How to select an appropriate pH indicator for titration?

How do we select an appropriate pH indicator for titration?

Let’s consider the same indicator as in the previous article: bromocresol green. If the pH of the analyte containing bromocresol green is between 0 and 3.8, it appears yellow; if the pH is between 5.4 and 14, it appears blue. Between 3.8 and 5.4, the colour changes from yellow to blue as the concentration ratio of [In]/[HIn] changes from 10 to 0.1. In fact, when [In] = [HIn], the colour of the titrand is green, indicating equal amounts of the yellow acid and the blue conjugate base.

Therefore, bromocresol green is a suitable indicator for a titration with a stoichiometric point (SP) that involves a sharp change of pH from less than 3.8 to more than 5.4 when about 0.1 cm3 of solution (about two drops) is added to the analyte. For example, the titration of 10 cm3 of 0.200 M of HCl with 0.100 M of NaOH sees a colour change of yellow to blue at the SP (see table and graph below).

One drop before SP

SP

One drop after SP

Volume, cm3

19.95

20.00

20.05

pH

3.78

7.00

10.22

In other words, a suitable pH indicator for titration is one whose pH range is narrower than and within the range of the change in pH of the analyte at the stoichiometric point (ideally with one or two drops of solution added).

Another example is the titration of 10 cm3 of 0.200 M of aqueous NH3 (Ka = 1.8 x 10-5) with 0.100 M of HCl, which sees a colour change from blue to yellow at the SP (see table and graph below).

One drop before SP

SP

One drop after SP

Volume, cm3

19.95

20.00

20.05

pH

6.65

5.22

3.78

The following table lists some common indicators and their pH ranges:

Indicator

Low pH colour pH range High pH colour

pKa

Thymol blue (first transition)

red 1.2 – 2.8 yellow

1.65

Thymol blue (second transition)

yellow 8.0 – 9.6 blue

8.96

Methyl yellow

red 2.9 – 4.0 yellow

3.30

Bromophenol blue

yellow 3.0 – 4.6 blue 3.85
Congo red blue-violet 3.0 – 5.0 red

4.10

Methyl orange

red 3.1 – 4.4 yellow 3.46

Screened methyl orange (second transition)

purple-grey 3.2 – 4.2 green 3.70

Bromocresol green

yellow 3.8 – 5.4 blue 4.66

Methyl red

red 4.4 – 6.2 yellow

5.00

Bromothymol blue (second transition)

yellow 6.0 – 7.6 blue

7.10

Phenol red yellow 6.4 – 8.0 red

7.81

Phenolphthalein (second transition)

colorless 8.3 – 10.0 purple-pink 9.30
Thymolphthalein (second transition) colorless 9.3 – 10.5 blue

9.90

Alizarine Yellow R

yellow 10.2 – 12.0 red 11.2
Indigo carmine blue 11.4 – 13.0 yellow

12.2

 

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