Concept: Vitamin C (ascorbic acid) is an antioxidant. The amount of vitamin C content in food can be found out by titrating with an oxidising agent, e.g. iodine. In the reaction, vitamin C is oxidised, while iodine is reduced to iodide ions. The endpoint is determined by the formation of blue-black starch-iodine complex when all vitamin C is oxidised and excess iodine is free to react with the starch solution added as indicator.
Introduction Vitamin C is an important component of a healthy diet. A well-balanced, healthy diet should ensure that there is enough vitamin C to prevent scurvy and other potential health problems. However, if taken in extreme excess, it may also cause harm to our body. Vitamin C is found in citrus fruits such as oranges, limes and grapefruits, and vegetables including tomatoes and green pepper. Vitamin C is easily damaged during food preparation. Moreover, the amount of vitamin C in most foods is usually high enough that the quantity remaining after processing is more than enough for a daily supply. DCPIP is often used as the reagent to test for vitamin C. The amount of vitamin C in a food sample can be found out by titrating with DCPIP as vitamin C can reduce blue DCPIP to colourless. However, it is sometimes difficult to ascertain the endpoint of the titration due to incomplete decolourisation of DCPIP. In this experiment, iodine and starch solutions are used instead of DCPIP to find out the amount of vitamin C in food sample. When iodine solution is added to the food sample, vitamin C is oxidised, while the iodine is reduced to iodide ions. During the reaction, iodine added is immediately reduced to iodide as long as there is any vitamin C present. Once all the vitamin C has been oxidised, the excess iodine will react with the starch indicator, giving a blue-black
starch-iodine complex. This is the endpoint and it allows you to estimate the amount of vitamin C in the food sample.