Question

The indicator used for titration of weak base and strong acid is: (a) Thymol blue (b) Methyl orange (c) Phenolpthalein (d) Fluorescein

Short answer

The correct indicator is (b) Methyl orange.

Step-by-step solution

Understand the Problem

We need to determine which indicator is appropriate for the titration of a weak base with a strong acid. The key is to understand the type of reaction and the corresponding pH range at the equivalence point.

Recall the Equivalence Point pH

In a titration of a weak base with a strong acid, the equivalence point pH will be less than 7 due to the formation of a salt that produces an acidic solution.

Analyze Indicator Color Change Regions

Recall the pH range where each indicator changes color: - Thymol blue changes color in the pH range of 1.2 to 2.8 and 8.0 to 9.6. - Methyl orange changes color in the pH range of 3.1 to 4.4. - Phenolphthalein changes color in the pH range of 8.2 to 10.0. - Fluorescein changes color around the pH range of 4.0 to 7.0.

Match the Indicator with Equivalence Point

Since the equivalence point pH for a weak base and strong acid titration is acidic (less than 7), we should select an indicator that changes color in an acidic pH range. Methyl orange (pH 3.1 to 4.4) is appropriate because it falls within this range.

Verify the Correct Choice

Check that the selected indicator appropriately matches the pH range for this titration. Methyl orange covers the expected pH range (3.1 to 4.4) after the equivalence point of a weak base and strong acid titration.

Key concepts

Weak Base-Strong Acid Titration

Understanding titrations where a weak base reacts with a strong acid is fundamental in chemistry. In such a titration, a weak base is gradually neutralized by the addition of a strong acid. The reaction proceeds until the acid fully consumes the base, reaching a point known as the equivalence point.
This type of titration is particularly interesting because the product is a salt, which creates an acidic solution when dissolved in water. Therefore, the solution's pH decreases as the base is neutralized, eventually resulting in an acidic pH at the equivalence point.

Equivalence Point pH

The equivalence point in a weak base-strong acid titration is where stoichiometrically equal amounts of the acid and base have reacted.
However, due to the production of a salt from a weak base and a strong acid, the resulting solution at this point is acidic, having a pH less than 7. This occurs because the ion formed from the weak base cannot completely neutralize the strong acid's acidic ions in the solution.
Thus, understanding the equivalence point's pH is crucial. It indicates the acidity of the solution, and helps in selecting an appropriate indicator that changes color around this pH range.

Methyl Orange Indicator

Methyl orange is a popular titration indicator used when dealing with strong acid reactions due to its clear and distinct color change. Unlike other indicators, methyl orange operates effectively in acidic environments, making it highly suitable for the titration of a weak base with a strong acid.
It transitions from red at a pH of approximately 3.1 to yellow at a pH of 4.4. This transition range encompasses the acidic pH typically found at the equivalence point of a weak base-strong acid titration, allowing accurate determination of the endpoint.
Methyl orange's sharp color change provides a precise visual cue for identifying the equivalence point, making it the preferred choice in such titrations.

pH Range of Indicators

Indicators are substances that change color based on the pH level of the solution in which they are placed. Different indicators have varying pH ranges where they change color.

• Thymol blue is effective between pH ranges of 1.2 to 2.8 and 8.0 to 9.6, suited for more extreme pH changes outside normal titration conditions.
• Methyl orange changes color within a range of 3.1 to 4.4, perfect for acidic environments.
• Phenolphthalein changes at a higher pH, from 8.2 to 10.0, suitable for basic conditions.
• Fluorescein is another option, with a color change around 4.0 to 7.0.

Choosing the correct indicator is vital for accurately determining the equivalence point during titrations.