GhostAliensFromMars
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77.5% formic acid
Hello
I have read through a number of threads regarding the drying of azeotropic formic acid to increase its concentration. All methods seem to be very
vague. I am interested in using dry cuso4 to dry it, has anyone attempted this method and can provide me with results? I only have access to 65%
formic and this can be distilled only to 77.5%
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Fery
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Industrially 85% formic acid is made by pressure distillation of azeotropic acid which shifts the azeotrope into higher concentration. This is not
easy to do in home lab unlike vacuum distillation. If you try any drying agent like anhydous CuSO4 please post here your results.
If someone has enough time and possibilities - this is just an idea, untested yet - maybe adding some entrainer like hexane/isohexanes could shift the
equilibrium - this is a way only if the contamination with relatively inert hexane/isohexanes does not cause any problem. If it works, then Dean-Stark
trap apparatus should be convenient way in home labs, the upper hexane layer will be returned back into distillation flask and the lower aqueous layer
could be periodically drained from the apparatus.
| Quote: | | b.p. 55-57°C with a composition of about 85-86% hexane, 13-14% formic acid, and 0.4-0.6% water. When distilled, this azeotrope separates into a
hexane-rich oil layer and a water-rich aqueous layer, allowing for the recovery of high-concentration formic acid through techniques like reactive
distillation |
"medicinal petrolether" should be available worldwide, it is hydrogenated fraction of isohexanes boiling at range 60-65 C (check the exact information
before you buy)... you certainly need hydrogenated fraction as any alkene present could be hydrated to alcohol or esterified in the presence of
relatively strong formic acid
medicinal petrolether is used for degreasing skin before applying adhesive transdermal patches and to clean the skin from remainders of adhesive
patches... note that also "spiritus vini cum benzino denat" could be used for that, so look for pure petrolether without ethanol
if you use isohexanes (petrolether with b.p. 60-65 C) the ternary mixture should distill at even lower temperatures, the mentioned 55-57 C is valid
for hexane which has b.p. 69 C, I recommend using an efficient condenser (if Liebig used then at least long enough) and very cold circulating water
here I found a patent with uncertain validity:
https://patents.google.com/patent/CN1287995A/en
and something which looks valid but not available for free:
https://www.sciencedirect.com/science/article/abs/pii/S00981...
there is also a very toxic small scale preparation by reacting Pb(HCOO)2 + H2S at 100 C:
https://chemistnotes.com/organic/laboratory-preparation-of-f...
if you are in EU you can buy cheap 99+% formic acid from laboratoriumdiscounter.nl
1 L 99+% for 27 EUR
https://www.laboratoriumdiscounter.nl/en/formic-acid-99-extr...
1 L 85% technical grade for 12 EUR
https://www.laboratoriumdiscounter.nl/en/formic-acid-85-tech...
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GhostAliensFromMars
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Alright, looks like I'm gonna have to try out the cuso4 drying. Quick question, if one had formic acid at a concentration above the azeotrope, eg 85%,
and attempted to distill this, would they end up with 77.5% or 85% acid? Under normal conditions.
Thinking I'm gonna have to first distill my 65% formic to get it to the 77.5% azeotrope, then add a bunch of anhydrous cuso4 to this, and distill off
what I hope to be greater than 77.5% formic in concentration(if my above question is correct. Will post results when my chemicals arrive.
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MrDoctor
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i think it would split into its anhydride, kind of how sulfuric acid above 98% does, forming a SO3 mist that immediately hydrolyzes again. but formic
anhydride is carbon monoxide and has no reverse mechanism, though pressurized CO does prevent further formation at least. its a shame you cant just
combine a stoichiometric quantity of formate and an inert salt forming acid.
I think i have heard though silica gel could work. As well as that the concentration under vacuum can go to 85 or 88% or something, youll have to look
into the azeotrope concentration under reduced pressure to confirm this.
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bnull
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From Purification of Laboratory Chemicals, 6th ed., p. 145:
| Quote: | | Anhydrous formic acid can be obtained by direct fractional distillation under reduced pressure, the receiver being cooled in ice-water. The use of
P2O5 or CaCl2 as dehydrating agents is unsatisfactory. Reagent grade 88% formic acid can be satisfactorily dried by
refluxing with phthalic anhydride for 6 hours and then distilling it. Alternatively, if it is left in contact with freshly prepared anhydrous
CuSO4 for several days about one half of the water is removed from 88% formic acid; distillation then removes the remainder. Boric
anhydride (prepared by melting boric acid in an oven at a high temperature, cooling in a desiccator, and powdering) is a suitable dehydrating agent
for 98% formic acid; after prolonged stirring with the anhydride the formic acid is distilled under vacuum. Formic acid can be further purified by
fractional crystallisation using partial freezing. |
Edit: Oops. Wrong button.
Distill your 65% acid to the azeotrope, add a lot of anhydrous copper sulfate to the distillate, let it rest for a week or so (you're not in a hurry,
are you?), then distill again. Fractional freezing should get rid of the rest of the water. Anhydrous magnesium sulfate may work, I don't know.
| Quote: | | i think it would split into its anhydride |
There is no sulfuric acid involved, hence no drastic dehydration.
[Edited on 25-9-2025 by bnull]
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GhostAliensFromMars
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"Distill your 65% acid to the azeotrope, add a lot of anhydrous copper sulfate to the distillate, let it rest for a week or so (you're not in a hurry,
are you?), then distill again. Fractional freezing should get rid of the rest of the water. Anhydrous magnesium sulfate may work, I don't know."
Should I distill the dried acid under aspirator vacuum or at atm pressure and temp? Or just try both seperately and compare results
[Edited on 26-9-2025 by GhostAliensFromMars]
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bnull
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| Quote: | | Should I distill the dried acid under aspirator vacuum or at atm pressure and temp? Or just try both seperately and compare results
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Vacuum. If you want to try both, I see no reason not to, as long as you watch the temperature behavior when distilling at 1 atm (what about making a
graph of temperature in function of time?).
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Precipitates
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Triple vacuum distill over anhydrous copper sulfate.
As the water is removed the copper sulfate will form a green, as opposed to blue, solution.
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Pumukli
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Interesting this CuSO4 drying. My gutfeeling was CuSO4 was incompatible with formic acid due to its oxidizing nature and the reducing formyl group
present in formic acid.
The long known Fehling's reaction comes to mind.
It seems (based on the cited sources) that is not the case. Maybe the low pH in this case makes it possible.
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GhostAliensFromMars
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Right I ran into a small problem during the distilation to get the 77.5% azeotrope
The acid was heated and started boiling, the vapor traveled up the column, it reached the top but it struggled to make it to the condenser, so I wrap
the column in foil and it came over. The temp was always at 107c, which was strange because I expected the small amount of water to come over first at
100c. I collected the first 10mls of distilate that came over and discarded it, assuming it was diluted formic acid. The next 60mls of formic acid
that came over was save and I assumed this was the azeotrope. Over the course of the distilation, the temperature was always at 107. I stopped when
there was 15mls of acid left in the boiling flask. I measured the density of the distiled acid, but weirdly the density was the same as the original
65% formic acid???
I know that formic acids density is close to water so small increases in concentration will not change the density very much, but I still feel as if
my distilled acid was not at 77.5% concentration. Maybe when I added the foil to the column I prevented reflux and the azeotropic acid mixed with the
water??? Maybe this is why the temp was constant?
Right now I have added some anhydrous cuso4 to the distilled acid of uncertain concentration. This will be redistiled under aspirator vacuum WITHOUT
column insulation.
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GhostAliensFromMars
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Right I ran into a small problem during the distilation to get the 77.5% azeotrope
The acid was heated and started boiling, the vapor traveled up the column, it reached the top but it struggled to make it to the condenser, so I wrap
the column in foil and it came over. The temp was always at 107c, which was strange because I expected the small amount of water to come over first at
100c. I collected the first 10mls of distilate that came over and discarded it, assuming it was diluted formic acid. The next 60mls of formic acid
that came over was save and I assumed this was the azeotrope. Over the course of the distilation, the temperature was always at 107. I stopped when
there was 15mls of acid left in the boiling flask. I measured the density of the distiled acid, but weirdly the density was the same as the original
65% formic acid???
I know that formic acids density is close to water so small increases in concentration will not change the density very much, but I still feel as if
my distilled acid was not at 77.5% concentration. Maybe when I added the foil to the column I prevented reflux and the azeotropic acid mixed with the
water??? Maybe this is why the temp was constant?
Right now I have added some anhydrous cuso4 to the distilled acid of uncertain concentration. This will be redistiled under aspirator vacuum WITHOUT
column insulation.
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zed
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Ummm. Personally, I have concerns about concentrating Formic Acid beyond normal concentrations. I have used concentrated Formic Acid as a solvent in
Peroxidation reactions. As I recall, Formic Acid precautions read like: Distill only under reduced pressure. Under some conditions, Formic Acid may
decompose spontaneously.
The "spontaneously" part, gives me pause for consideration. Spontaneously, is sometimes used as synonym for "explosively".
I'm going to check the net!
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bnull
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Quote: Originally posted by zed  | | The "spontaneously" part, gives me pause for consideration. Spontaneously, is sometimes used as synonym for "explosively". |
Not this time. According to Bretherick's, 98-100% formic acid slowly decomposes, releasing carbon monoxide, so storing it in an unvented bottle is not
a good idea.
But, it may decompose explosively on a nickel surface.
Edit: J. Falconer et al., The Explosive Decomposition of Formic Acid on Clean Ni(110), Jpn. J. Appl. Phys. 13, 525 (1974).
Attachment: Falconer_1974_Jpn._J._Appl._Phys._13_525.pdf (1.5MB)
This file has been downloaded 7 times
[Edited on 5-11-2025 by bnull]
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woelen
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I have worked quite a lot with formic acid and never had any explosive decomposition, not even with nickel compounds. Nickel formiate is a stable
salt. When it is heated to above 300 C, then it decomposes, leaving behind impure nickel metal. It does decompose fairly quickly, when added to
concentrated H2SO4, but this reaction needs some gentle heating. When heated to 60 C or so, you get a nice and fairly constant flow of CO gas, enough
to maintain a small blue flame, when it is ignited.
The slow decomposition, however, is a real concern. When the acid is at 98+ %, then it MUST be stored in a bottle with a vent-cap. Normal commercial
acid always is at concentrations up to at most 90%. With such acid, there is no risk of pressure buildup. The pressure problem only exists for
concentrations well above 90%. When you buy 98+ % acid, then it is sold in bottles with a special vent-cap.
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