What is the titration curve of glycine?

1 Answer
Jul 26, 2014

Answer:

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.

Glycine
(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.