Sciencemadness Discussion Board

Formic Acid Dehydration

Chemist1357 - 17-11-2022 at 23:04

Formic acid forms an azeotrope with water at 77.5%. I have this homemade but I need 85%-88%.

I can’t find any clear and easy way to dehydrate it further than 77.5% and I can’t buy 85% in Australia it just isn’t available anywhere and I prefer to keep things otc.

Can you do it under vacuum or something? There’s always dumping a huge amount of anhydrous Copper(II) sulfate..

Fery - 17-11-2022 at 23:57

Industrially they use distillation of azeotrope at reduced pressure, IIRC something like 0,3 atm. But I suppose you need also a good column (like insulated Hempel with Raschig rings) and variable adjustable reflux ratio distillation head for that.
https://www.researchgate.net/publication/343946284_Process_a...

I found also this for lab. preparation:
https://www.researchgate.net/post/How_do_I_remove_water_from...
You already have anhydrous CuSO4, other choice is B2O3

draculic acid69 - 19-11-2022 at 01:03

Possibly go with the classics: mgso4 or CaCl2

clearly_not_atara - 19-11-2022 at 10:05

According to a paper found by Klute, formic acid can be dehydrated over anhydrous oxalic acid:

http://www.sciencemadness.org/talk/viewthread.php?tid=7012

Oxalic acid however is purchased as the dihydrate which in turn means you need to dehydrate it first. One method involves heating at 60 C in a retort with a desiccant at the other end of the tube. Another is the use of alkanes with a Dean-Stark (toluene apparently does not work):

http://www.sciencemadness.org/talk/viewthread.php?tid=11153

Fery - 19-11-2022 at 11:05

If using anhydrous oxalic acid for concentrating formic acid works then you can maybe dehydrate oxalic acid dihydrate into anhydrous using silicagel and desiccator. I once did that with a hope to dry my wet dihydrate crystals but after some time the crystals decayed into dust. The silicagel was from a pack of effervescent vitamine tablets, nothing special.

Texium - 19-11-2022 at 11:35

In the past, I’ve dehydrated oxalic acid by azeotropic distillation with toluene and a Dean-Stark trap, followed by putting the resulting anhydrous oxalic acid in a desiccator over anhydrous CaCl2 and paraffin wax shavings to keep it dry while absorbing any residual toluene.

clearly_not_atara - 19-11-2022 at 20:28

Quote: Originally posted by Texium  
In the past, I’ve dehydrated oxalic acid by azeotropic distillation with toluene and a Dean-Stark trap, followed by putting the resulting anhydrous oxalic acid in a desiccator over anhydrous CaCl2 and paraffin wax shavings to keep it dry while absorbing any residual toluene.

Apparently toluene does work! :p

Boffis - 21-11-2022 at 09:50

Hi Chemist1357; I had this problem once and did some research into upgrading my formic acid and I found these two references I tried the azeotropic distillation but was a pain though it does work.


Attachment: Anhydrous formic acid AmCJ Garner et al 1911.pdf (452kB)
This file has been downloaded 213 times

Attachment: phptP4SsQ (660kB)
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Chemist1357 - 8-12-2022 at 07:26

Quote: Originally posted by Boffis  
Hi Chemist1357; I had this problem once and did some research into upgrading my formic acid and I found these two references I tried the azeotropic distillation but was a pain though it does work.

How did you go about doing it? How much mmhg do you have to pull and what was the obtained concentration?