Titrations and pH Curves Remember titration is commonly used to determine the amount of acid or base in a solution. This process involves a solution of known concentration (the titrant) delivered from a buret into the unknown solution until the substance being analyzed is just consumed. The progress of an acid-base titration can be monitored by plotting the pH of the solution being analyzed as a function of the amount of titrant added. Such a plot is called a pH curve or titration curve.

The figure below shows a plot of the pH of the solution in the receiving flask (HCl) versus the milliliters of the 0.10 M NaOH added – the titration curve for the titration of a strong acid with a strong base. Note the pH changes very gradually until the titration is close to the equivalence point, where a dramatic change occurs. Due to early in the titration there is a large amount of H+ and the addition of OH- thus produces only a small change in pH.

Near the equivalence point [H+] is relatively small, and the addition of a small amount of OH- produces a large change. This titration curve is typical of a titration of a strong acid with a strong base. The titration curve above is for the titration of a strong base with a strong acid (opposite). Eq. pt. still occurs at pH of 7.00.

Titrations of Weak Acids with Strong Bases Calculation of the titration curve for a titration of a weak acid with a strong base is a series of buffer problems. Important: even though the acid is weak, it reacts essentially to completion with the added strong base (OH-). Two step procedure: (1) stoichiometry problem and (2) equilibrium problem. Do these steps separately.

Important: The pH at the equivalence point of a titration of a titration of a weak acid with a strong base is always greater than 7. Calculating pH after the equivalence point is identical to the calculations for strong acid and strong base after the equivalence point. Notice here that the equivalence point occurs at a pH of approximately 9.0.

The titration of a weak base with a strong acid is very similar to the previous weak acid/strong base titration. The pH at the equivalence point for a weak base/strong acid titration occurs at a pH of less than 7.00. The calculations during the titrations can be treated using the procedures learned previously.