I have to reason whether silica or alumina would be a better choice of stationary phase for separating a mixture of pyridine and cyclohexanone with column chromatography.
I would expect both pyridine and cyclohexanone to be slightly polar (silica because it has the $Si(OH)$ groups and alumina I suppose because it has a metal attached to highly electronegative Oxygen), but the main difference I'm getting between the two is that pyridine (according to Wikipedia) is considered to be slightly basic, whereas cyclohexanone is neither really acidic or basic. So I suppose alumina is somehow a proton acceptor?
Another Google search shows that pyridine has a $pK_b \approx 8$, which doesn't seem particularly basic to me, but I suppose it's enough.
My intuition would tell me that since alumina is basic it would be best for separating basic pyridine from non-acidic/basic cyclohexanone. But I'm having some confusion wrapping my head around about why using an acidic or basic stationary phase would be helpful (or perhaps it's not helpful and I'm heading on the wrong track -- but even then I'm confused in justifying how it wouldn't be helpful). This brings me to my questions:
- How would having an acidic or basic stationary phase be useful for chromatography? Chromatography, as I understand it, relies in things adsorbing and sticking to the stationary phase for different retention times, whereas acid/base properties just have to do with proton transfer. If alumina is a basic stationary phase, I suppose it could be accepting protons from the solution -- but how it would this lead it to adsorb and retain one of pyridine vs cyclohexanone stronger? I would guess that maybe having a protonated stationary phase leads to the basic compound (pyridine) competing for those protons on alumina and somehow the pyridine ends up stuck to the stationary phase where all the protons are ... but I'm not sure if this would be logical.
- How does pH of the stationary phase affect the quality of separation? Retailers seem to be selling alumina stationary phases in various pH ranges: pH 6-7.5 (called neutral); pH 4-5.5 (called acidc...); pH 8-9.5 (called basic). How could the pH range which the alumina is kept affect its separation properties? I'm thinking of things in a "Henderson-Hasselbalch amino-acid titration way," but I suppose alumina may be more protonated at lower pHs than higher ones ... does this affect it's separation properties?