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Author: Subject: Help in preparation of phosphotungstic acid
vibbzlab
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[*] posted on 18-10-2020 at 06:35
Help in preparation of phosphotungstic acid


Hello all
I have like 300g of sodium tungstate with me and I really wanted to make phosphotungstic acid. I lookes a lot of books and articles and I found this

Phosphotungstic acid, stock: Dissolve 100 g sodium tungstate (Na2WO4, 2H2O) in 800 ml water in a round bottomed flask. Add 80 ml of 85% phosphoric acid, attach a reflux condenser and gently boil for 2 h. Cool. Dilute to 1L with water, mix and store in a brown bottle. Dilute 1 in 10 with water for use.

But this is actually for a stock solution. I wanted to get the solid product. Can i try getting it with this same procedure or someoneelse got any other easy methods? do share
thanks in advance





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Antigua
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[*] posted on 18-10-2020 at 06:37



From US Patent: https://patents.google.com/patent/US3288562A/en
Example I A solution of 103 g. of sodium tungstate,

Na2WO4x2H20 in 600 ml. of water was heated to about 90 C., and 347 g. of tungstic acid, H WO was added to the heated solution. Only a portion of the tungs-tic acid dissolved imediately, and heating of the suspension of the balance of the tungstic acid in the solution was continued for about minutes at about 90 C. The suspension was agitated continuously during this digestion operation. In the presence of the excess tungstic acid the sodium tungstate was converted to sodium metatungstate,

Undissolved tungstic acid was filtered from the sodium metatungstate solution and 17.3 g. of phosphoric acid (85% H PO was added to the clarified solution. The resulting solution of sodium phosphotun-gstate was diluted to a specific gravity of 1.290 (about 30% solids content) and the diluted solution was passed through an ion exchange column packed with Duolite C- resin on the hydrogen cycle at the rate of about rnL/minute. After the sodium ion content had been exchanged from the solution by contact with the resin it was concentrated by evaporation and the phosphotungstic acid was crystallized out. The crystalline product analyzed 92.1% W0 2.6% P 0 5.3% H 0 and less than 0.05% Na O
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clearly_not_atara
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[*] posted on 18-10-2020 at 15:12


That ion exchange resin in Antigua's procedure is crucial for obtaining the phosphotungstic acid in its neutral form. It is a very strong acid (pKa ~= -10) so it cannot be very easily prepared as an acid (rather than as its salts).

For example, this procedure:
"Phosphotungstic acid, stock: Dissolve 100 g sodium tungstate (Na2WO4, 2H2O) in 800 ml water in a round bottomed flask. Add 80 ml of 85% phosphoric acid, attach a reflux condenser and gently boil for 2 h. Cool. Dilute to 1L with water, mix and store in a brown bottle. Dilute 1 in 10 with water for use."
will probably give you the sodium salt Na3PW12O40.

You may be able to obtain the phosphotungstic acid by gassing the solution with excess HCl to precipitate NaCl. However precipitating sodium this way is inefficient.




[Edited on 04-20-1969 by clearly_not_atara]
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vibbzlab
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[*] posted on 19-10-2020 at 00:34


I dont have any ion exchange setup in my lab. Is there any other way?




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Bezaleel
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[*] posted on 19-10-2020 at 00:49


A thought: could thermal decomposition of the ammonium salt lead to the acid? I don't know how thermally stable the phosphotungstate ion is.

I suppose that the addition of sulphuric acid to a solution of sodium tungstate will give poorly soluble tungstic acid, so that would be a way to remove your sodium without the use of an ion exchange column.
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