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 :
- pink to colourless
- pinkish red to yellow
- colourless to pink
- yellow to pinkish red
Answer
(3) colourless to pink
Solution
In this titration NaOH (from the burette) is added to oxalic acid (in the flask) containing phenolphthalein.
In acidic medium phenolphthalein stays colourless; as per NCERT it changes colour in the pH range 8.2–10, so the first drop of excess alkali just past the equivalence point gives a pink colour.
∴ Colour change: colourless to pink
Answer: (3)