pH and Capacity Factor

[walker8476]

I'm having trouble with this question:

The capacity factor for nicotine separated on a C8 non-polar stationary phase is shown
below.


Capacity Factors for Nicotine at three Different pH values

pH3 pH5 pH7

0.00 0.13 3.11

Why does the capacity factor increase as the pH increases?

The pka of nicotine is 3.15, so at a pH above 3.15, nicotine will become deprotonated and therefore have a negative charge which makes it more attracted to the polar mobile phase which means it moves faster through the column and therefore the retention time will decrease and so capacity factor should decrease.

What is wrong with my thinking?

[tom jupille]

This may help: is nicotine an acid or a base?

[walker8476]

This may help: is nicotine an acid or a base?

Base

[walker8476]

This may help: is nicotine an acid or a base?

Base

OK I see where you're going. So nicotine is a base, so as you increase the pH, the nicotine molecules that are un-protonated will remain un-protonated and the nicotine molecules that are protonated will become un-protonated and therefore become neutral and therefore less polar, which means they will be more attracted to the non-polar stationary phase which increases the retention time and therefore the capacity factor increases also.

Is that better, or is this just as a bad as my first attempt?

[tom jupille]

You got it!

[walker8476]

You got it!

Can't believe that was right. Thanks for your help!