[macman104]

Hi all,

I'm not going to lie, this is 100% a question on a take-home final.  However, the book that we use, coupled with the professor, means I've not solved most of the problems on the take-home final through pouring over the book, but by google and doing my own research.  Anyway, after 9/10 questions successfully solved and answered, I'm stumped beyond all attempts to solve this last question.

This question is in regards to Ion-Exchange chromatography with Eluent Suppressor Columns.  We're told to view a figure in our book.  It shows the separation of the group I and group II metals using ion-chromatography.  The group I elements come off the column first and they're grouped into one peak.  Then individually the group II elements come off the column in the order, Mg, Ca, Sr, Ba.  We are then asked to explain why:
a)  The group I metals elute faster than group II metals
b) The group II metals elute in reverse order of their atomic numbers

First thoughts that came to mind were Z_eff, Ionic size, polarizability, and such.  However, when I was thinking about that, I realized that group I ions would probably have the largest Z_eff and would bind most tightly to the column.  So that idea was shot, polarizability...we've got bigger, more polarizable elements at the bottom, so..I'd think that they would probably elute slower, so that might be a reason why the group II metals elute in the order they do.  After googling everything I can imagine with regards to this question, and trying to find scholarly articles, I'm giving up.  I've got other finals to study for (PChem, ugh), and after this many hours, I'm just frustrated with this professor's exam.

Now, reading the rules, I fully realize that "my professor is bad.." and such isn't an excuse.  I understand that, and that's not my excuse, just one of the reasons I've decided to finally stop googling in vain, and come here in hopes someone can point me in the right direction.

I'm thinking A-Chem is the most appropriate section for this question, but if it isn't Mods, feel free to move this to the appropriate forum.

Thanks for your time.

BTW, take-home final is due on wednesday by noon (CST), just for reference.

[chiralic]

Hi macman:

Elution order in ion exchange chromatography is determined by the charge density (charge/radius) of the hydrated ion. In organic acids and bases the elution order is determined by their pKa or pKb (strength of acid or base)

For ion exchange selectivity, we need to consider the hydrated radius of the ions and not just the bare ionic radii.

The hydrated radius of an ion is the effective size of the ion or a molecule plus its tightly bound sheath of water molecules, which are attracted by the positive or negative charge of the ion. Smaller bare ions have larger hydrated radii because they attract water molecules more strongly

- Lager radius of naked ion -> more diffuse electric charge -> fewer water molecules
  surrounding the ion

- Greater ion charge -> increased solvent attraction -> greater the hydrated radius

More information: The Selectivity and retention time in Ion Chromatography depends on:

1- Valence of ions ( Retention time increases with increasing ionic charge )

2- Radius of hydrated ions ( Retention time increases with decreasing hydrated ionic radius,
   and increasing naked ionic radius )

3- Polarization of ions ( Ion Which can be polarized easily has longer retention)
4- Concentration of the mobile phase
5- Column Temperature
6- pH of the mobile phase

(Information from Shimadzu Power Point File)

Regards,

Chiralic

[macman104]

Awesome thanks for the info, so...if it's alright with you, I'm gonna sorta rephrase what you said, and apply to my question here for review:

Group I metals elute faster than group II metals:

They have a high effective nuclear charge, which increases the solvent attraction, which increases the hydrated radius, decreasing retention time.  They also have a smaller charge, compared to group II elements which decreases their retention time.

Group II elements elute in the order they do because:

They all have the same charge, however larger elements are more easily polarized, and increases their retention time.  Also, the smaller ions have a smaller hydrated radius which means it takes them less time to elute off the column.

Sound pretty decent, anything you think I'm really missing?

Again, thanks a ton for the help, none of this information is in the book...