# What is the titration curve of glycine?

Jul 26, 2014

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.

(from elte.prompt.hu)

Although we often write glycine as NH₂COOH, it is really a zwitterion, stackrel(+)("N")"H"_3"CH"_2"COO"^⁻.

The fully protonated form of glycine is stackrel(+)("N")"H"_3"CH"_2"COOH".

The protonated form of glycine ionizes in two steps:

Step 1 is the loss of ${\text{H}}^{+}$ from the carboxyl group.

stackrel(+)("N")"H"_3"CH"_2"COOH" + "H"_2"O" ⇌ stackrel(+)("N")"H"_3"CH"_2"COO"^⁻ + "H"_3"O"^+

Step 2 is the loss of ${\text{H}}^{+}$ from the less acidic ${\text{NH}}_{3}^{+}$ group.

stackrel(+)("N")"H"_3"CH"_2"COO"^⁻+ "H"_2"O" ⇌ "NH"_2"CH"_2"COO"^⁻ + "H"₃"O"^+

The first equivalence point, at 50 % titration, is at $\text{pH} = 5.97$.

Halfway between 0 % and 50 % titration (i.e. at 25 %) $\text{pH" = "p"K_"a1}$.

The second equivalence point, at 100 % titration, is at $\text{pH} = 11.30$.

Halfway between 50 % and 100 % (i.e. at 75 %), "pH" = "p"K_( "a2".

At 50 % titration, the glycine exists as a zwitterion.

This is the isoelectric point $\text{pI}$.

At this point, $\text{pH" ="pI}$.

"pI" = ½("p"K_"a1" + "p"K_"a2")

For glycine, $\text{p"K_"a1} = 2.34$, $\text{p"K_"a2} = 9.60$, and $\text{pI} = 5.97$.

Each amino acid has a characteristic set of $\text{p} K$ and $\text{pI}$ values.

Thus, you can use a titration curve to identify an unknown amino acid.