37 Interpretation of Titration Curves (M16Q5)

In the last section, we saw a couple of different titration curves, titrating a strong base with either a weak or a strong acid. We learned that the pH at the equivalence point is important for knowing what is reacting in solution, but there were also some other similarities or differences in the curves. In this section, we are going to learn how to interpret titration curves and use them to predict what is reacting in solution.

Strong Acid/Strong Base Titration Curves

Strong acid/strong base titrations both use the same chemical equation to describe what is happening during the titration:

H₃O⁺(aq)  +  OH^–(aq)   →   2 H₂O(l)

Figure 1 (below) shows a strong acid being titrated with a strong base. At the beginning of the reaction, only the strong acid is present, resulting in a pH < 7. As the reaction proceeds, some of the acid reacts with the added base to produce water, resulting in a lower [H₃O⁺] and a higher pH, although the pH is still less than 7. At around 20 mL, the pH sharply increases at the equivalence point, where the moles of acid are equal to the moles of added base. The pH at the equivalence point is equal to 7.00, since only water is present at this point. After the equivalence point, all of the acid has been used up and the pH is reflective of the excess base present.

Figure 2 shows the opposite titration, where a strong base is titrated with a strong acid. Once again, at the beginning of the reaction, only the strong base is present, resulting in a pH > 7. As the reaction proceeds, some of the base reacts with the added acid to produce water, resulting in lower [OH^–] and a lower pH, although the pH is still greater than 7. At around 20 mL, the pH sharply decreases at the equivalence point, where the moles of base are equal to the moles of added acid. The pH at the equivalence point is equal to 7.00, since only water is present at this point. After the equivalence point, all of the base has been used up and the pH is reflective of the excess acid present.

Weak Acid/Strong Base Titration Curves

Weak acid/strong base titrations are similar to strong acid/strong base titrations, but typically involve an extra step or two. When we add a weak acid to a strong base, we get the reaction:

HA(aq)  +  OH^–(aq)   ⇌   H₂O(l)  +  A^–(aq)

Figure 3 shows a weak acid being titrated with a strong base. At the beginning of the reaction, only the weak acid is present, resulting in a pH < 7. As the reaction proceeds, some of the weak acid reacts with the added strong base to produce water and the conjugate base. Between the beginning of the titration and the equivalence point, the added strong base is the limiting reactant and after the reaction, the weak acid and its conjugate base make a buffer solution. At around 20 mL, the pH sharply increases at the equivalence point, where the moles of weak acid are equal to the moles of added strong base. The pH at the equivalence point is greater than 7.00, since only water and the conjugate base are present at this point. After the equivalence point, all of the weak acid has been used up and the conjugate base and excess strong base remain. The pH can be calculated by the excess strong base alone, since the effect of the weak base on the [OH^–] is minimal. Compare the titration curves of Figure 1 and Figure 3 after the equivalence point – see how similar they are?

Weak Base/Strong Acid Titration Curves

Weak base/strong acid titrations are similar to weak acid/strong base titrations. When we add a weak base to a strong acid, we get the reaction:

A^–(aq)  +  H₃O⁺(aq)   ⇌   H₂O(l)  +  HA(aq)

Figure 4 shows a weak base being titrated with a strong acid. At the beginning of the reaction, only the weak base is present, resulting in a pH > 7. As the reaction proceeds, some of the weak base reacts with the added strong acid to produce water and the conjugate acid. Between the beginning of the titration and the equivalence point, the added strong acid is the limiting reactant and after the reaction, the weak base and its conjugate acid make a buffer solution. At around 20 mL, the pH sharply decreases at the equivalence point, where the moles of weak base are equal to the moles of added strong acid. The pH at the equivalence point is less than 7.00, since only water and the conjugate acid are present at this point. After the equivalence point, all of the weak base has been used up and the conjugate acid and excess strong acid remain. The pH can be calculated by the excess strong acid alone, since the effect of the weak acid on the [H₃O⁺] is minimal. Compare the titration curves of Figure 2 and Figure 4 after the equivalence point – see how similar they are?

Polyprotic Acid/Base Titration Curves

Polyprotic Acid/Base titrations are essentially multiple weak acid/strong base titrations put together and are only discussed briefly in this course. Figure 5 shows a weak polyprotic acid being titrated with a strong base. Notice how they are two equivalence points – this means that the polyprotic acid we start with must have two acidic hydrogens to start with, like H₂A (i.e. H₂CO₃, etc).

Key Concepts and Summary

Titration curves provide even more information than what is needed to find the concentration of the analyte. Two key markers in a titration curve help us identify whether the analyte and titrant in a titration is a strong or weak, acid or base. The first marker is if the initial pH is above or below 7. If the pH is above 7, the analyte is either a weak or strong base. If the pH is below 7, the analyte is either a weak or strong acid. The second marker is the pH at the equivalence point. If the pH is equal to 7, the titration involves both a strong acid and strong base. If the pH is above 7, the titration is between a weak acid and strong base. If the pH is below 7, the titration is between a weak base and strong acid.

Glossary

half equivalence point (or midpoint)
the pH when half of the volume of titrant has been added. In a weak/strong titration, this is the point where the pH = pKa of the weak acid in the flask.