Vitamin C Molecule Investigation
Vitamin C (ascorbic acid) is an essential component of human nutrition, as it is an antioxidant that the body requires. Having a deficiency in Vitamin C can lead to scurvy, a disease characterized by abnormalities in human bones and teeth. Fruits and vegetables are common sources of Vitamin C, notably oranges and citrus fruits. Cooking and the application of heat destroy the vitamin so such fruits must be raw in order to obtain the benefits from these sources. The investigation that is to be undertaken is to find the Vitamin C content from four different common sources; a Vitamin C tablet, two different common household orange juices, and a 99.9% pure lemon juice. In the conduction of this experiment, the use of redox titrations was essential. Acid-base titrations can also be used however they are not able to complete all of the reactions in the solution and do not interfere with the oxidation of ascorbic acid by iodine solution. Iodine is relatively insoluble, but this can be improved by complexing the iodine with iodide to form triiodide as the following formula suggests: I_(2(aq))+〖I^-〗_((aq))↔〖I^-〗_(3(aq))
This triiodide ion can be used to oxidize vitamin C to form dehydroascorbic acid. In this reaction the triiodide ion is reduced to iodide ion, and ascorbic acid, C6H8O6 is oxidized to dehydroascorbic acid, C6H6O6. Aim
The aim of this investigation is to discover the exact content of Vitamin C in a number of different common household products. Materials
1 x 0.05L Burette
1 x Retort Stand
1 x Burette Clamp
1 x 0.02L Pipette
1 x 0.5L Volumetric Flask
1 x Vitamin C tablet (0.74g)
0.06L ‘Just Juice’ Orange Juice
0.06L Brownes ‘Orange C’ Fruit Drink
0.06L Coles 99.9% Lemon Juice
Extensive Supply of Distilled Water
Extensive Supply of Tap Water
1-3 x 0.25L Conical Flask
1 x 0.1L Beaker
1 x Funnel
1 x Mortar and Pestle
1 x Pipette Filler
Extensive Amount of Detergent
Pen and Paper/Laptop
Preparing the Iodine Solution
Dissolve 5.00g potassium iodide (KI) and o.27g potassium iodate (KIO3) in 200ml of distilled water.
Add 30ml of 3 molL-1 sulfuric acid.
Pour this solution into a 500ml graduated cylinder and dilute it to a final volume of 500ml with distilled water.
Mix solution thoroughly and carefully.
Transfer the solution to a 500ml storage bottle. Label the bottle as your iodine solution. This will be needed throughout the entire experiment. Standardizing the Iodine solution
Add 25 ml of Vitamin C standard solution to a 250 ml conical flask.
Add 10 drops of 1% starch solution.
Rinse through the burette with a small volume of the iodine solution and then proceed to fill it with this solution. Record the initial volume.
Titrate the solution until the endpoint is reached. This will be when you see the first sign of blue colour that remains after 20 seconds of mixing the solution.
Record the final volume of iodine solution in the burette.
Repeat the titration at least twice more. The results should agree within 0.1ml
Calculate the concentration of the iodine using the data collected and the Vitamin C solution concentration. Titrating the Juice Sample(s)
Add 25 ml of juice sample to a 250 ml conical flask.
Titrate until the endpoint is reached. (Add iodine solution until you get a colour that persists longer than 20 seconds)
Repeat the titration until you have at least three measurements that agree to with 0.2ml.
Use your titration volumes to find the number of moles of iodine/tri-iodide and then determine the number of moles of ascorbic acid in the juice. Express your answer in mg L-1
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