Instant Notes: Analytical Chemistry

Indicators In general, indicators have two forms, which possess different colors. Indicators
for acid-base titrations are themselves weak acids or bases where the two forms
differ in color as shown in Figure 3of Topic C4 for methyl orange.

HIn (color 1) =H++In-(color 2)

The choice of indicator depends upon the reaction to be studied. As noted in

Topic C2, the equilibrium constant of the indicator must match the pH range, or

electrode potential range of the species being titrated. Table 1 shows a selection

of indicators for acid-base reactions.

As a general rule it is noted that the color change takes place over the range:

pH =pKIn± 1

Similarly for other reactions, the concentration at which the indicator changes

must match the concentrations at the end point.

For example, in the titration of the weak dibasic maleic acid with sodium

hydroxide, the first end point, corresponding to sodium hydrogen maleate

occurs at pH =3.5, while the second end point for disodium maleate is at

pH =9. Since pKIn=3.7 for methyl orange, this will change at the first end point,

while phenolphthalein would change color around pH =9.

C5 – Titrimetry I: acid–base titrations 83

12

10

8

6

4

2

0

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

0 10 20 30 40 50 60 0 10 20 30 40 50 60

Volume of acid (cm^3 ) Volume of acid (cm^3 )

pH

dpH/dV

(a) (b)

Fig. 2. (a) Titration of 25.00 cm^3 of 0.105 M ammonia with an ethanoic acid solution. (b) Derivative plot of d (pH)/dV
showing the end point as 26.80 cm^3.

Table 1. Typical visual indicators for acid-base titrimetry

Acid–base Low pH High pH pKIn

color color

Thymol blue Red Yellow 1.7

Methyl orange Red Yellow 3.7

Bromothymol blue Yellow Blue 7.0

Phenolphthalein Colorless Red 9.6

Alizarin yellow R Yellow Orange 11