Sodium Tungstates

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.

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° C., 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.

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

Solubility of sodium tungstate

The vapour pressures of solutions have been determined.

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.

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).

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

1. 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.12WO₃ in v litres, as follows:
   
   

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

   
   
   The following values show the difference in conductivities at 25° C. of freshly prepared cold solutions (Λ₁), and of solutions previously boiled (Λ₂).
   
   

   v =	25	50	100
   Λ1	61.2	71.2	78.9
   Λ2	82.4	92.1	99.7

   
   
   This considerable change, confirmed by Wells, supports Marignac's observation of change in solubility.
2. 5Na₂O.12WO₃.25H₂O. This hydrate is obtained when crystallisation takes place at about 60° to 80° C. as monoclinic prisms, with axial ratio,
   
   a:b:c = 0.8069:1:0.5328; and β = 120° 10'.
   
   When heated to 100° C. it loses 9.15 per cent, (corresponding to 18 molecules) of water.
3. 5Na₂O.12WO₃.21H₂O is formed when crystallisation takes place at 100° C. It yields octahedra of the triclinic system,
   
   a:b:c = 0.8695:1:1.2787: α = 91° 18', β = 86° 16', γ = 97° 59'.
   
   The substance is often contaminated with some of the 25-hydrate. At 100° C. it loses 15 molecules H₂O.
4. 3Na₂O.7WO₃.16H₂O is obtained, according to Marignac, by crystallisation from a solution of a paratungstate containing sodium carbonate, in short prismatic crystals, triclinic pinacoids,
   
   a:b:c = 0.6836:1:1.1802; α = 95° 3', β = 123° 42', γ = 91° 53'.
   
   Under analogous conditions Forcher obtained octahedral crystals to which he gave the formula 3Na₂O.7WO₃.15H₂O. At 100° C. it loses 12 molecules H₂O. It is probably a polymorphous form of the 28-hydrate described above.
5. 3Na₂O.7WO₃.21H₂O crystallises from solutions which have been boiled for some time, yielding prismatic crystals, triclinic pinacoids,
   
   a:b:c = 0.9296:1:0.5207; α = 92° 47', β = 96° 28', γ = 89° 40'.
   
   At 100° C. it loses 17 molecules H₂O.

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 tritungstate, Na₂O.3WO₃.4H₂O, is prepared, according to Lefort, by gradually adding a concentrated solution of the ditungstate to a boiling 50 per cent, solution of acetic acid. On cooling, a white precipitate results which dissolves in water, and the solution on evaporation yields long prismatic crystals. The existence of a tritungstate is denied by Kantschew.

Sodium tetratungstate, Na₂O.4WO₃, is obtained by the complete dehydration of sodium metatungstate, and is sometimes called "anhydrous sodium metatungstate." As will be seen, however, water is essential to the constitution of metatungstates. The salt may be obtained by heating the paratungstate and treating the residue with water. It is insoluble in water, but on prolonged heating with water at 120° C. it is converted into the metatungstate.

Sodium pentatungstate, Na₂O.5WO₃, is obtained by fusing together sodium tungstate and tungstic anhydride (1:2), or by heating sodium paratungstate to incipient fusion and extracting the fused mass with water, when it remains in brilliant plates or scales which are only slightly soluble in water.

Sodium hexatungstate, Na₂O.6WO₃.9H₂O, is obtained according to Marignac by prolonged boiling of tungstic acid with sodium paratungstate. Ullik, by decomposing a solution of sodium metatungstate with hydrochloric or nitric acid and allowing the solution to evaporate, obtained large yellowish crystals of what he considered to be the octa-tungstate, Na₂O.8WO₃.12H₂O, but Friedheim could not confirm his results, and Leontowitsch, using the reagents in different proportions, obtained crystals of the hexatungstate, of composition Na₂O.6WO₃.15H₂O. The anhydrous octatungstate, Na₂O.8WO₃, was obtained by von Knorre by oxidation of fused metatungstate at a bright red heat, and extraction of the mass with water, when lustrous scales of the octatungstate remain. The relation of these higher acid salts to one another and to metatungstic acid has not yet been determined.

The acid salts, 2Na₂O.3WO₃.7H₂O and 3Na₂O.8WO₃.17H₂O, have also been described.