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?chornedsnorkack - 22-7-2020 at 00:24
How can formic acid be concentrated? Its bp is same as water.
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.Tsjerk - 22-7-2020 at 00:50
Article on producing 99% formic acid from 85%.
Attachment: ritter1935.pdf (260kB) This file has been downloaded 445 timesunionised - 22-7-2020 at 01:47
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.chornedsnorkack - 22-7-2020 at 04:15
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?Cou - 22-7-2020 at 10:31
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]unionised - 22-7-2020 at 13:11
If you can do a reduced pressure distillation, you can break the azeotrope that way .
The azeotropic concentration is pressure dependent.Fery - 24-7-2020 at 11:13
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]foreign maple - 27-10-2020 at 15:41
possibly molecular sieves
[Edited on 27-10-2020 by foreign maple]Cou - 28-10-2020 at 10:21
