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I'm new with this concept and so I would need some advice.

I want to separate an acid and a base in a water solution. The acid has $\mathrm{p}K_\mathrm{a}(\ce{HA}) = 5$ and the corresponding acid of the base has $\mathrm{p}K_\mathrm{a}(\ce{HB}) = 7.$ The column is positively charged. What $\mathrm{pH}$ should I use in order to retain the acid in the stationary phase and the base passing the column?

Well, I'm just thinking that I would want the acid to be negatively charged in order to bind to the stationary phase, and so I would use a $\mathrm{pH} > \mathrm{p}K_\mathrm{a}(\ce{HA})$ because then the solution will gain a proton and leave the acid negatively charged. Right?

What about the base, should it be positively charged in order to escape the stationary phase and be attracted to the polar water?

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  • $\begingroup$ Is the base neutral before it picks up a proton?. $$\ce{B + H+ <=> BH+}$$ Maybe make a table with pH range (0-5, 5-7, 7-14) and the charge of the two species you want to separate. I'm not sure you can make one negatively charged and the other positively charged at the same pH. $\endgroup$ – Karsten Theis Jun 14 '19 at 17:00
  • $\begingroup$ Also, consider whether the charge of the column changes (weak anion exchange column, e.g. primary amines) or not (strong anion exchange column, e.g. quaternary ammonium) $\endgroup$ – Karsten Theis Jun 14 '19 at 17:09
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    $\begingroup$ Can you be specific with your acid and base? $\endgroup$ – Karl Jun 14 '19 at 19:47
  • $\begingroup$ I don't have more information than this, only that it is anion exchange. $\endgroup$ – Miana Jun 15 '19 at 10:12