Question

The iodeimetric titration of Vitamin C uses starch as the indicator, which will change from colorless to dark blue. The reason for this change is:

A. The excess of IO₃^- leads to a small change in pH, that triggers the change in structure in starch.

B. At the first excess, I₂ combines with I^- forming I₃^-, which can intercalate within the starch structure. The complex is not formed if IO₃^- or I^- is present.

C. The excess of IO₃^- can oxidize the starch and therefore its color change.

D. The natural color of starch is blue, but kept in a reduced stage by the presence of Vitamin C. Once the vitamin is consumed, the starch goes back to its original color.

Answer 1

Correct answer is : (B.) At the first excess, I₂ combines with I^- forming I₃^-, which can intercalate within the starch structure. The complex is not formed if IO₃^- or I^- is present.

Explanation

Starch is used as an indicator for iodometric titration because starch binds to the triiodide ion formed in the redox reaction of excess iodide with the oxidizing agent in the solution and it forms a starch-iodine complex which gives a dark blue color.

Formation of dark blue color is the indication that all the vitamin C has been consumed and only excess iodide ions remain.