Based on your structures above, provide explanations on why some of the observed retention factors change with pH of the mobile phase.

Principles of Modern Chemistry
8th Edition
ISBN:9781305079113
Author:David W. Oxtoby, H. Pat Gillis, Laurie J. Butler
Publisher:David W. Oxtoby, H. Pat Gillis, Laurie J. Butler
Chapter15: Acid–base Equilibria
Section: Chapter Questions
Problem 99AP
Question

Part B only 

LC analysis are sometime performed using buffers as mobile phase to control the pH. The following table lists the
retention factors (k) for three analytes separated on a C8 non-
polar column. The mobile phase was a citrate buffer: methanol
mixture adjusted to given pHs with NH3.
a. With given structures of acetophenone, nicotine,
and salicylic acid, fill out the following table with
appropriate major species at each pH.
Structure
Acetophenone (no
pka)
Nicotine
(pka1 = 3.15,
pka2 = 7.85)
Salicylic acid
(pka1 = 2.97,
pKa2 = 13.60)
CH3
CH3
OH
OH
pH 2
-CH3
CHS
OH
Ha
Analyte
Acetophenone
Nicotine
Salicylic Acid
pH 5
pH 2
8.5
0.0
6.0
ott
CH3
CHS
jó
pH 5
8.5
0.2
1.1
pH 8
CH3
N
pH 8
8.5
6.1
1.1
CH3
Of
b. Based on your structures above, provide explanations on why some of the observed retention factors
change with pH of the mobile phase.
Transcribed Image Text:LC analysis are sometime performed using buffers as mobile phase to control the pH. The following table lists the retention factors (k) for three analytes separated on a C8 non- polar column. The mobile phase was a citrate buffer: methanol mixture adjusted to given pHs with NH3. a. With given structures of acetophenone, nicotine, and salicylic acid, fill out the following table with appropriate major species at each pH. Structure Acetophenone (no pka) Nicotine (pka1 = 3.15, pka2 = 7.85) Salicylic acid (pka1 = 2.97, pKa2 = 13.60) CH3 CH3 OH OH pH 2 -CH3 CHS OH Ha Analyte Acetophenone Nicotine Salicylic Acid pH 5 pH 2 8.5 0.0 6.0 ott CH3 CHS jó pH 5 8.5 0.2 1.1 pH 8 CH3 N pH 8 8.5 6.1 1.1 CH3 Of b. Based on your structures above, provide explanations on why some of the observed retention factors change with pH of the mobile phase.
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