What is the titration curve of glycine?

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    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 NHCOOH it is really a zwitterion ##stackrel(+)(N)H_3CH_2COO^##.
    The fully protonated form of glycine is ##stackrel(+)(N)H_3CH_2COOH##.
    The protonated form of glycine ionizes in two steps:
    Step 1 is the loss of ##H^+## from the carboxyl group.
    ##stackrel(+)(N)H_3CH_2COOH + H_2O stackrel(+)(N)H_3CH_2COO^ + H_3O^+##
    Step 2 is the loss of ##H^+## from the less acidic ##NH_3^+## group.
    ##stackrel(+)(N)H_3CH_2COO^+ H_2O NH_2CH_2COO^ + HO^+##
    The first equivalence point at 50 % titration is at ##pH = 5.97##.
    Halfway between 0 % and 50 % titration (i.e. at 25 %) ##pH = pK_a1##.
    The second equivalence point at 100 % titration is at ##pH = 11.30##.
    Halfway between 50 % and 100 % (i.e. at 75 %) ##pH = pK_(
    a2##.
    At 50 % titration the glycine exists as a zwitterion.
    This is the isoelectric point ##pI##.
    At this point ##pH =pI##.
    ##pI = (pK_a1 + pK_a2)##
    For glycine ##pK_a1 = 2.34## ##pK_a2 = 9.60## and ##pI = 5.97##.
    Each amino acid has a characteristic set of ##pK## and ##pI## values.
    Thus you can use a titration curve to identify an unknown amino acid.

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