A student determines the % NaOCl in a bleach by reacting 5.00 mL of bleach (density 1.05 g/mL) with excess KI in acidic solution. The liberated I2 is titrated to a starch endpoint (blue->colorless) with 0.1000 M Na2S2O3, requiring 26.45 mL. Reactions: OCl- + 2 I- + 2 H+ -> Cl- + I2 + H2O; I2 + 2 S2O3^2- -> 2 I- + S4O6^2-.
Why is starch added near the endpoint rather than at the beginning?
- A
Starch turns colorless in the presence of OCl-
- B
Starch reacts with thiosulfate
- C
Starch is destroyed by I2 if added too soon
- Dcheck_circle
Starch-iodine complex is reversible only when [I2] is low; adding starch early can cause it to bind strongly and slowly release I2, giving an indistinct endpoint
Explanation
At high [I2], starch forms a tightly bound, slow-to-release complex; adding it when titration is nearly complete gives a sharp blue->colorless endpoint.