Phenolphthalein indicator is used in the pH range:

(A) 3.2 - 4.4

(B) 4.8 - 6.0

(C) 6.8 - 8.4

(D) 8.2 - 10.0

The phenolphthalein indicator is commonly used in acid-base titrations due to its distinct color change at a specific pH range. To answer your question directly, phenolphthalein is effective in the pH range of (D) 8.2 - 10.0. This means it transitions from colorless in acidic solutions to pink in basic solutions, making it particularly useful for detecting the endpoint of titrations involving strong bases.

Understanding Phenolphthalein's Functionality

Phenolphthalein is a pH indicator that changes color based on the acidity or basicity of a solution. In acidic environments (pH below 8.2), it remains colorless. As the pH rises and crosses the threshold of approximately 8.2, it begins to turn pink, indicating a basic environment. This transition continues until about pH 10, where it remains pink in more alkaline conditions.

Why the pH Range Matters

The choice of pH range for an indicator is crucial in titration experiments. If an indicator changes color outside the desired pH range of the reaction, it may lead to inaccurate results. For example, if you were to use phenolphthalein in a titration involving a strong acid and a weak base, the endpoint might not be clearly indicated, as the pH would not reach the necessary level for the color change.

Examples of Usage

• Strong Acid vs. Strong Base: In a titration of hydrochloric acid (HCl) with sodium hydroxide (NaOH), phenolphthalein is ideal because the pH at the equivalence point is around 7-8, allowing for a clear color change.
• Weak Acid vs. Strong Base: In a titration of acetic acid (a weak acid) with sodium hydroxide, phenolphthalein can still be used, but the endpoint will be more basic, around pH 8.7, which is still within the effective range of the indicator.

Color Change Mechanism

The color change of phenolphthalein is due to the structural changes in the molecule as it gains or loses protons (H⁺). In acidic solutions, the molecule is protonated and remains colorless. As the pH increases and the concentration of hydroxide ions (OH^-) rises, phenolphthalein loses a proton, resulting in a structural change that produces the pink color.

Practical Applications

In laboratories, phenolphthalein is frequently utilized in titrations to determine the concentration of an unknown acid or base. Its clear visual cue allows chemists to easily identify when the reaction has reached its endpoint, ensuring accurate measurements and results.

In summary, phenolphthalein is a valuable tool in chemistry, particularly for titrations involving strong bases, due to its effective pH range of 8.2 to 10.0. Understanding its properties and applications can greatly enhance your ability to conduct experiments and interpret results accurately.