Analysis of Vitamin C Content in Processed Fruit Juices

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This experiment is to determine the vitamin C content in different brands of processed fruit juices. Hence, compare the vitamin C content in the experimental results with the manufacturer's claim and to find out the best buy.

Vitamin C (ascorbic acid) is a reducing agent, the vitamin C content of processed fruit juices can be determined by oxidizing ascorbic acid, C6H8O6 to dehydro-L-ascorbic acid, C6H6O6.

A reagent that is particularly good for the oxidation is an aqueous solution of iodine, I2. The brown iodine solution can be reduced by vitamin C (ascorbic acid) to form colourless iodide ions.

However, I2 solution is not normally prepared directly by dissolving iodine in water because iodine is too volatile so it is almost impossible to avoid loss while the solution is being prepared. Therefore iodine is prepared in situ by mixing pure potassium iodate (KIO3 ) and potassium iodide (KI) in acidified medium. IO3- + 5I- + 6H+ → 3I2 + 3H2O

The excess iodine that has not reacted with ascorbic acid in fruit juices (vitamin C) is then back titrated by standardized sodium thiosulphate solution. Iodine can be decolourized by thiosulphate ion. 2S2O32- + I2 → S4O62- + 2I-

Starch solution, indicator of the titration, is added near the end of the titration when the solution is straw-coloured. Any iodine present will react with starch to form a blue-black solution. When all available iodine has been reacted, the blue-black solution becomes colourless which signals the end-point.

The number of moles of excess iodine can be determined and the mass of ascorbic acid reacted by iodine can be calculated. Hence the best buy could be found.

Brands Chosen
First Choice orange juice < It was not used as there were too many pulp in the juice> Qoo mango juice
you.c1000 (Lemon)
Popper Juice (Apple)
Just Juice (Orange)
Pure J (Apple) < Cape apple juice was used instead because Pure J was out of stock>...
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