Question

The best indicator for detection of end point in titration of a weak acid and strong base is: (a) Methyl red ( 5 to 6 ) (b) Methyl orange \((3\) to 4\()\) (c) Phenolphthalein \((8\) to \(9.6\) ) (d) Bromothymol blue ( 6 to \(7.5\) )

Short answer

Phenolphthalein is the best indicator for this titration.

Step-by-step solution

Identify the Nature of the Titration

In this exercise, a weak acid is being titrated with a strong base. This typically results in an equivalence point that is at a pH greater than 7 because the conjugate base from the weak acid is present in the solution and contributes to the pH by undergoing hydrolysis.

Determine the pH Range of the Equivalence Point

For a weak acid-strong base titration, the pH at the equivalence point usually ranges from about 8 to 9. This occurs due to the production of a basic solution at the end of the titration when the weak acid is fully neutralized by the strong base.

Match Indicators with Equivalence Point

Identify the pH range over which each indicator changes color: - Methyl red: 5 to 6 - Methyl orange: 3 to 4 - Phenolphthalein: 8 to 9.6 - Bromothymol blue: 6 to 7.5 Given the equivalence point range of 8 to 9 for a weak acid and strong base titration, the indicator should ideally cover this range.

Choose the Most Suitable Indicator

Phenolphthalein, with a pH range of 8 to 9.6, covers the expected equivalence point range for a weak acid and strong base titration. This makes it the most suitable indicator for detecting the endpoint in this scenario.

Key concepts

Weak Acid-Strong Base Titration

In a weak acid-strong base titration, a weak acid is gradually neutralized by a strong base. This type of titration is characterized by an equivalence point where the solution becomes neutralized, but it is important to remember that it doesn't result in a neutral pH of 7. Instead, the pH at the equivalence point is typically greater than 7 because the conjugate base of the weak acid remains in the solution.
This conjugate base undergoes hydrolysis, releasing hydroxide ions which make the solution basic. When the weak acid is fully neutralized, the resulting solution is basic due to the presence of this conjugate base.
The determination of the equivalence point in this titration is key, as it marks the point at which the amount of acid equals the amount of base, leading to a distinctive pH change.

Equivalence Point Determination

The equivalence point in a titration is the stage where equal amounts of acid and base have reacted. In the case of weak acid-strong base titrations, the equivalence point is significant due to the basic nature of the solution at this stage.
Because the weak acid is neutralized by the strong base, the resultant solution consists predominantly of the conjugate base of the acid. This contributes to the solution's basicity. This unique characteristic shifts the equivalence point to a pH typical range between 8 and 9.
It is important for the indicator used to accurately highlight this pH change; hence selecting the right one is crucial.

pH Range of Indicators

To successfully determine the endpoint in a titration, the selection of an appropriate indicator is essential. Each pH indicator has a specific color change range where it shifts from one color to another. This color change occurs over a narrow pH range that typically spans about two units.
The choice of indicator is pivotal; it must change color at a pH that matches the titration's equivalence point.

• Methyl red changes color between a pH of 5 to 6.
• Methyl orange shifts between pH 3 to 4.
• Phenolphthalein changes between pH 8 to 9.6.
• Bromothymol blue transitions between 6 to 7.5.

This reveals that for weak acid-strong base titrations, selecting an indicator that encompasses the equivalence point pH is necessary for precise endpoint recognition.

Phenolphthalein Usage

Phenolphthalein is an ideal indicator in the context of weak acid-strong base titrations. It is known for its distinctive transition from colorless to pink over a pH range of 8 to 9.6.
This range is perfect for this kind of titration since it overlaps the expected pH shift that occurs at the equivalence point. Its usage ensures reliable and clear visualization of the endpoint, which is the moment when the last amount of weak acid is neutralized by the strong base.
Besides its clear color change, phenolphthalein is preferred in many titrations because it is easy to use and provides consistent results. Upon reaching the point where the pink hue appears and persists, it signifies successful titration completion.