Which indicator is used in the titration of sodium hydroxide against oxalic acid and what is the colour change at the end point?
- Phenolphthalein, pink to yellow
- Alkaline KMnO4, colourless to pink
- Phenolphthalein, colourless to pink
- Methyl orange, yellow to pinkish red colour
Answer
(3) Phenolphthalein, colourless to pink
Solution
Oxalic acid is a weak acid and NaOH is a strong base; the end point lies in the basic range (pH about 8–10), so phenolphthalein is the suitable indicator.
With oxalic acid in the flask and NaOH in the burette, the solution is colourless and turns pink with the first excess drop of NaOH.
Answer: (3)