Savag3Savant
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Formic Acid Purification?
Helllloo everybody,
I've been doing a bit of home chemistry, and one of my endeavors requires the use of formic acid and 30% H202 to create performic acid. Living in the
US of A, formic acid was a bit difficult to obtain.. However, it was accomplished, but still needs to be separated from a mess of water. The water is
at least 50% of the precious sample, and I have already attempted freezing the product to just below 8 degrees C (Formic acid's melting point) to
attempt freezing just the desired compound from the soln. Do I need to use a smaller sample to achieve a faster freeze? I'm using approximately 700 ml
of soln as of now.. Any suggestions on what I could do? Unfortunately, distillation is out of the question because the compounds differ by only 1
degree C. Thanks in advance!
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leu
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Either distill it under a vacuum as in JACS 50 2166 (1928) to obtain a 77% concentration or azeotropically using butyric
acid as in British Patent 597078 for higher concentrations up to 99% 
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Savag3Savant
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Butyric acid, eh? Sounds somewhat accessible.. How did you go about obtaining this? Also, I found the two-page British patent, and it mentions
'n-propyl formate' and 'isopropyl ether' as the only suggested azeotroping agents.. Could you refer me to you source for the switch? Be it logic, or
pdf .. heh Thanks again! 
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leu
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After downloading and reading that patent, it's clear that while the author was one of the leading authorities in the world when he wrote that
paragraph that he still made a typographcal error  Another good source of
information states that formic and butyiric acid don't form an azeotrope 
Isopropyl ether is extremely dangerous to work with, but it may very well be the best way to obtain highly concentrated formic acid 
[Edited on 30-6-2006 by leu]
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Nicodem
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| Quote: | Originally posted by Savag3Savant
The water is at least 50% of the precious sample, and I have already attempted freezing the product to just below 8 degrees C (Formic acid's melting
point) to attempt freezing just the desired compound from the soln. Do I need to use a smaller sample to achieve a faster freeze?
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That does not make sense. A 50% solution of HCOOH(aq) has a freezing point well bellow 0°C. Mixtures behave like mixtures and their properties
obviously differ from the properties of pure compounds.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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S.C. Wack
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Simple distillation of 50% acid is not out of the question, it's just not perfect, giving weaker and stronger acid. The weaker acid can be put to good
use after neutralization:
http://www.sciencemadness.org/talk/viewthread.php?tid=1782
That German patent works well, though of course the temperature is a little high for a mass of bisulfate. I set a CO detector at the retort outlet and
got nothing at any time.
http://www.erowid.org/archive/rhodium/chemistry/formic.acid....
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Savag3Savant
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So the bp's (while in the mixture) are not close enough to completely screw a simple distillation? Or will it be like you said, simply "not perfect?"
What do you mean when you say it will yield a weaker and a stronger acid? Do you mean that since the boiling point of formic acid is higher - outside
of the sample - that it will be the first to come off to yield (in the receiving glass) a more concentrated sample.. while the distilling flask will
contain most of the water... Sorry I'm a bit slow..
And seeing how this will inevitably be thrown into a soln. of 30% H2O2 ... Can't I use a few shortcuts to diminish the necessity of perfectly
purifying the formic acid? Since I've never actually performed this synth, here is a list of q's I have on my mind:
1. Is the reaction violent? -- I'm sure it is just a bit.. heh
2. Is there a visual cue to help determine when the soln. has settled and therefore fully reacted?
3. Is there a particular ratio to keep in mind while creating a peracid? Excess of H2O2? Excess of Formic Acid? Or perfect molar mix?
4. My intentions are to oxidatively cleave an alkene into an aldehyde.. Does this affect the proportions to which I should mix these?
If these questions are a bit general.. please ask me anything. I've done a share of reading, and am in my third year of university (studying
chemistry).. thanks again guys (or gals..... hah don't we wish) for all the input!
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leu
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The attached paper Ind. Eng. Chem. 27 1224 (1935) uses fractional crystallization by freezing to produce highly pure formic
acid, but the cooling process is used to remove impurities; the formic acid is isolated by distillaton 
Attachment: ritter.zip (31kB)
This file has been downloaded 625 times
Chemistry is our Covalent Bond
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