Question

What is the titration curve of glycine?

Answer 1

The titration curve for glycine looks like the titration curve for a weak diprotic acid.
Explanation:
Below is a typical curve for the titration of glycine with $NaOH$.
Although we often write glycine as $N{H}_{2}COOH$, itis really a zwitterion ${}^{+}N{H}_{3}C{H}_{2}CO{O}^{-}$
The fully protonated from a glucime is ${}^{+}N{H}_{3}C{H}_{2}COOH$
The protonated form of glycine ionize in two steps:
Step $1$ is the loss of $H^{+}$ form the carboxyl group.
${}^{+}N{H}_{3}C{H}_{2}COOH+H_{2}O{\rightleftharpoons }^{+}N{H}_{3}C{H}_{2}CO{O}^{-}+H_3{O}^{+}$
Step $2$ is teh loss of $H^{+}$ form the less acidic $N{H}_3^{+}$ group.
${}^{+}N{H}_{3}C{H}_{2}CO{O}^{-}+H_{2}O{\rightleftharpoons }^{+}N{H}_{3}C{H}_{2}CO{O}^{-}+H_3{O}^{+}$
The first equivalence point, at $50\%$ titration is at $pH=5.97$
Halfway between $0\%$ and $50\%$ titration (ie. at $25\%$) $pH=P{K}_{a1}$
The second equivalence point, at $100\%$ titration, is at $pH=11.30$
Halfway between $50\%$ and $100\%$ (ie ar $75\%$) $pH=p{K}_{a_2}$
At $50\%$ titration, the glycine exists as a zwitterion.
This the isoelectric point $pI$
At this point, $pH=pI$
$pI=1/2(p{K}_{a_1}+p{K}_{a_2})$
Fro glycine, $p{K}_{a_1}=2.34,p{K}_{a_2}=9.60$a nd $pI=5.97$
Each amino acid has a characteristic set of $pK$ and $pI$ values.
THus, you can use a titration curve t identify an unknown amino acid.