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Sauron
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Drying Oxalic Acid Dihydrate to Anhydrous
There appears to be substantial interest in various compounds (oxalyl chloride, diethyl oxalate, ethylene oxalate) derived from anhydrous oxalic acid.
Unfortunately the OTC sources are all, AFAIK, to the dihydrate. In any case the dihydrate is much cheaper than the commercial anhydrous oxalic acid,
even after compensating for the fact that 36 g out of every 126 g (1 mol) of the dihydrate is pesky water.
A number of members have related their adventures with oven drying oxalic acid. This can be a mess and is usually incomplete, which I believe is
partially responsible for the modest yields encountered in both the PCl5 and TCT preps of oxalyl chloride.
One of the Org.Syn. methods uses CCl4 as a medium for dehydrating oxalic acid, but unfortunately since that time CCl4 has become a rara avis.
Certainly I can't buy it, where I am. I must make my own and when I do I won't use it for this purpose.
The same monograph mentions using conc H2SO4 as dehydrating agent, cites references and then dismisses the method as having proved unsatisfactory.
Actually they said "less than satisfactory" and did not elaborate. Driven by curiosity I sought out one of the references in English. It is attached.
The H2SO4 method per se is indeed not very interesting as it involves letting the mixture stand several months.
However the author describes a simple efficient and quick method. He talks about a vacuum retort connected to a receiver loaded with conc H2SO4. The
retort is charged with oxalic acid dihydrate and evacuated with a water aspirator (I'd interpose a guard tube) and the retort heated on a water bath
to 60 C. In a few hours the theoretical amount of water is absorbed in the H2SO4.
Neat! Anhydrous oxalic acid without sublimation, without caking, without the chore of cleaning the drying oven, and without CCl4.
For "retort" I'd substitute "rotavap" or "Kugelrohr" and I would opine that by using a pump capable of higher vacuum, the temperature can be gotten
close to ambient so that a vacuum dessicator could do this job with conc H2SO4 as dessicant. Certainly the water aspirator is not going to get below
20-30 C depending on water temperature, 60 C and that torr can be extrapolated back from 100 C/760 mm and further to whatever vacuum you can pull and
your dessicator can handle.
I think this is a splendid technique. The citation is Fisher, Proceeding of the Royal Chemistry Society 8, 186-187 (1892).
[Edited on 9-9-2008 by Sauron]
Attachment: Pages from pl8920800185.pdf (166kB)
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<!-- bfesser_edit_tag -->[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: replaced
"2H20" in subject with "Dihydrate"]
[Edited on 16.2.14 by bfesser]
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not_important
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Good reference, I knew of this method but not the origin (it worked, so I was lazy).
It's similar to the method of dehydrating B(OH)3 to B2O3, although with the addition of the H2SO4 trap and the absence of a slow temperature ramp-up.
Vacuum aided drying often gives a higher yield in a more desirable physical state, a loose or very slightly sinter porous product that is easy to
crush.
While pushing it for the home lab, a fluidised bed with microwave heating also seems to work well for some substances, the drop in absorption as the
material dries protecting against overheating while the gas flow and turbulence of the fluidised bed both speeds water removal and helps prevent
formation of compact masses. There may even be lab-sized commercial versions by now.
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Klute
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I have started a thread on the subject, where I relate the use of light pet ether in a Dean Stark to dry oxalic acid in a few hours, with very good
results.
The acid doesn't cake at all, and remains a fluid , easily decanted supension when dry.
Most of the time, I simply left it to decant, decanted the pet ether off as much as possible, and added IPA or MeOH to obtain a clear solution (so
insolubles from the technical grade used).
It could be just as practical to remove the remaining alkanes by applying vacuum to the flask, and scrapping the acid out, or directly continuing
with the procedure (esterification, etc).
I really recommend this method, seems less wastefull than using the H2SO4, as long and more practical (just need a dean stark), and you get your acid
ready-to-use in a dry flask/system 
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Sauron
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Nothing against your method, Klute. But the method described here, shares most or all of those merits. The acid never contacts anything but vacuum
and glass. The H2SO4 never touches anything but water it absorbs. The rotavap or Kugelrohr are luxuries and can be replaced by a simple setup. The
H2SO4 is recoverable. It isn't wasted any more than light pet.ether is wasted in your method. Whether or not one makes the effort to recover these is
simply a matter of resources and priorities.
Either method will get the job done.
What struck me was thst in a recent thread I made an off the cuff remark about doing this with toluene and a Dean-Stark. On reviewing the matter in
Org Syn I was abashed to find that toluene (and benzene) were not so great for this application. That was what started me off on this paper chase. I
had not seen your thread.
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Klute
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Of course not I just thought you hadn't seen it (which was the case apparently).
I choose pet ether becasue of th epossible sublimation with toluene, but a memeber reported to me than he sued toluene a few times and that
sublimation wasn't a real problem (a little solid in the condenser).
So what would be the setup? A reflux setup with a H2SO4 wash bottle and warm water in the condenser?
It would be a good idea to see how long this procedure takes and compare the two. It might be shorter than the dean stark, and even the slightly
dilute H2SO4 can be sued for lots of things..
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Sauron
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Certainly not a reflux setup. Just a RB flask heated in a water bath, a connection to receiver with vacuum takeoff, a guard tube and then the pump. If
the pump is an aspirator, then the bath is held at 60 C. The vacuum takeoff adapter need not have a drip tube down into the acid.
The charge of acid should be adequate to absorb the theoretical amount of water from the charge of oxalic acid dihydrate. One Kg oif that is almost
exactly 8 mols, and 16 mols of water are present. 288 mls. Therefore I would certainly put a condenser in there, well cooled, and I would also cool
the receiver because the conc H2SO4 is going to not be happy about all that H2). I would use not less than a Kg of acid (which is only about 600 ml)
and the use of a magnetic stirrer in the receiver would also be a good idea.
On this scale, a rotating pot like a rotavap is very helpful at turning over the upper layer of oxalic acid. In a simpler setup I would seriously
consider a high torque stirrer at slow speed with a blade strong enough to do the job.
The final weight of anhydrous oxalic acid ought to be 712 g starting from a Kg of dihydrate. It should be nicely crystalline as described in Fisher's
paper. You can recrystallize it from glacial acetic acid but the product will be powdery rather than crystalline.
Authorities differ about the problem of hygroscopicity. Some say this is rapid, others say slow, attributing the slowness to a protective surface
later of dihydrate.
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panziandi
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I used to dry oxalic acid in a vacuum dessicator over (I think H2SO4) it has been many years since I did it. I only dried small quantities at a time.
Recently I have been thinking of making about 100g or so of the anhydrous acid and have recently gotten a new vacuum pump so I may try drying it under
HiVac over H2SO4 in a rotating flask.
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kmno4
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I really do not unerstand all these efforts.... Warming oxalic acid x 2H2O on water bath ( so a little less than 100 C) gives anhydrous compound -
nothing easier. This is procedure from a book I have ( laboratory manual). And it works  .
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Klute
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For how long? At which temps? Indeed it would be much more simple, but at low temps it might be toolong, and hotter you get too much sublimation....
I'd be willing to try that out next time, and see how dry it gets, and compare the efficienty/time to the Dean Stark method. Of course, it would be
much better to just heat the oxalic acid for 30-60min...
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kmno4
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Propably I used to do this, about 15 years ago (at school) but I do not remember exactly how long it should be dried. This anhydrous acid was a
component for preparation of diethyl oxalate. Whole time, reserved for all preparations, was about 4 hours. So same drying cannot last longer than 30
- 60 minutes.
From 63g of hydrate, 45g anhydrous acid is obtained (example from book).
I have bought recently 2kg of hydrated oxalic acid (good quality, price about 4$ / kg) and I am going to (re)check this drying procedure.
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Sauron
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The point of both the hydrocarbon/Dean-Stark method and the vacuun + heating method with a dessicant in receiver is to assist the removal of the water
of hydration and to keep it sequestered away from the anhydrous acid so that rehydration can't occur.
If you are just drying 10-20 g then of course you can do lots of simpler things but if you want to rigorously dry 500-1000 g then klute's method, or
Fisher's with or without my modifications are better and the completion can be verified quantitatively by 1) weight loss of oxalid acid, 36 g/mol
lost, and 2) weight gained of H2SO4, 36 g/mol (oxalic dihydrate basis). For 500 g dihydrate that's 144 g lost and gained, and for 1 Kg dihydrate
that's 288 g lost and gained.
We know 100 C at 760 mm Hg works and we are told 60 C at aspirator pressure works (say 25 mm Hg.) Get out your temperature/pressure nomograph or
program and plug in whatever final vacuum a better pump will do. 1 mm Hg? 0.1 mm Hg? You can instantly see what the temperature ought to be for
efficient vacuum dessication of oxalic acid dihydrate over H2SO4.
This technique amounts to the same thing as a "drying pistol" but scaled up. Doing this on a rotavap or Kugelrohr, you have all the bits integrated
plus rotation to agitate the acid being dried.
Knbo4, 63 g is half a mol and so is 45 g (dihydrate and anhydrous respectively.) Entirely consistent with the numbers I have been using upthread.
[Edited on 10-9-2008 by Sauron]
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kmno4
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If simple drying really works, then in my opnion scaling up to 500g is much effective, less time consuming and no additional aparaturus/componenst
need comparing to methods given earlier.
As far as I know, oxalic acid is not especially volatile and your fears about its sublimation are exaggerated. It is not especially hygroscopic too. I
would play with "hydrocarbon/Dean-Stark method and the vacuun + heating" at scale up to 10g just to see how it all works, for fun only.
Effects of my drying I will give tomorrow, but now I can say that indeed loss of H2O seems to correspond to all amount water from dihydrate. So using
more sophisticated equipment than water bath or just carefuly heating hydrated acid can be simply needless.
Experimental.
10,4 g dihydrate was placed in a small crucible, then crucible placed in boiling water bath. After ~4 hours of heating loss of weight was 2,3 g. Total
amount of water in dihydrate is 3,0g (2,97g). Propably temperature of bath was too low.
The same crucible was placed directly on a steel plate and heated very small flame. After ~1 hour loss of weight was 3,1g and during next hour mass
did not change.
Temperature at the bottom of crucible was about ~100 C.
Very simple preparation of anhydrous oxalic acid and very easy to scale up with aid, for example, stainless pot.
Heating at the begining should be not too vigrous, because dihydrate can melt itself (m.p. 101 C). Heating can be increased and loss of water detected
by weighting from time to time. Heating can be stoped when theoretic amount of water is relised and/or there is no dew on the cover placed upon pot.
Temperature should be kept around 110 C I think, to remove all adsorbed water. There are many variants of such drying: termostated oil bath,
electrical
heating... etc.
In all preparations, important thing is stirring (from time to time) with a spoon etc. It shortens time of dehydrating.
[Edited on 11-9-2008 by kmno4]
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Sauron
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Did you by any chance actually read the Org.Syn. monograph and the Fisher paper from Proc.Chem.Soc.?
Sublimation is a relatively minor problem at 100 C and a major problem at 150 C.
Drying by heat alone tends to produce incompletely dried, caked, white material. The caked acid is dry on outside and hydrated on inside. This problem
in scaleup gets worse with thickness of the bed. Dry on top, hydrated below. Thin layers work better. So do agitated layers, either rotating or
fluidized.
The Org.Syn. and Fisher documents graphically describe these shortcomings.
A brotavap is not an uncommon piece of equipment. A Kugelrohr is. A nonrotating setup using same glassware as simple distillation with vacuum takeoff,
a water bath for heating, and a cooling bath for the H2SO4 in receiver, is neither complex nor unusual. Adding a mechanical (overhead) stirrer to turn
over the acid being dried is not exotic. So two out of three of my suggested setups are commonplace. And they result, according to the literature,
in a free flowing crystalline product devoid of caking and rigorously dried.
The same is true of the hydrocarbon methos (klute's use of light pet.ether and a Deak-Stark). Quantitative, unambiguous and complete drying without
caking.
My best estimate is that the simple heating method will give results similar to oven drying.
But, try it and see and report back.
The question of hygroscopicity is inconsistently reported in the literature but can be settled easily with a simple experiment. Place a tared beaker
on a balance and add anhydrous oxalic acid. Note the weight of acid. One mol would be nice (90 g).
Now just take measurements of weight increase over time at regular intervals. When 126 g is contained, the rehydration is complete. How long does that
take? A plot of weight over time will indicate how much care needs be taken when transferring the anhydrous acid. There's nothing subtle about 36
g/mol water of hydration. You might want to make a note of the relative humidity.
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kmno4
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From my experimet I got quantitative water removing from hydrated acid. If you do not overheat it, no caking take place.
From white powder you get another withe powder - stirring (from time to time) is important, if you want to have it "at once".
I do not need additional literature in this case - I am writing about experimental facts only and that it is enogh for me. Drying this acid is as
simple as drying CuSO4x5H2O or K4[Fe(CN)6]x3H2O.
If you want use another technics for drying - you are welcome, it is not my business.
ps. I left crucible with dehydrated acid in open air, during ~10 hours. Mass increased in 0,5 g. As it shows, this acid is truly hygroscopic. Besides,
as many substances does. I am not interested in making additional measurements of gaining mass, because I am not going to keep it in an open jar.
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Sauron
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And that was on a 500 mmol scale? Or what?
5-10 mol may be somewhat different.
Anyway, have fun.
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garage chemist
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A few weeks back, I dried some oxalic acid myself.
Since I had a liter of CCl4, I first tried the method from orgsyn with azeotropic removal of water.
As I didn't have a water separator for heavier-than-water solvents, I used a pressure equalized dropping funnel with a dimroth condenser on top of it
as a makeshift water separator- the water phase collects on top of the CCl4, and the CCl4 continuously drips back into the vessel when the stopcock is
opened just the right amount.
Guess what: The process takes so much time (I ran it for several hours and there still was water very slowly accumulating) that had to be spent
watching the apparatus and adjusting the stopcock from time to time that I stopped this endeavour, distilled off the CCl4, and did the oven method
with the residue.
At 100°C with forced air circulation in the oven, there was a lot of sublimation. The colder parts on the front of the oven got covered with a
crystalline sublimate.
After a further two hours, I assumed the acid to finally be dry, and put it into a tightly closing bottle for storage.
And now, get this:
A batch of oxalyl chloride with 9g of this acid and 25g selfmade PCl5 of good quality gave a lot of HCl evolution directly after mixing despite
cooling, the mixture NEVER liquified even after two weeks (it was guarded with a drying tube at all times) and gave ZERO oxalyl chloride upon
distillation, only a small amount of POCl3.
I assume this result to stem from the acid still being partially hydrated.
Yesterday, I tried this: In a test tube I mixed a small amount of my "anhydrous" acid with a few ml of thionyl chloride.
Gas evolution began almost immediately.
I warmed the mixture until gas evolution ceased and distilled off the excess of SOCl2. A crystalline residue of about the same volume as the starting
amount of oxalic acid remained.
Therefore, I assume that SOCl2 does not react with anhydrous oxalic acid, making the addition of SOCl2 a good method of getting the last bit of water
out of previously dried oxalic acid before using it in the oxalyl chloride synthesis.
I will now always treat my oxalic acid this way if a procedure calls for anhydrous oxalic acid- pre-dry in the oven at 100°C, and then add SOCl2 for
complete moisture removal.
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Klute
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It's strange CCl4 requires really long distn times... With pet ether, the theoretic amount of water was collected in 3-4H max, and no water at all
was seen in the condenser after another 30min to get those last tiny drops in the setup off...
How vigorous was your reflux? i usually maintain a pretty striong reflux (2-3 drops/sec) using a 110°C oil bath...
What kind of stirring did you have? Maybe insufficient stirring reduces the efficienty considerably...
I would suggets you try with pet ether or any avaible low boiling alkanes and compare the volume of SOCl2 required to neutralize any remaining water
with that of oven-dried oxalic acid... Of course, my uses of anhydrous oxalic didn't require absolute absence of water, I based myself on the amount
of hydrated oxalic acid introduced and maximum amount of water generated... I don't think oxalic acid absorbs that much more than 2mols of H2o, and
in any case the hydrated acid is kept in a sealed container and handled in a home-made glove box (because of the very fine powder burning nose and
mouth).
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Jor
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Garage chemist, isn't a waste to use SOCl2 for drying oxalic acid? SOCl2 is hard to get, and anhydrous oxalic is also hard to get, but not as hard as
SOCl2. Or is SOCl2 readily available for you?
But if you only use it to remove traces of water, you would ofcourse need only a very small amount compared to the amount of oxalic.
[Edited on 24-9-2008 by Jor]
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garage chemist
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Yes, of course I use as little SOCl2 as possible by dehydrating the oxalic acid as good as possible in the oven, and distilling and recycling the
excess of SOCl2.
It's just that PCl5 is much more valuable than SOCl2 to me since I have to make it myself, so I choose to use some SOCl2 to make better use of the
PCl5.
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Klute
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Deviating a bit, but I suppose you made it from red P and Cl2? Do you do 25g batches, or more? this is really something I would like to try one day...
Do you want to make oxalyl chloride because it is more practical than PCl5, or just for the challenge? Or maybe an application that requires oxalyl
chloride to work?
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garage chemist
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Yes, I make it from red P and Cl2 in chloroform suspension.
A detailed synthesis with pictures is available on www.versuchschemie.de .
I did a batch with 10g red P once, yielding ca. 65g PCl5, and used 25g of this for the failed oxalyl chloride synthesis.
The quality of the remainder is not that good after the long storage time, though- it has become wet and clumpy due to POCl3. This can easily be
distilled off in vacuum, though.
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kmno4
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| Quote: | Originally posted by garage chemist
(...)
At 100°C with forced air circulation in the oven, there was a lot of sublimation. The colder parts on the front of the oven got covered with a
crystalline sublimate.
After a further two hours, I assumed the acid to finally be dry, and put it into a tightly closing bottle for storage.
And now, get this:
A batch of oxalyl chloride with 9g of this acid and 25g selfmade PCl5 of good quality gave a lot of HCl evolution directly after mixing despite
cooling, the mixture NEVER liquified even after two weeks (it was guarded with a drying tube at all times) and gave ZERO oxalyl chloride upon
distillation, only a small amount of POCl3.
I assume this result to stem from the acid still being partially hydrated.
Yesterday, I tried this: In a test tube I mixed a small amount of my "anhydrous" acid with a few ml of thionyl chloride.
Gas evolution began almost immediately.
I warmed the mixture until gas evolution ceased and distilled off the excess of SOCl2. A crystalline residue of about the same volume as the starting
amount of oxalic acid remained.
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As you said - your acid seems to be not fully dehydrated, I do not know why.
I also do not belive in sublimation at ~100°C.
That is why I decide to repeat my experiment in more "scientific" way and take few pictures.
ps. my previously dehydrated sample of acid after ~5 days rehydrated itself in ~95% (mass increase almost the same as water losses before).
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kmno4
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24,4g H2C2O4x2H2O was placed in a beaker immersed in an oil bath at 95°C, loosely covered with watchglass
Dehydratation started soon and droplets of water can be seen on watchglass. This water gives acidic reaction, pH ~3.
This temperature was kept during ~2h without stirring. From time to time, droples of water have been shaken off.
After this time, temperature was set at 101°C and kept one hour.
Water were still present on watchglass.
Temperature was set at 105°C and kept ~ 3 hours. Still water...
After this time I had enough - watchglass was removed and I started stirring acid from time to time.
Stirring was continued during ~ 4 hours, till no more water could be seen on watchglass when covered beaker with it.
At the end, small amouths of sublimated acid started to appear on glass. Then acid was removed from beaker and weighted :
17,3 g. Theory: 17,4 g. Losses by sublimation are very small and stirring is important factor.
On the next picture:
HI-TECH aparaturus for dehydrating oxalic acid
This pot is ready to use water bath, originally using (probably) for warming of milk.
In this pot I placed 106,7g H2C2O4x2H2O, filled pot with water (inside) and put it on gas burner (very small flame).
In this moment stirring with a spoon is very important - in another case (as Suron predicted), cakinkg take place at the bottom and
later it is hard to crush hard layer on the botton.
Heating and stirring (at the begining every 5 minutes, later can be every 20-30 minutes) was continuated for ~5 hours (the more often stirring, the
quicker dehydration).
Degree of dehydration was indicated as described earlier.
After ~4 hours acid was weighted (traces of water were stil detecable): 77.8 g.
After ~5 hours acid was weighted again (water was not detecable):
75,9 g.
Theory: 76,2 g. For me it is perfect result.
Yesterday I found an article:
Journal of Thermal Analysis, Vol. 29 (1984) 1115-1122
THE KINETIC STUDY OF THERMAL DEHYDRATION OF OXALIC ACID DIHYDRATE
It clearly shows that dehydration take place even below 60°C.
So my old manual laboratory book said the truth and water bath is good solution.
Of course, the higher temperature the quicker dehydration.
Even below 100°C anhydrous acid sublimates, but it is very slow process, comparing to dehydratation. Losses of acid (caused by sublimation) are
propably much smaler than 1%.
Unfortunately I have not got SOCl2 to check my dehydrated acid.
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Klute
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Nice! thanks for the report... I think I will try this with a hotplate next time, and see if its more practical/short than using a Dean Stark.
That one-piece water-bath a neat pice of equipment! Can the water compartiment been used under pressure? (Steam bath?)
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Sauron
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I concurr, however the scale is submolar. I want to dry 10-20 mols in one run and do it fast and unequivocally. So I am going to try Fisher's method
as modified by myself.
I can use either a 4 liter (old model) Kugelrohr with air oven set to 60 C or a 3 liter Buchi rotavapor with a water bath at same temperature.
Rotation provides agitation of the oxalic acid, Eyela dual water aspirator ptovides vacuum (with a guard tube) and conc H2SO4 in 3 L receiver flask
(on Buchi) or a series of 1 L bulbs (on Kugelrohr) traps the water. The exact scale will dependm on the amount of dihydrate that I can place in the
put. In either case the usual liquid fill is 1.5 L for a 3 L Buchi or 1.3 L for a 4 L old model Kugelrohr and I am vague about the bulk density of the
dihydrate.
The rotavap is probably the better of the two for this purpose.
The H2SO4 likely needs chilling . 20 moles dihydrate will give up 720 mls water = a lot of heat to remove.
I am not saying this is the "only" way, I am just saying that this looks like a good way to proceed on a larger scale, IF you happen to have the
rotavap or Kugelrohr at hand.
No one is going to go buy a rotavap, much less a Kugelrohr, just for this purpose but let's not pretend that no one has these things already. A
rotavap particularly is a rather standard lab tool and not a luxury.
[Edited on 27-9-2008 by Sauron]
Sic gorgeamus a los subjectatus nunc.
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