Question

Place the following compounds in the order they will elute from the column when separated using reverse-phase liquid chromatography (RPLC) with a C18 stationary phase and the mobile phase buffered at pH = 2. The neutral species of each compound is shown below. Compounds are Salicyclic acid (pka1 of 2.97) , nicotine (pka1 of 3.15 and pka2 of 7.85) and acetophenone. Ranks from First elutes to Last to elute.

Answer 1

The order of elution is Nicotine, Salicyic acid and acetophenone.

Step-by-step explanation:

The order of elution can be described as:

In case of reverse phase liquid chromatography the more polar molecules will elute first being having more affinity for mobile phase.

The given compounds are nicotine, acetophenone and salicylic acid.

The mobile phase pH =2

The
`Pk_(a)1 and Pk_(a)2` of each compound is as follows.

Nicotine= 3.15 and 7.85

salicylic acid= 2.97

Here nicotine is ionic form at the given pH (buffered at pH 2), acetophenone is hydrophobic and salicylic acid is in neutral form.

Therefore, The order of elution is first Nicotine, Salicyic acid and then acetophenone.