Kaplan MCAT General Chemistry Review

Titrations are accomplished by reacting a known volume of a solution of unknown concentration (called the titrand) with a known volume of a solution of known concentration (called the titrant). In acid-base titration, the equivalence point is reached when the number of acid equivalents present in the original solution equals the number of base equivalents added, or vice versa. It is important to emphasize that, while a strong acid/strong base titration will have an equivalence point at pH 7, the equivalence point need not always occur at pH 7. Also, when titrating polyprotic acids or bases (discussed later in this chapter), there are several equivalence points, as each different acidic or basic conjugate species is titrated separately. In problems involving titration or neutralization of acids and bases, the equation to remember is

N_aV_a = N_bV_b

where N_a and N_b are the acid and base normalities, respectively, and V_a and V_b are the volumes of acid and base solutions, respectively. (Note that as long as both volumes use the same units, the units used do not have to be liters.)

Bridge

This formula should remind you of Chapter 9 ... If not, turn back and take a look!

The equivalence point in an acid-base titration is determined in two common ways: either evaluated by using a graphical method, plotting the pH of the titrand solution as a function of added titrant by using a pH meter (see Figure 10.1), or estimated by watching for a color change of an added indicator. Indicators are weak organic acids or bases that have different colors in their protonated and deprotonated states. Because they are highly colored, indicators can be used in low concentrations and therefore do not significantly alter the equivalence point. The indicator must always be a weaker acid or base than the acid or base being titrated; otherwise, the indicator would be titrated first! The point at which the indicator actually changes color is not the equivalence point but rather the end point. If the indicator is chosen correctly and the titration is performed well, the volume difference (and therefore the error) between the end point and the equivalence point is usually small and may be corrected for or simply ignored.

Bridge

A useful set of compounds (indicators) will change color as it goes between its conjugate acid and base forms:

This allows us to use it to follow a titration, and we can see that since it is an equilibrium process that we can apply Le Châtelier’s principle. Adding H⁺ shifts equilibrium to the left. Adding OH^- removes H⁺ and therefore shifts equilibrium to the right.

Acid-base titrations can be performed for different combinations of strong and weak acids and bases. The most useful combinations are strong acid/strong base, weak acid/strong base, and weak base/strong acid. Weak acid/weak base titrations can be done but are not usually accurate (and therefore almost never performed), because the pH curve for the titration of a weak acid and weak base lacks the sharp change that normally indicates the equivalence point. Furthermore, indicators are less useful because the pH change is more gradual.

STRONG ACID AND STRONG BASE

Let’s consider the titration of 10 mL of a 0.1 N solution of HCl with a 0.1 N solution of NaOH. Plotting the pH of the reaction solution versus the quantity of NaOH added gives the curve shown in Figure 10.1.

Figure 10.1

Key Concept

Our tug-of-war analogy for a bond between two atoms can be recycled in a different way with titrations. Whichever is stronger (our acid or our base) will determine the equivalence point for the titration. Here they are equal, so the equivalence point is at a neutral pH.

Because HCl is a strong acid and NaOH is a strong base, the equivalence point of the titration will be at pH 7, and the solution will be neutral. Note that the end point shown is close to, but not exactly equal to, the equivalence point; selection of a better indicator, one that changes colors at, say, pH 8, would have given a better approximation. Still, the amount of error introduced by the use of an indicator that changes color around pH 11 rather than, say, pH 8 is not especially significant: a mere fraction of a milliliter of excess NaOH solution.