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Determination of the Amount of Vitamin C in a Commercial Tablets

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DETERMINATION OF THE AMOUNT OF VITAMIN C IN A COMMERCIAL TABLETS AIM To find out the concentration of a given Na2S2O3 solution by titrating against standard KIO3 solution in excess KI and H2SO4 solution. To find out the amount of vitamin C in a commercial tablets with adding excess but known amount of standard KIO3 and excess KI and H2SO4 solution. Finding the amount of I2 left and amount of vitamin C used by back titration against standard Na2S2O3 solution. RESULT Mass of KIO3 weighed by electric balance: 0.69g Mass of Vitamin C commercial tablet: 4.48g Titration result of standard KIO3 solution in excess KI and H2SO4 solution against given Na2S2O3 Trial 1 2 3 Final burette reading (cm3) 20.92 40.42 28.10 47.50 Initial burette reading (cm3) 1.08 20.92 8.64 28.10 Volume of Na2S2O3 added (cm3) 19.84 19.5 19.46 19.40 Mean volume of Na2S2O3 used = ( 19.5 + 19.46 + 19.4)/3 = 19.45 cm3 Titration result of excess I2 solution (after reacted with vitamin C) ...read more.


It is because starch could form a blue-black complex with iodine. As the colour change of iodine to iodide is insignificant and difficult to observe, so starch is used as the indicator. However, Starch solution is being added in the later stage of titration. It is because starch would combine with iodine irreversibly when iodine is at a very high concentration. At the endpoint, the blue-black colour would completely disappear. REASON OF STANDARDIZING SODIUM THIOSULPHATE SOLUTION We have to standardize the given sodium thiosulphate solution before use. It is because thiosulphate is unstable in acidic medium, it would decompose easily. Also, it would be unstable in the presence of microorganism, Cu2+ and sunlight. So the concentration of the sodium thiosulphate solution might be inaccurate. Therefore, we have to standardize it before use. REASON OF NOT USING IODINE SOLUTION DIRECTLY In this experiment, we dissolve known mass of KIO3 into excess KI and H2SO4 solution to form iodine instead of using iodine solution directly. IO3- + 5I- + 6H+ --> 3I2 + 3H2O This is because iodine is volatile, I2 would escape and cause inaccurate concentration of the solution. ...read more.


However, after adding distilled water, the colour of the solution becomes paler. We can hardly make comparison of solution colour with the previous one. We have to add less distilled water to wash the droplets back into the flask next time. ERROR IN CONCENTRATION OF SOLUTIONS After standardizing the sodium thiosulphate solution, it might decompose by air and sunlight. As a result, the concentration of the solution would be lower than we expected and the calculated amount of vitamin C would be smaller. OTHER CHEMICALS IN THE TABLET MIGHT REACT In this experiment, we have assumed that only vitamin C in the tablet would react with I2. However, we don't know whether other chemicals and ingredients inside the tablet would react with iodine solution. If some of the ingredients would react with iodine solution, the calculated amount of vitamin C in the tablet would be greater. CONCLUSION The mass of vitamin C inside the tablet is 0.9624 g, the tag on the vitamin C tablet claimed that it contain 1.0000 g vitamin C. The percentage error is 3.76%, which is insignificant. So the claim of the tablet maker is justified. ?? ?? ?? ?? Lam Hei Yan (24) 6A ...read more.

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