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Chapter 9 Titrimetric Methods of Analysis 289

–
whenever the concentration of HIn is ten times less than that of In . Substi- pH
tuting these inequalities into equation 9.6 In –
1 pH = pK + 1
K
1
pH = p a +log = p a K - a,HIn
10 Indicator’s
pH = pK a,HIn color transition
10 region
pH = p a +log = p a K + pH = pK – 1
1
K
1 a,HIn
shows that an indicator changes color over a pH range of ±1 units on either
HIn
side of its pK a (Figure 9.11). Thus, the indicator will be the color of HIn when
–
the pH is less than pK a – 1, and the color of In for pHs greater than pK a +1. Figure 9.11
The pH range of an indicator does not have to be equally distributed on either Ladder diagram showing the range of pH
side of the indicator’s pK a . For some indicators only the weak acid or weak base is col- levels over which a typical acid–base
indicator changes color.
ored. For other indicators both the weak acid and weak base are colored, but one
form may be easier to see. In either case, the pH range is skewed toward those pH lev-
els for which the less colored form of the indicator is present in higher concentration.
A list of several common acid–base indicators, along with their pK a s, color
changes, and pH ranges, is provided in the top portion of Table 9.4. In some cases,

9 4
Table . Properties of Selected Indicators, Mixed Indicators,
and Screened Indicators for Acid–Base Titrations

Indicator Acid Color Base Color pH Range pK a
cresol red red yellow 0.2–1.8 —
thymol blue red yellow 1.2–2.8 1.7
bromophenol blue yellow blue 3.0–4.6 4.1
methyl orange red orange 3.1–4.4 3.7
Congo red blue red 3.0–5.0 —
bromocresol green yellow blue 3.8–5.4 4.7
methyl red red yellow 4.2–6.3 5.0
bromocresol purple yellow purple 5.2–6.8 6.1
litmus red blue 5.0–8.0 —
bromothymol blue yellow blue 6.0–7.6 7.1
phenol red yellow red 6.8–8.4 7.8
cresol red yellow red 7.2–8.8 8.2
thymol blue yellow blue 8.0–9.6 8.9
phenolphthalein colorless red 8.3–10.0 9.6
alizarin yellow R yellow orange/red 10.1–12.0 —
Mixed Indicator Acid Color Base Color pH Range

bromocresol green and methyl orange orange blue-green 3.5–4.3
bromocresol green and chlorophenol red yellow-green blue-violet 5.4–6.2
bromothymol blue and phenol red yellow violet 7.2–7.6
Screened Indicator Acid Color Base Color pH Range
dimethyl yellow and methylene blue blue-violet green 3.2–3.4
methyl red and methylene blue red-violet green 5.2–5.6
neutral red and methylene blue violet-blue green 6.8–7.3