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QUESTION

# What is the titration curve of glycine?

The titration curve for glycine looks like the titration curve for a weak diprotic acid.

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 "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 "H"^+ from the less acidic "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 "pH" = 5.97.

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

The second equivalence point, at 100 % titration, is at "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 "pI".

At this point, "pH" ="pI".

"pI" = ½("p"K_"a1" + "p"K_"a2")

For glycine, "p"K_"a1" = 2.34, "p"K_"a2" = 9.60, and "pI" = 5.97.

Each amino acid has a characteristic set of "p"K and "pI" values.

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