2. Test For Vitamin C

 

Quantitative Test for Vitamin C

 

Introduction

This method determines the quantity of Vitamin C in a solution by a redox reaction, using iodine.

The ascorbic acid reacts once in contact with iodine, during titration, and it is oxidised to dehydroascorbic acid. The iodine is reduced to iodide ions.

 

                 ascorbic acid   +   I2         →  dehydroascorbic acid     +   2I-

                  C6H8O6(aq)     +   I2(aq)   →        C6H6O6(aq)             +  2HI(aq)

 

Because of this reaction, all the ascorbic acid reacts and iodide is formed, until excess iodine is added (from the burette). The excess iodine is now free to react with the starch indicator solution, forming a blue-black colour, which is therefore the sign that all the ascorbic acid has reacted. This is the endpoint of the titration.

 

Apparatus and Materials

• Burette and stand

• 100 mL volumetric flask

• 20 mL pipette

• 250 mL conical flasks

• 10 mL and 100 mL measuring cylinders

• Food processor

• Different foods to be tested

• Distilled water

 

A. Preparing the Solutions Needed

 

Preparation of 0.005M Iodine Solution

1. 2g of potassium iodide were weighed and transferred to a 100ml beaker.

2. 1.3g of iodine crystals were weighed and added to the same beaker.

3. About 50mL of distilled water were added and the solution was swirled for a few minutes until all the iodine dissolved.

4. The iodine solution was transferred to a 1L volumetric flask, making sure that all traces of the solution were rinsed into the volumetric flask using distilled water.

5. Distilled water was added to the solution until the 1Lł mark was reached.

 

Starch Indicator Solution

1. 0.25g of soluble starch was weighed and added to 50 ml of nearly boiling water in a 100ml conical flask.

2. The solution was stirred to help dissolution of starch and then cooled prior to use.

 

 

B. Titration (testing the presence and quantity of the food sample)

1. 25ml of the sample liquid extract was placed inside a conical flask together with 150ml distilled water and 1ml starch indicator solution.

2. The sample was titrated together with 0.005M of iodine solution

3. The burette tap was closed as soon as a tinge of blue/black colour appeared in the conical flask

4. The titration was repeated with further portions of the food sample until concordant results were obtained.