[ma002]

Why does triiodide react with ascorbic acid before it reacts with a starch indicator in a redox titration?

[Borek]

It doesn't.

The reaction with the starch is reversible, which can't be said about the reaction with ascorbic acid.

[ma002]

To clarify, a redox titration was performed in which starch was used as an indicator. The purpose of the titration was to determine the concentration of ascorbic acid in various substances such as orange juice. You know the titration is complete when the solution turns into a blue-black colour, meaning that all of the ascorbic acid in the solution has been reacted with and what's left for the excess iodine to react with is the starch indicator. My question is, why does the iodine react with vitamin C before it reacts with the starch? Specifically looking for answers relating to the properties of ascorbic acid and starch as I need to explain this concept within a lab report. Any help would be appreciated, thanks!

[Borek]

I told you: the reaction with the starch is reversible. That means adsorbed iodine is in a constant equilibrium with the iodine in the solution. Even if the iodine reacts with the starch it can leave the starch surface any moment and react with the ascorbic acid. Iodine will be able to produce stable colored compound with starch only when the reducing agent is no longer present.

[ma002]

Got it, thanks!