|Chemistry 8th Edition / Chang|
|Student Study Guide
TITRATION CURVES AND INDICATORS (16.4 16.5)
Titration Curves. Acid-base titrations were discussed in Chapter 4. With the introduction to pH in the previous chapter, it is informative to follow the pH as a function of the progress of the titration. A graph of pH versus volume of titrant added is called a titration curve. Initially, the pH is that of the unknown solution. As titrant is added, the pH becomes that of a partially neutralized solution of unknown plus titrant. The pH at the equivalence point refers to the H+ ion concentration when just enough titrant has been added to completely neutralize the unknown. If more titrant is added after the equivalence point has been reached, the pH assumes a value consistent with the pH of excess titrant. We will review three types of titration curves.
Strong acid-strong base titrations. Figure 16.3 of the text shows a titration curve for the addition of a strong base to a strong acid. The main features of this curve can be stated briefly. 1. The pH starts out quite low because this is the pH of the pure acid solution. 2. As base is slowly added, it is neutralized, and the pH is determined by the unreacted excess acid. Near the equivalence point, the pH begins to rise more rapidly. 3. At the equivalence point the pH changes sharply, increasing about 5.0 units upon the addition of only two drops of base. 4. Beyond the equivalence point the pH is determined by the amount of excess base that is added and is higher than 7.
The pH at the equivalence point of an acid-base titration is the pH of the salt solution that is formed by neutralization. NaCl, NaNO3, NaBr, KCl, and KI are examples of salts that can be formed in titrations of strong acids and strong bases. These salts yield ions that do not cause hydrolysis (Chapter 15). Therefore, the pH at the equivalence point in a titration of a strong acid with a strong base is 7.
Weak acid-strong base titrations. Figure 16.4 of the text shows a titration curve for the addition of a strong base to a weak acid. Because the acid is a weak acid, the initial pH is greater than in the titration of a strong acid. At the equivalence point, the pH is above 7.0 because of salt hydrolysis. The anion of the salt is the conjugate base of the weak acid used in the titration. In the titration of acetic acid with sodium hydroxide the salt produced is CH3COONa. The Na+ ion does not hydrolyze, but CH3COO does:
CH3COO + H2O CH3COOH + OH
Hydrolysis of acetate ions makes the solution basic at the equivalence point.
Strong acid-weak base titrations. Figure 16.5 of the text shows a titration curve for the addition of a weak base to a strong acid. The first part of the curve is the same as in the strong acid versus strong base titration. However, the pH at the equivalence point is below 7.0 because the cation of the salt is the conjugate acid of the weak base used in the titration. Hydrolysis of this salt yields an acidic solution. In the titration of hydrochloric acid with ammonia, the salt produced is NH4Cl. The Cl ion does not hydrolyze, but does:
NH4+ + H2O NH3 + H3O+
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