Quick NaOH Removal

[AddisonJ]

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?

[tom jupille]

First of all, as a rule of thumb it takes about ten column volumes just to get complete *physical* washout. That's independent of equilibration issues (to put that in perspective, imagine a coffee cup full of something bitter like a quinine solution; empty it out and refill with an equal volume of water, then taste it; keep emptying and refilling until you can't taste the quinine any more).

Secondly, hydroxide is very weakly bound; almost anything (except fluoride) will displace it. For monovalent anions, the order is F- < OH- < OAc- < H2PO4- < HCO3- < Cl- < NO2- < HSO3- < CN- < Br- < NO3-

For monovalent cations, the order is Li+ < H+ < Na+ < NH4+ < K+ < Rb+ < Cs+ < Ag+

[AddisonJ]

So it sounds like I wouldn't gain anything by rinsing with a concentrated buffer salt before rinsing with water since the salt would be bound to the column just as tightly if not more tightly. Maybe I should be asking about the importance of removing the NaOH completely then. Would residual OH- still bound to the column (quaternized polyethyleneimine or quaternary ammonium) 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?

[tom jupille]

Ion *exchange* is called that for a reason; all you can do is to exchange one counterion for another. So no, ethanol will not remove OH- unless there is some other anion present to replace it. As to which ionic form is best for storage, you should check with the manufacturer to find out what ionic form they use when they ship it to you. That said, I've tended to use acetate, since it is fairly weakly bound.