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.