The best indicator for detection of end point in titration of a weak acid and a strong base is

Options

A. Methy1 orange (3 to 4)
B. Methy1 red (5 to 6)
C. Bromotymol blue (6 to 7.5)
D. Phenolphthalein (8 to 9.6)

Answer

To determine the best indicator for the titration of a weak acid and a strong base, we can follow these steps:

### Step 1: Understand the Nature of the Reactants
- In this titration, we have a weak acid (HA) and a strong base (BOH).
- Weak acids partially dissociate in water, while strong bases fully dissociate.

### Step 2: Identify the Products of the Reaction
- When a weak acid reacts with a strong base, the products are water (H2O) and a salt (BA).
- The salt formed from a weak acid and strong base will be basic in nature.

### Step 3: Determine the pH at the Endpoint
- The pH of the solution at the equivalence point of a weak acid and strong base titration will be greater than 7, typically around 8 or higher.
- This is because the resulting salt hydrolyzes in water to produce hydroxide ions (OH⁻), making the solution basic.

### Step 4: Choose an Appropriate Indicator
- Indicators are substances that change color at specific pH ranges.
- For a weak acid-strong base titration, we need an indicator that changes color at a pH greater than 7, ideally around pH 8 or higher.

### Step 5: Evaluate Common Indicators
- Common indicators and their pH ranges:
- **Phenolphthalein**: Changes color at pH 8.2 to 10.0 (suitable for this titration).
- **Methyl orange**: Changes color at pH 3.1 to 4.4 (not suitable).
- **Bromothymol blue**: Changes color at pH 6.0 to 7.6 (not suitable).
- **Litmus**: Changes color around pH 7 (not ideal for detecting a basic endpoint).

### Conclusion
- The best indicator for detecting the endpoint in the titration of a weak acid and a strong base is **Phenolphthalein**, as it changes color in the pH range suitable for this reaction.