Sodium Tungstate Physical Properties

Molecular formula : Na2WO4
Molecular Weight: 329.86 g/mole
Molar mass：293.82 g·mol−1
Physical state and appearance: Solid.
Melting Point: 692.22℃ (1278°F)
Specific Gravity: 3.25 - 4.15(Water = 1)

Nature : a white crystalline powder, rhomboidal, mineral spinel structure. The temperature of 698 ℃, density 4.179g/cm3. In dry air weathering. Air heated to 100 ℃ or concentrated sulfuric acid can be stripped dry crystalline water. Three tungsten oxide and liquid caustic soda or soda ash by heating system in concentration crystallization. 10 hydrate colorless crystals, the 6℃ easy dehydration two hydrate. Preparation for the metal tungsten and tungsten products, the other intermediate materials, and for the production of paints and pigments Light, heavier fabrics agent, waterproof fabric and fire, as alkaloids precipitant.

Synergistic Inhibition Effect of Sodium Tungstate and Hexamethylene Tetramine on Reinforcing Steel Corrosion

Sodium tungstate (Na₂WO₄) and hexamethylene tetramine (HMTA) are both eco-friendly corrosion inhibitors. In this work, their synergistic corrosion inhibition effects on reinforcing steel in the simulated polluted concrete pore solution containing Cl⁻ were studied by electrochemical techniques including electrochemical impedance spectroscopy and potentiodynamic anodic polarization curve measurements. The morphologies and compositions of the steel surface were characterized by Electron Micro-Probe Analyzer, X-ray photoelectron spectroscopy, and Raman spectroscopy. The results showed that the serious steel corrosion took place in the solution with pH 11.00 and 0.5 M NaCl. However, a stable passive region occurred in the anodic polarization curve of the steel and its corrosion current density decreased dramatically after addition of a mixed inhibitor with 0.01 M Na₂WO₄ and 0.01 M HMTA to the solution. The inhibition efficiency of the mixed inhibitor reached 97.1%. The surface analyses revealed that a protective composite film was formed on the steel in the solution with the mixed inhibitor, which indicated that the mixed inhibitor had a synergistic inhibition effect on the steel corrosion. Our study also indicated that the mixed inhibitor could effectively control corrosion of the reinforcing steel in cement mortar.

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.

 

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