Sciencemadness Discussion Board

Drying oxalic acid

Klute - 29-7-2008 at 15:27

I woul dlike to dry commercial oxalic acid dihydrate to anhydrous. My first thought was drying it in a oven at 100°C, but I realized it will sublime to a certain extent, which I cannot allow in a oven i cook food in :) And excessive heating can readily lead to decomposition..

Curiously, I haven't found any related info by searching the net.

So has anyone ever dried oxalic acid himself? I was thinking about placing the acid in a dessicator over CaCl2 for 24-48h, or NaOH. Drying in a vacuum is a little less practical, so I might try the dessicator option if nobody has any advice on this.

Ritter - 29-7-2008 at 15:33

Quote:
Originally posted by Klute
I woul dlike to dry commercial oxalic acid dihydrate to anhydrous. .


The Org Syn procedure employs azeoptropic distillation with CCl4 but states that toluene has also been used. See http://www.orgsyn.org/orgsyn/pdfs/CV1P0421.pdf.

not_important - 29-7-2008 at 16:23

Quote:
Originally posted by Ritter
The Org Syn procedure employs azeoptropic distillation with CCl4 but states that toluene has also been used...


Note that the preparation states
Quote:
Carbon tetrachloride is in every way the most suitable liquid. It is not inflammable; the boiling temperature is such that water is readily evolved but sublimation of anhydrous acid is slight; and, inasmuch as the density is near to that of oxalic acid, an efficient mixing can be obtained. In benzene, the first and third conditions are not fulfilled; in toluene, apparently none of them, for although the water is more rapidly driven off than with carbon tetrachloride the sublimation is so great as to be troublesome, and, owing to the less efficient stirring in the liquid of lower density, the acid shows a great tendency to clump. However, toluene has been used.


Tech hexanes would be a better choice, as the boiling point range is lower. You will still need a good fast stirrer to keep the acid in suspension

That preparation also has a second method using an oven and non-reactive trays. As some oxalic acid sublimates it is a good idea not to use an oven also used at other times for cooking.

One more, likely less satisfactory as it is much older than the OrgSyn ref and they don't suggest it:
Quote:
353. Anhydrous Oxalic Acid. — To obviate the uncertainty of the degree of hydration, anhydrous oxalic acid has been somewhat used. Drying at 60° to 80° for several hours is said to secure an anhydrous product. At 100° considerable oxalic acid is volatilized.
As the anhydrous material is quite hydroscopic, it must be cooled in a desiccator and weighed as soon as cool.

QUANTITATIVE CHEMICAL ANALYSIS BY GRAVIMETRIC, ELECTROLYTIC, VOLUMETRIC AND GASOMETRIC METHODS

John Charles Oslen

S.C. Wack - 29-7-2008 at 16:34

I have done it in toluene and sublimation is not a problem at all. The major problem in the amounts that I used (huge excesses of toluene prevent problems) is clumping of the acid. This works great with overhead stirring, but magnetic stirring is too weak to prevent clumping with large relative amounts of acid.

len1 - 29-7-2008 at 16:54

How about drying in a sealed dessicator in an oven at 80-90C (to speed up the kinetics) over conc H2SO4? Its got a much lower H2O partial pressure than CaCl2 - especially at elevated temperature, with the advantage over NaOH that it will keep the oxalic acid vapour pressure up. Sublimation of the acid should cease once its vapour pressure comes to equilibrium with the solid.

Klute - 29-7-2008 at 18:23

Thanks for all your replies; I can't beleive I didn't find the orgsyn preparation....

I think i will try the toluene method with a dean stark, as I plan on making a solution in IPA to form amine oxalates anyway, I can just aswell dilute with dry IPA once the acid is dry and half the toluene (or hexane) is removed, until a solution is obtained, and then add that to the freebase in toluene.

I will report on how it goes.

AT the same time I will leave some hydrate in a dessicator over some H2SO4 and see how that goe stoo, it might be easier to just forget about it a day or two and directly dissolve it in IPA.

Klute - 5-8-2008 at 16:06

A 3H reflux with pet ether (50-65°C) with a dean stark was enough to completly dry the oxalic acid, with only minimal sublimation (a few traces in the bottom of the condenser).

I think oxalic acid can be a very practical substitute to conventional means of crystallizing amines. HCl requires dry condtions, and often involves depleasant and unpractical HCl gas, H2SO4 is delicate to use as any excess can form a mess and redissolve any salt, and the sulfates often require a carefull recrystallization even for short term storage of an intermediate to avoid decomposition, etc. I'm not familiar with phosphate salts, but I ahve been told that their formation is often troublesome, forming amorphous semi-solids, etc

Oxalic acid is nice in the sense that it can be used in excesswithout problems, it is readily soluble in IPA, MeOH, Et2O and possibly other solvents (AcOEt? ), the oxalate salts are much less hygroscopic that hydrochlorides and sulfates (from wjhat I have witnessed so far), their formation is immediate, and they are more easily recrystallized with alcohols then sulfates and hydrochlorides.

A known amount of powdered commercial oxalic acid dihydrate was placed in a wide neck RBF, and covered with pet ether. The mixture was stirred strongly and heated to a steady reflux with a dean stark trap, protected with a CaCl2 guard. After 3h, the theoritical amount of water had been collected, and no more was evolved. The oxalic acid had not clumped up at all, and was still as a loose, fine white powder, quickly decanting from the colorless solvent. After cooling down (some traces of acid were visible at the bottom of the condenser, but not higher), the flask was disconnected, and the solvent was carefully decnated as much as possible.
A precise volume of IPA (technical, not specially dried) was then added, the flask stoppered and swirled. The white solid dissolved very quickly, giving a clar, very slightly milky solution. The solution had a concentration of 1mol/L,a nd was kept in a capped amber bottle.

60 mmolof th eoslutionw as placed in a wide neck, RBF, and with stirring a solution of ~50mmol of crude n-methyl-2-phenethylamine in toluene was added in a slwo stream. A voluminous beige solid formed immediatly. Once all the solutionw as added, a thick slurry of the oxalate salt was formed. It was stirred for 15min a RT, then placed in a fridge for another 15min.
The gelatinous slurry was vacuum filtered, the solid washed with IPA/acetone 1:1, then pet ether and dried by suction.

Filtration was rather long because of the gelatinous natur eof the salt, but this is caracteristic of this particluiar compound, and not just the oxalates (the sulfate is very similar, very long to filter and properly dry). Triacetoneamine oxalate is a loose, granular solid for example, very easy to filter.

I think 15min stirring is recommended to let the acid and amine fully react. Also, having alarge proportion of alcohol is necessary to avoid precipitating excess oxalic acid (don't flood with pet ether during filtration, only once most of the mother liquor has been filtered). In a lot of procedures, only MeOH or IPA is used to crystallize the amines, but cooling to -20°C is recommended for maximum recovery of the products. Adding acetone or AcOET is adequate proportions should able to find the balance between maximum recovery and not precipitation of the oxalic acid.


I think a quicker, or at least more practical way of drying the oxalic acid could give acces to a stable, easily stored solution of known concentration (titration could be a good idea), and really simplify the crystallization and isolation of amines. the solution seems to be less hygroscopic than HCl in IPA (although that could be just an impression), but it cna surely be kept over molecular seives or another suitable drying agent. In any case traces of water seem less detrimental that with hdydrochlorides..
Oxalic acid could be dried by standing over H2SO4, or even CaCl2?
But drying 1 mol of oxalic acid by azeotropic reflux every once in a while isn't that bothersome, less so than gassing IPA with HCl in any case, and doesn't require much attention.


All in all, I'm very happy of trying out this procedure for crysatllizing amines. In the past it was always a annoying proceudre, using HCl gas, or obtaining junk and having to try again (sulfates..), and now its just so simple. I really recommend trying this out.