Why do metal ions with large radii have smaller Rf values in paper chromatography.?
Hi Jacqui: This is how it works in ion exchange columns so is probably the same for paper chromatography (hope you’re using water). Consider elution of Li+ and Rb+ down an anion exchange column. The Li+ ion elutes first contrary to expected. The small Li+ ion should bind more tightly to the -ve sites in the column material and elute last. But we have forgotten that we are eluting M+(aq) ions and the smaller Li+ ion with its more concentrated electrostatic field binds far more water in its primary (bound to Li+) and secondary hydration spheres. It is estimated that there are ~25 H2O molecules bound to Li+ in H2O and this is also thought true in the hydrated resin. The Li+(aq) can’t bind tightly to the -ve sites and so it is eluted quickly. On the other hand the Rb+(aq) is estimated to have only ~10 H2O bound to it and so Rb+(aq) is a smaller in H2O than Li+(aq). It can therefore bind more tightly to the column material (silica has Si-Oδ- and alumina is similar) and is thus held up on th
