Lecture 8 summary

After wrapping up our discussion of buffers, we turn our attention to titrations, a fundamental technique of analytical chemistry. We define some basic terminology, and then proceed with a discussion of three types of titrations: Titration of a strong acid with a strong base, titration of a weak acid with a strong base, and titration of a weak base with a strong acid. We will want to be able to interpret and construct the graphs of pH versus amount of titrant added, which we refer to as titration curves. Provided we have a facility with the types of acid-base chemistry calculations we have spent the past week or so learning, and a knowledge of which regions in the titration curve a particular calculation applies, we will have little trouble constructing the titration curves for the types of titrations we are considering.

Definitions

As our text states at the outset of Chapter 10, "(i)n a titration, we measure the quantity of a known reagent required to react with an unknown sample" (Harris, p.193). We will learn the following terms and apply them in our discussion and analysis of acid-base titrations.

• Analyte: The substance (in this context, acid or base) whose concentration we are trying to determine.
• Titrant: The standardized (i.e. the concentration is known to high accuracy) strong acid or strong base that we add to the analyte in a carefully measured manner.
• Titration curve: The graph obtained when pH is plotted versus the amount of titrant added to the analyte.
• Equivalence point: The point at which the amount of titrant added is exactly equal to the amount of analyte with which it is capable of reacting. The analyte and titrant are at stoichiometric equivalence.
• End point: The observed amount of titrant at which we ascertain the equivalence point has been reached. In acid-base titrations, we look for the end point by means of the color change of an indicator, or the readout of a pH meter.

Titration of a strong acid with a strong base

(in progress)

Titration of a weak acid with a strong base

In the case of a titration of a weak acid HA with a strong base such as NaOH, we see a curve similar to that for the titration of a strong acid with a strong base. However, the curve for the titration of a weak acid can be distinguished from that for a strong acid. The principal difference is that the equivalence point is no longer at pH 7, but instead occurs at a pH, on the alkaline side of 7, determined by the strength of the conjugate base A^–.

As a specific example, let us consider the titration of the weak acid MES [2-(N-morpholino)ethanesulfonic acid] with the strong base NaOH (sodium hydroxide). The reaction for this titration is shown below. Note that it is the reverse of the base hydrolysis reaction for the conjugate base of MES.

Shown at right is a calculated titration curve for the titration of 50.00 mL of MES at a concentration of 0.02000 M, using 0.1000 M NaOH as the titrant. This example corresponds to that given in §10.2 of Harris (see p.199). Following the discussion presented there, we distinguish several regions in the graph. These correspond to four different types of calculations used to determine the pH for a given amount of strong base added. (1) When no base has been added (V_b = 0), we have a solution of weak acid. We perform a weak acid calculation, using the formal concentration F and the K_a of the acid. (2) Once some base has been added, some of the HA has been converted into its conjugate base form A^–. We can apply the Henderson-Hasselbalch equation in this region, which we call the buffer region, using the pK_a for the acid and amounts of HA and A^–. (3) The equivalence point, reached when the amount of strong base added is stoichiometrically equivalent to the amount of HA present to begin with, corresponds to the the complete conversion of HA to A^–. Therefore, the pH is calculated as a weak base problem, using a formal concentration F' and K_b of the conjugate base of the weak acid. (4) Past the equivalence point, we are no longer titrating HA, but we are adding excess hydroxide, making pH calculations in this region those we would use for a strong base.

Titration of a weak base with a strong acid

(in progress)