Oxidation of Vitamin C

Your I₃^- source in this experiment is provided by the reaction of IO₃^- with I^- in acidic solution:

5 I^- + IO₃^- + 6 H⁺ → 3 I₂ + 3 H₂O

3 I₂ + 3 I^- → 3 I₃^-

The endpoint of the titration is determined using a starch indicator. Starch in the presence of iodine forms a complex that yields a dark-purple solution. Before the endpoint is reached in the titration, there is an excess of Vitamin C and as a consequence, no free I₂ to complex with the starch. However, once past the endpoint, excess I₂ persists, the starch-I₂ complex is formed, and a dark-purple solution appears.