The document describes a titration curve showing the pH changes as hydrochloric acid is added to an amino acid. There are two steep drops in pH indicating two equivalence points, suggesting the amino acid has two pKa values of approximately 12.2 and 4.6. The flat portions of the curve near these points indicate the two pKa values.

1. Titration Curve
14
12 The amino acid has been
adjusted to basic pH, so
10 the titration curve starts
high.
pH
8
6
4
2
0 50 100 150 200 250
Volume HCl (mL)

2. Titration Curve
14
12
10
The “flat” portion of
the curve suggests a
pH
8
pKa≈12.2
6
4
2
0 50 100 150 200 250
Volume HCl (mL)

3. Titration Curve
14
12
10 The steep drop in
pH indicates that
pH
8 we are passing an
equivalence point
6
4
2
0 50 100 150 200 250
Volume HCl (mL)

4. Titration Curve
14
12
10
pH
8 The curve
again
6 flattens, sugge
sting a
4 pKa≈4.6
2
0 50 100 150 200 250
Volume HCl (mL)

5. Titration Curve
14
12
10
pH
8
6
If we look carefully, we
4 observe a second
equivalence point
2
0 50 100 150 200 250
Volume HCl (mL)

6. Titration Curve
14
12
10
pH
8 pKa2=12.2
pKa2=4.6
6 Unknown N = Cytosine
4
2
0 50 100 150 200 250
Volume HCl (mL)