Sciencemadness Discussion Board

removal of pyridine

andre178 - 13-1-2010 at 22:16

Hello, I am trying to rid an aqueous solution from pyridine contaminants. I am trying to go the cleanest possible way without adding other things that I cannot remove. I read that extraction with ether might solve my problem, but I tried it today and pyridine still remained in my aq. solution.

Any suggestions?



Andre

chemrox - 13-1-2010 at 22:56

what else is in the solution? depending on what else you've got you might run it through a resin column... but a lot hinges on what you have and wish to keep..

JohnWW - 14-1-2010 at 00:13

If there are no other amines present, and you want to extract the pyridine from an aprotic organic solvent, a likely way could be to add some HCl or H2SO4 to form a pyridinium salt, and then extract the salt with water.

Nicodem - 14-1-2010 at 01:12

JohnWWW, "aqueous solution" means a solution in water as solvent. Obviously water is not an "aprotic organic solvent".
Quote: Originally posted by andre178  
Hello, I am trying to rid an aqueous solution from pyridine contaminants. I am trying to go the cleanest possible way without adding other things that I cannot remove. I read that extraction with ether might solve my problem, but I tried it today and pyridine still remained in my aq. solution.

Any suggestions?

First of all you need to tell us the pH of the solution.

You can not remove pyridine from an aqueous solution with diethyl ether. I don't know who told you that, but the H2O/Et2O partition of pyridine is not enough in favour of Et2O.

If the pH is above 6 then you might be able to remove most of the pyridine by washing several times with ethyl acetate (If the pH is much lower than that then you can forget about removing it with washes). Then concentrate the aq. solution on a rotavap to half volume. This will remove the dissolved ethyl acetate and the remaining pyridine which is gone via the pyridine/water azeotrope. Now dilute with water up to the original volume and you should have your pyridineless original solution. If traces of pyridine are still present, then rotavap again to half volume and dilute again. If the original concentration of pyridine is low enough, then you can skip the ethyl acetate washes and do the removal by azeotropic distillation only.

andre178 - 14-1-2010 at 11:32

Thank you all for the answers, to summarize,

The other "stuff" in solution is functionalized fullerenes (ie. fullerenes with malonic acids stuck on them, enough to make it soluble in water).

the pH is basic.... ~11. I cannot add sulfuric acid because I don't know how to get rid of that... the fullerenes are somewhat temp sensitive so I can't raise the mixture to anything above 60*C. I have a lot of sodium from the initial NaOH I added, so if I add HCl, it would get me a lot of NaCl residues which again I don't want.


The Et2O extraction I got from my old college chem notebook in purification of pyridine from pyridinium chloride by making the solution basic >8pH then 3x Et2O extraction, drying w/ NaSO4, then Hickman distillation to obtain pure Pyridine. <<==== I really do appreciate your help but saying "who told you that...." doesn't get me far. It would've been more more helpful to point me to why pyridine would not have a preference for ether over water; I understand that the compounds go by dielectric index in choosing solvents, is that correct?


I chose the ethyl acetate wash according to your advice since the pH is high, and then I will get rid of ethyl acetate by rotovapping. The fullerene was processed in ethyl acetate before and with the help of the rotovap we were able to obtain pure compounds ~95% level.



[Edited on 1-14-2010 by andre178]

Panache - 15-1-2010 at 04:08

Continuous liquid-liquid extraction for a day using ethyl acetate followed by the rotovap seems like much less work and better efficacy (in relation to the extraction, given your success already with the afermentioned advice re ethyl acetate), just 'set and forget'.

Nicodem - 15-1-2010 at 05:36

Quote: Originally posted by andre178  

The Et2O extraction I got from my old college chem notebook in purification of pyridine from pyridinium chloride by making the solution basic >8pH then 3x Et2O extraction, drying w/ NaSO4, then Hickman distillation to obtain pure Pyridine. <<==== I really do appreciate your help but saying "who told you that...." doesn't get me far. It would've been more more helpful to point me to why pyridine would not have a preference for ether over water; I understand that the compounds go by dielectric index in choosing solvents, is that correct?

Not really. The dielectric constant is just one of the molecular properties that affects the liquid/liquid distribution of a certain compound. Many others can have a much more important role (H-bond donors/acceptors, ionizability, VdW volumes, flexibility, polarity of functional groups, (a)symmetry: the whole structure in general). Good computational models exist for calculating the distribution coefficient for water/octanol, but since all these effects are solvent specific, no general models exist for calculation of just any liquid/liquid system (water/octanol was chosen as a model due to biochemists).
Anyway, the partition coefficient of pyridine for Et2O/H2O is 1.2 (Acta Chem. Scand., 3, 717-747). This means that in a biphasic system composed of equal volumes H2O and Et2O, the amount of pyridine partitioned in Et2O will be only 1.2 as much as that remaining in H2O. Therefore there is no way to quantitatively extract pyridine from its aqueous solution using diethyl ether, except for a continuous extraction. After a single wash 45% of the original pyridine amount still remains in the aq. solution. After the second wash it is 20%. Third wash gives leaves about 9% and so on. So just imagine that more than six diethyl ether washes are needed to reduce the pyridine content bellow 1% of the original, never minding a quantitative removal.

PS: Are you sure the sodium salt of fulerenyldene malonic acid does not get extracted in ethyl acetate at least to some degree? I would expect this to be the case given that it might have some limited surfactant properties. If so, then rather skip the washes and instead just do a few rotavaps to half volume and back.

Barium - 15-1-2010 at 10:04

Reduce the volume of the solution to half, two or three times and it should be gone. In essence steam distillation is the way to go.
At a pressure of about 70mbar water boils at 40 deg C so you are well below your limit of 60 deg C.