Titration Curve of Amino Acid

Let's take a look at this question where we need to sketch the titration curve when amino acid alanine at pH=1 is titrated with NaOH.

titration curve of amino acid question

Since the pKa values of alanine is given, we can assign these values to the alpha acid (pKa=2.34) and alpha amino (pKa=9.69) groups first.

titration curve of amino acid assign pKa values for alanine

Deduce Structure at pH=1

Next we need to deduce what alanine would exist as at pH=1.

pH 1 will be acidic with respect to the alpha acid group hence it would remain unchanged as an acid.

pH 1 is also acidic with respect to the alpha amino group and it would be converted to conjugate acid.

titration curve of amino acid deduce structure at pH1

Notice now this structure has now 2 acidic groups, hence it is considered as a diprotic acid.

Diprotic acid will react with NaOH in 2 stages.

titration curve of amino acid reactions with NaOH

The stronger acid (alpha acid with smaller pKa) will react first and at the end of this distinct reaction, we have a distinct equivalence point.

The weaker acid (alpha ammonium with bigger pKa) will react second and at the end of reaction 2, there is another distinct equivalence point.

Hence we will see 2 distinct equivalence points in the titration curve of alanine with NaOH.

Deduce Buffer Region in Reaction 1

During reaction 1, we have a mixture of structures A and B which are a conjugate acid-base pair.

Therefore this is a buffer solution and we can determine the maximum buffering capacity at:

pH = pKa1 = 2.34
Vol = 0.5 Veq = 10 cm3

titration curve of amino acid deduce first buffer region

Maximum buffering capacity is a point we can plot on the titration curve and the gradient at that point is zero.

titration curve of amino acid plotting first max buffering capacity

Deduce Buffer Region in Reaction 2

Similarly, there is a buffer solution involving mixture of conjugate acid-base pair B and C in the second reaction.

Maximum buffering capacity for the second buffer is at:

pH = pKa2 = 9.69
Vol = 0.5 Veq = 30 cm3

titration curve of amino acid deduce second buffer region

We can then plot the second mbc and deduce the rest of the titration curve.

titration curve of amino acid plotting second mbc and rest of titration curve

Remember for titration curve involving amino acids and other diprotic acids, the maximum buffering capacities are at pKa1 and pKa2.

These 2 points are very easy to plot and they are well spaced out, so the titration curve will look really nice.

Topic: Nitrogen Compounds, Organic Chemistry, A Level Chemistry, Singapore

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