What is the titration curve of glycine?

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Answer 1 · 2014-07-26T16:46:36

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.

Glycine 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.