Determination of Vitamin C Content in Fruit Juices Methodology

METHODOLOGY

Redox titration with iodine is the method used to determine the Vitamin C content in fruit juices. The apparatus is set up as shown in the diagram below:

Fresh fruits are squeezed or blended into fresh fruit juices. Fresh juices and commercial juices are poured into conical flasks. The juices are weighed. Iodine solution is filled into a burette for the purpose of titrating the juices in conical flasks. The initial reading of burette is recorded. Iodine solution is added drop by drop into the conical flask with juice and the conical flask is swirled gently throughout the experiment. 1 cm3 of starch solution is added into the conical flask when the brown colour of iodine turns pale yellow(near the end point of titration). As iodine is added into the solution during titration, the iodine is reduced to iodide ions while the ascorbic acid is oxidized to dehydroascorbic acid.

The formula of the chemical reaction is shown as below:

ascorbic acid + I2 ---------( 2 I- + dehydroascorbic acid

As long as ascorbic acid is present in the solution, iodine will react with it and become iodide ion. However, when the all the ascorbic acid is oxidized, iodine will react with the starch to form a blue-black complex. Once the solution turns blue black and the colour remains for at least 30 seconds, the titration is stopped. The final reading of burette is recorded. Total volume of iodine used in the titration is calculated to determine the concentration of ascorbic acid in that fruit juice. The experiment is repeated for another two times to get the average volume of iodine used which is more accurate. The results obtained from the freshly-squeezed and refrigerated fruit juices and those of the commercial fruit juices are compared according to their types respectively.