Weak bases Titration curves

In this section, you examined acid-base titration curves for combinations of strong and weak acids and bases. You may have noticed the absence of a curve for the reaction of a weak acid with a weak base. A weak acid-weak base titration curve is difficult to describe quantitatively, because it has competing equilibria. You may learn about this curve in future chemistry courses. 

Weak Acid/Weak Base Titration Curves... 

Understand how an acid-base indicator works, how the equivalence point and end point in an acid-base titration differ, and how strong acid-strong base, weak acid-strong base, and strong acid-weak base titration curves differ explain the significance of the... 

The calculation of values along a weak acid/weak base titration curve (other than the initial pH and the equivalence point) is beyond the scope of this text. 

Automatic titrators that are electronically equipped to compute the first and second derivative of pH with respect to time, and hence reagent flow, can automatically locate the equivalence point. The first derivative, which is the slope of the titration curve, reaches a maximum at the equivalence points. The second derivative changes sign at the equivalence points. A plot of first derivative helps in the control system design because it is indicative of the pH process sensitivity. Figure 3-2a shows a plot of the first derivatives for the weak acid and weak base titration curv e in Figure 3-le. For more information on automatic titrators, consult Reference 3.3. 

A slrang acid-weak base titration. The curve represents Ihe titration of 50.00 mL of 1.000 M ammonia, a weak base, with 1.000 M HCI. The solution at the equivalence point is acidic because of the NfV ion. Methyl red is a suitable indicator phenolphthalein would change color much too early. 

In overall form this equation resembles that for the glass electrode (Chapter 6) and a pM-EDTA curve resembles an acid-base titration curve. The mercury electrode is most usefully employed when coloured or turbid solutions are being titrated, or when dilute solutions and weak complexes lead to poor colour changes. 

FIGURE 5.1 Acid-base titration curves (a) 0.10 M HCI (strong acid) titrated with 0.10 M NaOH (strong base), (b) 0.010 M HCI titrated with 0.010 M NaOH, and (c) 0.10 M acetic acid (weak acid) titrated with 0.10 M NaOH. 

FIGURE 5.2 A family of acid-base titration curves for a 0.10 M strong acid (HC1) and three weak acids, as indicated (0.10 M each), titrated with 0.10 M NaOH (strong base). HAc is a representation of acetic acid. 

Of considerable difference with the titrations of weak acids and weak bases are the buffering effects of the conjugate salts. The titration curves (see Figure 14.6) all contain a buffering region near the equivalence point where most of the solution consists of the conjugate base (for a weak acid titration a conjugate acid for a weak base titration). 

Figure 20-11. Spreadsheet for weak acid-strong base titration curve.

The curve for a weak acid/weak base titration is the composite of the curves for a weak acid/ strong base and weak base/strong acid titrations, as indicated (Figure 6.5). 

Calculating acid-base titration curves Strong acids, strong bases (Table 8.1), p. 266 Spreadsheet calculations, p. 269 Weak acids, weak bases (Table 8.2), p. 272 Spreadsheet calculations, p. 277 Indicators (key equations 8.4, 8.5), p. 270 Titration of Na2C03, p. 280 Titration of polyprotic acids (Table 8.3), p. 281 Titration of amino acids, p. 286... 

Features of the Curve When we compare this weak acid-strong base titration curve with the strong acid-strong base titration curve (dotted curve portion in Figure 19.8 corresponds to bottom half of curve in Figure 19.7), four key regions appear, and the first three differ from the strong acid case ... 

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