ABC - Weak Acid and Strong Base Titrations (Lesson)

Weak Acid and Strong Base Titrations

The titration curve representing the titration of a weak acid with a strong base shares many similarities with the example discussed in the previous lesson of the titration of a strong acid with a strong base. For example, consider the titration of weak acetic acid with the strong base NaOH shown below. The titration curve of a strong acid and strong base is shown to the left for the sake of comparison. Each scenario involves the exact same initial concentrations of acid and the concentration of the sodium hydroxide remains constant as well.

As previously mentioned, a few comparisons and contrasts can be drawn between these two scenarios.

  1. Notice that the initial pH for the weak acid is significantly higher than the strong acid shown to the left. This speaks directly to the strength of the acid and the fact that because acetic acid is a weak acid, it dissociates much less than 100%, and thus the pH is higher. 
  2. Even though the pH at the equivalence point is different, the equivalence point is reached after the exact same volume of NaOH is added. The amount of base necessary to reach the equivalence point is not affected by the strength of the acid that is being titrated. Rather, this is based on the mole to mole ratios from the reaction. 
  3. Although the pH at the equivalence point for a strong acid/strong base titration is equal to 7.00, that is not true in the case of weak acids. For weak acid/strong base titrations, the pH at the equivalence point will not be equal to 7.00, and in fact will always be higher than 7.00 (basic). This point will be explained further in the videos linked below.
  4. Also pointed out in the video is a point on the curve referred to as the half-equivalence point. As the name implies, this is the point half-way to the point where the number of moles of acid and base are equal. This point is important because it can be used to estimate the pKa of a weak acid because at the half-equivalence point, the pH of the solution is equal to the pKa of the weak acid.

Titration of Weak Acid with Strong Base Video, Part One

Titration of Weak Acid with Strong Base Video, Part Two

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