour pressures of solutions have been determined.

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.

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.