Ion Exchange-Removing NaOH

[AddisonJ]

Hi all. I use 0.5 M NaOH for cleaning anion and cation exchange resins and need a quick/efficient way of removing the NaOH before storage in 20% ethanol. H2O does not work quickly enough - can take more than 10 column volumes. I think I need a buffer at high concentration (1M) that can displace the OH- ions quickly, then I would follow with H2O (so salts won't precipitate when ethanol is used later for storage). I'm hoping to use maybe 1 column volume of high concentration buffer and 2-3 column volumes of H2O. Which buffer(s) would work well? I would prefer to use the same buffer for all cation and anion resins if possible for convenience. Any advice?

[BabcockHall]

Truthfully, I don't understand the question. Suppose you are cleaning a Dowex 50 column with NaOH. The Dowex will now be in the Na⁺ form. If you add a buffer with a different counterion, your exchanger will be in a different form. Even if you make sure you don't run afoul of this problem, you will still need water to rinse out the buffer. Why would water rinse out buffer salts any more efficiently than it does NaOH?

[AddisonJ]

I think I see your point. Whatever I replace the NaOH with will also need to be replaced later by the water. And if the buffer salt binds just as tightly to the column as the NaOH, then there's no advantage to rinsing with salt first. I was assuming that the NaOH is binding very tightly, but somebody pointed out to me that it binds more weakly than most salts would.

 

So maybe I should be asking about the importance of removing the NaOH completely. Would residual OH- still bound to the column (quaternized polyethyleneimine or quaternary ammonium ligand) due to a low rinse volume cause any damage over time? And would the 20% ethanol that is added for storage pull any residual OH- off of the column since they are both polar?

[BabcockHall]

Yes, hydroxide ion binds very weakly to Dowex 1 (quaternary ammonium ion), more weakly than formate and much more weakly than chloride ion. Many manufacturers give suggestions for how many column volumes of water to use and how to test for completeness. One such guideline is to use four column volumes of water and see that the pH has returned to less than 9. Ethanol is not an ion and should not displace hydroxide ion.

Edited February 25, 2013 by BabcockHall