366
Acid-Base Equilibria
ENDPOINT INDICATORS
When a pH meter is used to locate the endpoint of a titration, the midpoint of
the nearly vertical region of the curve (Figure 23-1) is taken as the endpoint.
When an indicator is used to locate the endpoint, you must select one that
changes color at the pH corresponding to the pure salt formed in the titration
(points 7, 8, and 9 of the pH summary).
Acid-base indicators are themselves weak acids (or bases) that possess an
intense color in the acid form (HIn) and a different intense color in the salt
form (In~). The equilibrium between the two color forms is
HIn *± H+ + Inand the equilibrium constant expression is
_ [
1 ~ [HIn]
If the equilibrium lies far to either side, the solution will have the color
corresponding to the predominant form; the eye will be unable to perceive the
small amount of the other form. When the equilibrium is such that there are
equal concentrations of the two forms, [HIn] = [In~], the eye will detect a
rapid change in color if the ratio changes slightly. This is the situation where
the indicator will be useful, and the [H
+ ] that corresponds to the point of most
sensitive color change is just equal to the AT, of the indicator.
To select a proper indicator, first calculate the [H
+ ] that will exist at the
endpoint (points 7, 8, and 9 of the pH summary), then select an indicator
whose K, is as nearly the same as possible as the calculated [H
+ ]. At the
endpoint of the addition of NaOH to HC 2 H 3 0 2 (see p 361), the pH is 8.79,
corresponding to [H
+ ] = 1.62 x 1(T
9
. You can see from Table 23-2 that
phenolphthalein (K { = 1.6 x 10~
9
) would be an ideal indicator for this titration.
TABLE 23-2
Characteristics of Some Acid-Base Indicators
Colors
Indicator
Methyl yellow
Bromcresol green
Methyl red
Bromthymol blue
Cresol purple
Phenolphthalein
Thymolphthalem
*i
35 x 10-"
25 x 105
56 x 10-"
16x10-'
63 x 10-"
1 6 x 10-"
1 2 x 10-'°
Acid form
Red
Yellow
Red
Yellow
Yellow
Colorless
Colorless
Base form
Yellow
Blue
Yellow
Blue
Purple
Magenta
Blue
Acid-Base Equilibria
ENDPOINT INDICATORS
When a pH meter is used to locate the endpoint of a titration, the midpoint of
the nearly vertical region of the curve (Figure 23-1) is taken as the endpoint.
When an indicator is used to locate the endpoint, you must select one that
changes color at the pH corresponding to the pure salt formed in the titration
(points 7, 8, and 9 of the pH summary).
Acid-base indicators are themselves weak acids (or bases) that possess an
intense color in the acid form (HIn) and a different intense color in the salt
form (In~). The equilibrium between the two color forms is
HIn *± H+ + Inand the equilibrium constant expression is
_ [
1 ~ [HIn]
If the equilibrium lies far to either side, the solution will have the color
corresponding to the predominant form; the eye will be unable to perceive the
small amount of the other form. When the equilibrium is such that there are
equal concentrations of the two forms, [HIn] = [In~], the eye will detect a
rapid change in color if the ratio changes slightly. This is the situation where
the indicator will be useful, and the [H
+ ] that corresponds to the point of most
sensitive color change is just equal to the AT, of the indicator.
To select a proper indicator, first calculate the [H
+ ] that will exist at the
endpoint (points 7, 8, and 9 of the pH summary), then select an indicator
whose K, is as nearly the same as possible as the calculated [H
+ ]. At the
endpoint of the addition of NaOH to HC 2 H 3 0 2 (see p 361), the pH is 8.79,
corresponding to [H
+ ] = 1.62 x 1(T
9
. You can see from Table 23-2 that
phenolphthalein (K { = 1.6 x 10~
9
) would be an ideal indicator for this titration.
TABLE 23-2
Characteristics of Some Acid-Base Indicators
Colors
Indicator
Methyl yellow
Bromcresol green
Methyl red
Bromthymol blue
Cresol purple
Phenolphthalein
Thymolphthalem
*i
35 x 10-"
25 x 105
56 x 10-"
16x10-'
63 x 10-"
1 6 x 10-"
1 2 x 10-'°
Acid form
Red
Yellow
Red
Yellow
Yellow
Colorless
Colorless
Base form
Yellow
Blue
Yellow
Blue
Purple
Magenta
Blue