Chapter 23, Problem 23.1P

(a)

Interpretation Introduction

Interpretation:

Principal species that will be eluted and order of elution at $\text{pH}$12 has to be explained.

Concept Introduction:

If $\text{pH}$ is greater than ${\text{pK}}_{\text{a}}$ then deprotonation occurs and if $\text{pH}$ is less than ${\text{pK}}_{\text{b}}$ then protonation occurs.

Cation exchangers contain negative bound groups that attract solute cation. And anion exchangers contain positive bound groups that attract solute anion.

Explanation of Solution

${\text{pK}}_{\text{a}}$ of hexanoic acid is 5.09 and ${\text{pK}}_{\text{a}}$ of $1\text{-aminohexane}$ is 10.21, hence at $\text{pH}$ 12.0, the reaction occurs as follows:

At this $\text{pH}$, $1\text{-aminohexane}$ remains as it is. Therefore at $\text{pH}$ 12, the mobile phase contains ${\text{C}}_{\text{6}}{\text{H}}_{\text{11}}{\text{O}}_2^{-}$, ${\text{Na}}^{+}$, ${\text{OH}}^{-}$, and ${\text{C}}_{\text{6}}{\text{H}}_{\text{15}}\text{N}$ as principal species.

As cation exchanger attracts the solute cation, thus all directly passes through the cation exchange column.

(b)

Interpretation Introduction

Interpretation:

Principal species that will be eluted and order of elution at $\text{pH}$3 has to be explained.

Concept Introduction:

Refer to part (a).

Explanation of Solution

At $\text{pH}$ 3.0, reaction occurs as follows:

At this $\text{pH}$, hexanoic acid remains as it is.

Therefore at $\text{pH}$ 3.0, the mobile phase contains ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_2$, ${\text{H}}^{+}$, ${\text{Cl}}^{-}$, and ${\text{C}}_{\text{6}}{\text{H}}_{\text{16}}{\text{N}}^{+}$ as principal species. As cation exchanger attracts the solute cation, thus all directly passes through the cation exchange column except that of ${\text{C}}_{\text{6}}{\text{H}}_{\text{16}}{\text{N}}^{+}$.

So, the elute mixture contains ${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_2$, ${\text{H}}^{+}$, and ${\text{Cl}}^{-}$.