Selection of Indicators for Acid Base Titration

Selection of Indicators for Acid Base Titration

Selection of Indicators for Acid Base Titration

Selection of Indicators for Acid Base Titration

The acid-base indicator (is a dye that changes colour when pH changes) is usually an organic compound that is itself a weak acid or weak base. So, they have their own pH values. They change their colors within a definite pH range. For example Methylorange change its color at the pH range 3 - 4.5 and phenolphthalein change its color at the pH range 8 - 10. On the other hand pH at the equivalent point of acid-base react may not be 7 always. In some case it is greater than 7, less than 7 or in some case it is equal to 7 and it is decided by the nature of acid and base present in the reaction.

Selection of Indicators for Acid Base Titration

In order to determine the accurate end point (when the indicator changes colour during a titration) of acid-base titration, the pH indicator should be selected in such a way that the pH range for the color change of the indicator must coincide with the pH at the equivalent point (the amount of acid and of base is just sufficient to cause complete consumption of both the acid and the base. i.e. neither the acid nor the base is in excess or neither the acid nor the base is the limiting reagent) of reaction. Litmus is not used in titrations because the pH range over which it changes colour is too great (pH range is 5.0 - 8.0)


Indicator NamepH RangeColor Change
Methyl Orange3.1 - 4.4Red → Yellow
Bromothymol Blue6.0 - 7.6Yellow → Blue
Phenolphthalein8.3 -10.0Colourless → Pink
Acid-Base indicators are selected by classifying different acid-base reaction of the titration in different groups as follows-

1. Strong Acid Titration

HCl + NaOH → NaCl + H2O
The pH at this type of reaction is 7 (neutral) because the salt does not undergo hydrolysis with water. The pH change at the end of this type of titration is 3 - 10 approx. So, either methyl orange (3 - 4.5) or phenolphthalein (8 - 10) can be used to find the actual end point of this type of titration.

2. Strong Acid-Weak Base titration

HCl + NH4OH ⇋ NH4Cl + H2O
The pH at the equivalent point of this type of reaction is less than 7 (acidic) because the salt undergoes hydrolysis to give strong acid and weak base. The pH change at the end point of this type of titration is 3 - 7 approx. So, methyl orange (3 - 4.5) is good indicator for this type of titration.

3. Weak Acid-Strong Base titration

CH3COOH + NaOH ⇋ CH3COONa + H2O
The pH at the equivalent point of this type of reaction is more than 7 (basic) because the salt undergoes hydrolysis to give Strong base and weak acid. The pH change at the end point of this type of titration is 7 - 10 approx. The indicator used to find the end point of this type of titration should give the color change in basic range. Phenolphthalein is good indicator for this type of titration.

4. Weak Acid-Weak Base titration

CH3COOH + NH4OH ⇋ CH3COONH4 + H2O The pH at the equivalent point of this type of reaction may be either slightly greater than 7, less than 7 or equal to 7 and it is decided by the relative extent of ionization of Weak Acid and weak base produced after the hydrolysis of the salt. The pH change at the end point of this type of titration is not sharp and wide because the salt acts as a buffer. Because of no sharp and wide pH change at the end point of this indicator method of titration is not so accurate for weak acid and weak base titration.


Theory of Acid Base Indicators

Question Based on Selection of Indicators for Acid Base Titration




1. In the titration of a weak acid with a strong base, which indicator would be the best choice?

2. Definition and Examples of Acid-Base Indicator

3. How to choose an indicator for an acid-base titration?

4. On what factor the choice of an indicator in acid-base titration depends?

5. What is the role of an indicator in an acid-base titration?

6. Explain how an indicator works.

7. What is general criteria of choosing a suitable indicator for a given titration?

8. What should I consider when choosing an indicator for an acid-base titration?

9. Why is universal indicator not used in titration?

10. What is a natural and artificial indicator? Give some examples.

11. What happens if you use the wrong indicator in titration?