Determination of Vitamin C by Redox Titration with Iodate

Introduction

This method determines the vitamin C concentration in a solution by a redox titration with potassium iodate in the presence of potassium iodide. Vitamin C, more properly called ascorbic acid, is an essential antioxidant needed by the human body (see additional notes).

When iodate ions (IO^3-) are added to an acidic solution containing iodide ions (I^-), an oxidation-reduction reaction occurs;

- the iodate ions are reduced to form iodine

IO₃^- + 6 H⁺ + 5 e^- --> ½ I₂ + 3 H₂O

- while the iodide ions are oxidised to form iodine.

2 I^- --> I₂ + 2 e^-

Combining these half-equations demonstrates the reaction between iodate and iodide

2 IO₃^- + 10 I^- + 12 H⁺ --> 6 I₂ + 6 H₂O

It is the iodine formed by this reaction that oxidises the ascorbic acid to dehydroascorbic acid as the iodine is reduced to iodide ions.

ascorbic acid + I₂ --> 2 I^- + dehydroascorbic acid

Due to this reaction the iodine formed is immediately reduced to iodide as long as there is any ascorbic acid present. Once all the ascorbic acid has been oxidised, the excess iodine is free to react with the starch indicator, forming the blue-black starch-iodine complex. This is the endpoint of the titration.
The method is suitable for use with Vitamin C tablets, fresh or packaged fruit juices and solid fruits and vegetables.

NB: This method is really the same as titrating ascorbic acid directly with iodine solution (see Vitamin C Method using Iodine). However, this method is more reliable as the potassium iodate solution is more stable than iodine as a primary standard.

Method

To download a printable version of this experiment (in pdf format) use the link below.

Vitamin C Titration with Iodate (PDF 100Kb)