Titration Curves

A titration curve is a graphical representation of how the pH of an acidic or basic solution changes with addition of a base or an acid. The reaction followed in a titration curve is a neutralization reaction. There are two important examples of neutral reactions that we use titration curves to describe;

1. Strong acid/strong base reaction

2. Weak acid/strong base reaction

We could also generate a titration curve for a third neutralization reaction;

3. Strong acid/weak base reaction

But this latter example is not as important to us this semester.

The following titration curve shows a plot of pH versus milliliters of base added for a titration of a strong acid and strong base (in purple), and for a titration of a weak acid and a strong base (in black).

Important features of titration curves for strong acids and bases;

1. The initial pH is typically low and can be determined knowing the concentration of the strong acid;

2. The slope of the pH curve increases slowly until very close to the equivalence point;

3. The pH at the equivalence point is always equal to 7;

4. The pH continues to increase approaching the pH of the strong base added to the acid.

 

 

 

 

Important features of titration curves for weak acids and bases;

1. The initial pH is typically higher for a weak acid and can be determined knowing the concentration of the weak acid;

2. The slope of the pH curve increases more rapidly after adding a small amount of base, but falls off as more base is added, until very close to the equivalence point;

3. The pH at the half-equivalence point (at a volume equal to half the volume required to reach the equivalence point) equals the pK of the acid;

4. The pH at the equivalence point is always greater than 7;

5. The pH continues to increase approaching the pH of the strong base added to the acid.

In this discussion of titration curves it is important to be able to calculate the pH at any point on the curve.

Example 1: (see if you can do the calculation before looking at the answer.)

A titration is performed using by adding 0.100 M NaOH to 40.0 mL of 0.1 M HCl.

a) Calculate the pH before addition of any NaOH. Answer

b) Calculate the pH after addition of 10.0 mL of 0.100 M NaOH. Answer

b') Calculate the pH after addition of 20.0 mL of 0.100 M NaOH. Answer

c) Calculate the volume of base needed to reach the equivalence point. Answer

d) Calculate the pH at the equivalence point. Answer

e) Calculate the pH after addition of 10.0 mL of 0.100 M NaOH past the endpoint. Answer