Analytical Chem istry - DePauw University

430 Analytical Chemistry 2.0You may wonder why an indicator’s pHrange, such as that for phenolphthalein, isnot equally distributed around its pK a value.The explanation is simple. Figure 9.12presents an idealized view of an indicatorin which our sensitivity to the indicator’stwo colors is equal. For some indicatorsonly the weak acid or the weak base is colored.For other indicators both the weakacid and the weak base are colored, butone form is easier to see. In either case,the indicator’s pH range is skewed in thedirection of the indicator’s less coloredform. Thus, phenolphthalein’s pH rangeis skewed in the direction of its colorlessform, shifting the pH range to values lowerthan those suggested by Figure 9.12.Table 9.4 Properties of Selected Acid–Base IndicatorsIndicatorAcidColorBaseColor pH Range pK acresol red red yellow 0.2–1.8 –thymol blue red yellow 1.2–2.8 1.7bromophenol blue yellow blue 3.0–4.6 4.1methyl orange red yellow 3.1–4.4 3.7Congo red blue red 3.0–5.0 –bromocresol green yellow blue 3.8–5.4 4.7methyl red red yellow 4.2–6.3 5.0bromocresol purple yellow purple 5.2–6.8 6.1litmus red blue 5.0–8.0 –bromothymol blue yellow blue 6.0–7.6 7.1phenol red yellow blue 6.8–8.4 7.8cresol red yellow red 7.2–8.8 8.2thymol blue yellow red 8.0–9.6 8.9phenolphthalein colorless red 8.3–10.0 9.6alizarin yellow R yellow orange–red 10.1–12.0 –For pHs between pK a – 1 and pK a + 1 the indicator’s color passes throughvarious shades of orange. The properties of several common acid–base indicatorsare shown in Table 9.4.The relatively broad range of pHs over which an indicator changes colorplaces additional limitations on its feasibility for signaling a titration’s endpoint. To minimize a determinate titration error, an indicator’s entire pHrange must fall within the rapid change in pH at the equivalence point.For example, in Figure 9.13 we see that phenolphthalein is an appropriateindicator for the titration of 50.0 mL of 0.050 M acetic acid with 0.10 MNaOH. Bromothymol blue, on the other hand, is an inappropriate indicatorbecause its change in color begins before the initial sharp rise in pH,and, as a result, spans a relatively large range of volumes. The early changein color increases the probability of obtaining inaccurate results, while therange of possible end point volumes increases the probability of obtainingimprecise results.Practice Exercise 9.5Suggest a suitable indicator for the titration of 25.0 mL of 0.125 M NH 3with 0.0625 M NaOH. You constructed a titration curve for this titrationin Practice Exercise 9.2 and Practice Exercise 9.3.Click here to review your answer to this exercise.