Refinery
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How to concentrate formic acid?
How can formic acid be concentrated? Its bp is same as water. Distilling from H2SO4? Drying with something that doesnt react or absorb the HCOOH?
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chornedsnorkack
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1) It forms a high boiling azeotrope with water (unlike acetic acid). Boiling point 107,5 Celsius, 77,6 % formic acid by weight. Making about 57 %
formic acid by mole.
https://onlinelibrary.wiley.com/doi/pdf/10.1002/14356007.a12...
2) Once you are at the boiling point extremum, it is not generally a freezing point extremum. Formic acid forms a simple eutectic, which is for some
reason given in mole % as 41 %. 57 % by mole (the azeotrope) should have a freezing point somewhere between -28 and -23, though formic acid has a
tendency to supercool.
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Tsjerk
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Article on producing 99% formic acid from 85%.
Attachment: ritter1935.pdf (260kB)
This file has been downloaded 531 times
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unionised
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Beware of trying to use H2SO4 as a dessicant for HCOOH
You might do better than you hoped and dehydrate it to the remarkably toxic, carbon monoxide.
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chornedsnorkack
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Formic acid is itself unstable for two different decay reactions:
HCOOH <-> CO+H2O
HCOOH -> CO2+H2
At 25 Celsius, the first reaction would come into equilibrium with 1 bar CO in headspace at about 3 % formic acid, but is slow. 99 % formic acid
decomposes less than 0,1 % in 3 months... but this would be enough to build up carbon monoxide in headspace of a bottle left with 100 % formic acid
for months.
Would it be better to store 100 % formic acid as a solid, under 8 Celsius?
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Cou
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Anhydrous copper (ii) sulfate is a suitable drying agent for carboxylic acids. Add anhydrous CuSO4 until new additions no longer clump together, then
distill under reduced pressure (formic acid decomposes at high temperatures)
[Edited on 7-22-2020 by Cou]
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unionised
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If you can do a reduced pressure distillation, you can break the azeotrope that way .
The azeotropic concentration is pressure dependent.
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Fery
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unionised is right, IIRC distillation at 0,2-0,3 atm produces 85% acid from 77% azeotrope which boils at 1 atm (higher vacuum is not helpful)
[Edited on 24-7-2020 by Fery]
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foreign maple
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possibly molecular sieves
[Edited on 27-10-2020 by foreign maple]
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Cou
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Molecular sieves are not compatible with acids
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