Acid-base indicators are either weak organic acids or weak organic bases. Indicator change colour in dilute solution when the hydonium ion concentration reaches a particular colour For example. Phenolphthalein is a coloureless substance in any aqueous solution with a pH less than 8.3 In between the pH range 8.3 to 10, transition of colour (colourless to pink ) takes place and if pH of solution is greater than 10 solution is dark pink. Considering an acid indicator Hln, the equilibrium involving it and its conjgate base `In^(-)` can be represented as :
`" " underset("acidic from")(HIn)hArrH^(+)underset("basic from")(In^(-))`
pH of solution can be computed as :
`" " pH=pK_(In)+log.([IN^(-)])/([HIn])`
In general, transition of colour takes place in between the pH range `pK_(In+-1`
Which of the following indicator is most suitable for titration of HB with strong base :

Options

A. Phenolphthalein (8.3-10)
B. Bromothhmol blue (6-7.6)
C. Methyl red (4.2-6.3)
D. Malachite green (11.4-13)

Answer

To determine the most suitable indicator for the titration of a weak acid (HB) with a strong base, we need to analyze the pH range of the indicator and how it corresponds to the pH at the equivalence point of the titration.

### Step-by-Step Solution:

1. **Understand the Titration Process**:
- In a titration of a weak acid (HB) with a strong base (like NaOH), the pH at the equivalence point will be greater than 7. This is because the weak acid does not completely dissociate, and the resulting solution will contain the conjugate base of the weak acid.

2. **Identify the pH Range of Indicators**:
- We need to select an indicator that changes color in a pH range that includes the pH at the equivalence point of the titration. The transition range of the indicator should ideally be around the pH of the equivalence point.

3. **Consider Phenolphthalein**:
- Phenolphthalein is a common indicator that changes color from colorless to pink as the pH increases from 8.3 to 10.
- At pH values below 8.3, it is colorless, and at pH values above 10, it becomes dark pink. This makes it suitable for titrations where the pH at the equivalence point is above 8.3.

4. **Conclusion**:
- Since the pH at the equivalence point of the titration of a weak acid with a strong base is typically above 7, and since phenolphthalein changes color in the range of 8.3 to 10, it is the most suitable indicator for this titration.

### Final Answer:
- The most suitable indicator for the titration of HB with a strong base is **Phenolphthalein**.