Question

Give the pOH range for the isoelectric point of the atmospheric ion of an amino acid.
A 5.5 to 6.3
B 2.5 to 5.0
C 7.7 to 8.5
D 9.0 to 10.7

Answer 1

Hint: In aqueous solution, the amino acids are present in the form of zwitterions. At the isoelectric point, the amino acid molecule is neutral. The pH value of the acids which do not readily liberate H ions in the aqueous solution have the values in between 4 and 6.

Complete Step by step answer: An isoelectric point is the pH value at an electric field cannot induce the migration of the the amino acid. This isoelectric point depends on the functional groups present in the amino acid. For the Zwitter ionic form of the amino acid, the formula for the pH value is \[{\text{pI = }}\dfrac{{{\text{p}}{{\text{K}}_{{\text{a1}}}} + {\text{p}}{{\text{K}}_{{\text{a2}}}}}}{2}\] . Here, \[{\text{pI}}\] is the isoelectric point, \[{\text{p}}{{\text{K}}_{{\text{a1}}}}\] is the negative logarithm of the equilibrium constant for the carboxylic functional group and \[{\text{p}}{{\text{K}}_{{\text{a2}}}}\] is the negative logarithm of the equilibrium constant for the amino functional group. The pH of the surrounding environment influences the net charge on the amino acid molecule. The value of the isoelectric point affects the solubility of the amino acid molecule at a given pH.
For an amino acid, the pH range is 5.5 – 6.3.
For the pH 5.5, calculate the pOH.
\[
  {\text{pOH = 14}} - {\text{pH}} \\
 \Rightarrow {\text{pOH = 14}} - 5.5 \\
  \Rightarrow{\text{pOH = 8}}{\text{.5}} \\
 \]
For the pH 6.3, calculate the pOH.
\[
  {\text{pOH = 14}} - {\text{pH}} \\
  \Rightarrow{\text{pOH = 14}} - 6.3 \\
 \Rightarrow {\text{pOH = 7}}{\text{.7}} \\
 \]
Hence, the pOH range is 7.7 to 8.5.

Note: In the zwitterion form of the amino acid, the carboxylic group is present as the carboxylate ion and the amino group is present as ammonium ion. Carboxylic group loses a proton that is gained by the amino group.