Question

Phenolphthalein is used as an indicator for titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change observed at an alkaline pH close to equivalence point of this titration is:

• A. Colourless to pink
• B. Pinkish red to yellow
• C. Pink to colourless
• D. Yellow to pinkish red

Hint:

Phenolphthalein is colourless in acidic solution and pink in basic solution.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This problem focuses on an acid-base titration between Sodium Hydroxide ($NaOH$), a strong base, and Oxalic Acid ($H_2C_2O_4$), a weak acid. Specifically, we need to identify the visual signal (color change) provided by the indicator phenolphthalein at the endpoint of the titration. It is important to know the starting medium and the final medium to determine the direction of the color change.
Step 2: Key Formula or Approach:

Phenolphthalein range: It is colorless in acidic and neutral solutions ($pH<8.3$) and turns bright pink in basic solutions ($pH>10$).
Titration Setup: Usually, the acid (analyte) is in the conical flask, and the base (titrant) is added from the burette.
Step 3: Detailed Explanation:

Initial Conditions: The conical flask contains Oxalic acid and a few drops of phenolphthalein. Because the solution is acidic, the phenolphthalein is colorless.
During Titration: As $NaOH$ is added dropwise, it neutralizes the oxalic acid. The $pH$ slowly rises.
Equivalence Point: At the equivalence point for a weak acid vs. strong base titration, the resulting salt (sodium oxalate) undergoes hydrolysis, creating a slightly basic environment ($pH>7$).
End Point: The very first excess drop of $NaOH$ pushes the $pH$ into the range where phenolphthalein becomes active. The indicator then undergoes a structural change that results in a pink color.
Thus, the visible change is from a colorless solution to one that is permanently light pink.
Step 4: Final Answer:
The color change observed is from colourless to pink.