2015年10月27日星期二
Sodium Pentatungstate/Sodium Hexatungstate
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 compositionNa2O.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.
Sodium Tritungstate /Sodium Tetratungstate
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 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.
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:
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
|
2015年9月27日星期日
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).
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.
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:
The equivalent conductivities of solutions containing ½Na2WO4 in v litres at 25° C. are as follows:
The vapour pressures of solutions have been determined.
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.
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