Silicotungstic acid preparation

Procedure for production and isolation of Silicotungstic acid
H4SiW12O40.30H2O

Procedure largely following reference given in Merck Index 11th edition. Adapted by a friend and used successfully for the production of Silicotungstic acid solution.

Key reagents

Sodium silicate - commonly known as 'water glass' and readily available from pottery suppliers.

Ether - only small amounts required, obtainable from some 'Starting Fluid' by spraying down a tube into vessel in an ice bath. Can be distilled WITH EXTREME CARE to purify to lowest boiling fraction, approx. 34C. ETHER IS A HIGHLY EXPLOSIVE SUBSTANCE, OPEN FLAMES AND SPARKS CAN CAUSE EXPLOSIONS. USE SEALED ELECTRICAL ELEMENTS AND WATER BATHS FOR ANY APPARATUS. KEEP COLLECTION VESSEL AND ANY VENT FROM IT BELOW HEAT SOURCE, ETHER IS HEAVIER THAN AIR AND SINKS.

Hydrochloric acid conc HCl (37%) - only this acid should be used as sulphuric and nitric acids are not readily absorbed by the ether / silicotungstic acid adduct and the hydrochloric acid can also be removed more readily than the others by simple means such as having warm dry air drawn over the crystals.

Sodium tungstate (Na2WO4.2H2O) - prepared as given here.
Starting with Tungsten or Tungsten carbide powder, available at non-ferrous metal suppliers, Tungsten carbide powder being used as a grinding abrasive.
The powder is placed on heat proof surface (fire brick) and treated with a gas (butane) torch and made to glow red. Once initiated an oxidizing process will take hold and the heat will work it's way through the powder and reduce most of it to a yellow-green powder, Tungsten oxide (WO3), and expand in volume. the powder can be stirred and again set glowing to completely oxidize the tungsten. Tungsten also exists in a lower oxidation state (WO2), which is a brown powder.
The Tungsten oxide is now added to a solution of caustic soda (NaOH)

Mol Wt WO3 = 231.92
Mol wt NaOH = 40.01

WO3 + 2 NaOH = Na2WO4 + H2O

therefore approx. 80 gm (NaOH) to 230 gm (WO3)
or 8 gm to 23 gm

There should be a small excess of caustic soda.
The solution is boiled (may be somewhat violent boiling so best with large container) until generally insoluble tungsten oxide is dissolved.
The liquid is reduced until crystals in the form of plates appear on the surface, removed from the heat and allowed to crystallize. Sodium tungstate being less soluble than Sodium hydroxide will crystallize first. The Tungstate is removed and allowed to dry, with the remaining small amount of liquid with some caustic soda is discarded.

Sulfuric acid 98% - Only available from chemical suppliers, it's use is only required in one process outlined.

From procedure given (p1717-18, ISO- and HETROPOLYACIDS AND THEIR SALTS, B. Gruttner & G. Jander.)
citing Z. anorg. allg. Chem #101:240, 1917, Rosenhiem and Jaenicke. Checked by authors above.

A solution of 50 gm of Sodium Tungstate in 400 ml cold water is prepared, and treated drop wise with 6N HCl, until neutral to litmus. Any white precipitate that forms is re-dissolved with swirling the flask or vessel. An excess of freshly precipitated silicic acid hydrate (H4SiO4) is now added to the solution.
Fresh Silicic acid hydrate is prepared by dissolving Sodium silicate in a minimum of cold water and made neutral to litmus by drop wise addition of HCl. After 15 min a small excess of HCl is added. The solution decanted and the precipitate is washed once or twice with cold water, which is likewise decanted. (it is best not to use good glassware for this process, the Silicic acid will possibly fuse with glass and become impossible to remove. Use an old clean jar. The precipitated Silicic acid hydrate should be removed from the container)

Mol wt H4SiO4 = 96.09
Mol wt Na2WO4.2H2O = 329.95
One mole of Silicic acid for every twelve moles of Sodium Tungstate required.

Therefore for each mole of Sodium tungstate only one twelf of a mole of Silicic acid is required. For Silicic acid hydrate, 96 (mol wt) / 12 = 8. Only 8gms of Silicic acid hydrate (which becomes silicic acid, SiO2 in solution) for every 330 (approx.) of Sodium tungstate is required, based on the relative molecular weights. An excess is required, and will be later filtered out anyway.

reaction

12Na2WO4 (aq.) + SiO2 + 20 HCl = 2Na2O2.SiO2.12WO3(aq) + 20NaCl
sodium tungstate + silicic acid + hydrochloric acid = sodium silicotungstate + sodium chloride (common salt)

The mixture of Sodium tungstate and Silicic acid is boiled for about 2 hours, with liquid being kept acidic by the periodic addition of small amounts of HCl. Until the solution no longer precipitates tungistic acid hydrate on addition of dilute HCl.
The solution is filtered to remove undissolved SiO2.

The free silicotungistic acid can be obtained by two slight variations in procedure.
(p 1700-01, ISO- and HETROPOLYACIDS AND THEIR SALTS, B. Gruttner & G. Jander.)
Isolation by the method of Drechsel.

Firstly the sodium salt solution is concentrated as far as possible, then shaken in a funnel with about 1/3 it's volume of ether. Ice-cold conc. (37%) HCl is added in small quantities and shaken vigorously after each addition. Try to avoid the solution heating up too much, can use an ice or water bath to maintain cool temperature.
The liberated acid immediately forms an oily adduct with the ether and sinks to the bottom of the funnel and forms an oily layer. The reaction is complete when addition of HCl no longer produces any droplets of the oily adduct when added to the solution.
Allowed to clarify and then separated from the aqueous and ether phase, the adduct can be treated in two ways to obtain the free acid.
The adduct is shaken with equal volumes of clean H2O and a stream of air driven through the mixture drives the ether off. The residual clear aqueous solution is then placed in a dessicator over Sulfuric acid to remove the water, and crystallization of the free acid takes place. DO NOT DRY IT OUT BY USING HEAT TO DRIVE OFF THE WATER. Any residual HCl can also be driven off by passing a stream of air through a tube containing the crystals. Whilst there may be some contamination, the largest crystals are relatively pure and can be used as is. Re-dissolving in water, filtering and recrystallisation can be done to obtain a cleaner product.

Alternatively, the oily adduct can be decomposed at 40C, the ether removed by heating to this temp over an extended period. The excess HCl can also be removed by drawing a stream of air through the slowly solidifying residue in a tube over time. Then re-dissolved in water, filtered and recrystallised as above.

These crystals or residue is free Silicotungstic acid.
Used as reagent for alkaloid testing at the rate of 12 gm Silicotungstic acid to 100 ml of water.