NEET 2026, Chemistry Q40: Principles Of Qualitative Analysis
Phenolphthalein is used as an indicator for the titration of sodium hydroxide solution against a standard solution of oxalic acid. The colour change that is observed at an alkaline pH close to the equivalence point during this titration is :
- A
pink to colourless
- B
pinkish red to yellow
- C
colourless to pink
- D
yellow to pinkish red
Phenolphthalein, HPh, exists in equilibrium with its ionised form Ph: HPh (colourless) H + Ph (pink).
In oxalic acid solution (acidic), the equilibrium lies to the left and the indicator is colourless. As NaOH is added (titrant in the burette), the solution becomes alkaline near the equivalence point, the equilibrium shifts to the right, and the solution turns pink. So the colour change is colourless to pink.
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