Geo-Scientist Prelims 2023 Chemistry Q40: During iodometric titration of CuSO₄ solution with Na₂S₂O₃…

During iodometric titration of CuSO₄ solution with Na₂S₂O₃ solution, a student used starch solution as the indicator. However the blue black colour obtained with the indicator did not disappear even after adding the required amount of Na₂S₂O₃ from the burette (end point). What possibly could be the mistake done in this titration ?

1. (a) Starch solution is not a suitable indicator for iodometric titration
2. (b) Freshly prepared starch solution was not used
3. (c) Starch solution was added in the beginning of the titration
4. (d) The reaction medium was acidic