Question

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Min.

1.In the following chromatogram, a mixture of pyridine (1), phenol (2), and toluene (3) was separated. Determine the retention times of the components. Given this order of elution, would you expect the stationary phase to be polar or non-polar?   Explain why the toluene peak is much shorter than the other two peaks even though it is approximately the same concentration.

2. Stationary phases come in different varieties. Which of the two analytes listed for each stationary phase has the greater retention time (or is retained more)? Explain your answer.

Stationary Phase	Retention mechanism	Analytes
Polar Solid	adsorption	H2O and benzene
Nonpolar Liquid	solubility	H2O and benzene
Ionic	electrostatic interaction	Na+ and Mg2+
Porous solid	pore penetration	ethylene and polyethylene

Answer 1

1.

1.4 min 28 min 3 12.3 min 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Min.

The retension time of pyridine(1) is 1.4min, phenol(2) is 2.8 min and that of toluene (3) is 12.3 min as shown in the chromatogram given above.

Pyridine and phenol are polar molecules compared with toluene, which in non polar. In the given chromatogram polar molecules are eluted first and non-polar molecules are eluted later. Thus, polar molecules are not retained and non polar molecules are retained for long time. The non-polar stationary phase retains nonpolar molecules in mobile phase. Therefore, stationary phase used to obtained given chromatogram must be nonpolar.

Pyridine and phenol are not retained by nonpolar stationary phase. Hence, pyridine and phenol gets eluted quickly giving sharp peaks in the chromatogram. Tolune being nonpolar is retained longer by the stationary phase. It takes more time to elute it from stationary phase. Therefore, peak of toluene is short as compared with pyridine and phenol despite of having same concentration.

2.

Polar solid stationary phase will have stronger interaction with polar analytes. H2O is more polar than benzene. Thus, H2O will be retained strongly by polar solid stationary phase. Therefore, retention time of H2O will be greater than that of benzene.

Nonpolar liquid stationary phase will have stronger interaction with nonpolar anlaytes. Benzene being less polar compared with H2O, will be retained more by nonpolar liquid stationary phase.Thus, retention time of benzene will be greater than that of water on nonpolar liquid stationary phase.

Ionic stationary phase will have stronger interaction with analytes carrying more charge compared with those carrying less charge. Thus, Mg2+ carrying 2+ charge will be retained strongly compared with Na+ carrying 1+ charge. Therefore, Mg2+ will have greater retention time than Na+

Porous solid stationary phase will trap small molecules insides the pores and large molecules will pass through stationary phase easily. Thus, ethylene being small compared with its polymeric form polyethylene will remain trapped in pores for longer time. Therefore, ethylene will have greater retention time compared with polyethylene..