Determination of Vitamin C Concentration by Titration

Determination of Vitamin C Concentration by Titration

(Redox Titration Using Iodine Solution)

Safety

Lab coats, safety glasses and enclosed footwear must
be worn at all times in the laboratory.

Introduction

This method determines the vitamin C concentration in a solution by a redox titration using iodine. Vitamin C, more properly called ascorbic acid, is an essential antioxidant needed by the human body (see additional notes). As the iodine is added during the titration, the ascorbic acid is oxidised to dehydroascorbic acid, while 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 more straight forward than the alternative method using potassium iodate, but as the potassium iodate solution is more stable than the iodine as a primary standard, the alternative method is more reliable.

Equipment Needed

burette and stand
100 mL or 200 mL volumetric flask
20 mL pipette
10 mL and 100 mL measuring cylinders
250 mL conical flasks

Solutions Needed

Iodine solution: (0.005 mol L−1). Weigh 2 g of potassium iodide into a 100 mL beaker. Weigh 1.3 g of iodine and add it into the same beaker. Add a few mL of distilled water and swirl for a few minutes until iodine is dissolved. Transfer iodine solution to a 1 L volumetric flask, making sure to rinse all traces of solution into the
volumetric flask using distilled water. Make the solution
up to the 1 L mark with distilled water.
Starch indicator solution: (0.5%). Weigh 0.25 g of soluble starch and add it to 50 mL of near boiling water in a 100 mL conical flask. Stir to dissolve and cool before using.

Source: https://www.canterbury.ac.nz/media/documents/science-outreach/vitaminc_iodine.pdf

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