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Introduction

Title: Determination the amount of ascorbic acid(vitamin c). Objective: To determine the vitamin C content in a given commercial effervescent Vitamin C tablets by the reaction between ascorbic acid and bromine, then follow by the addition of potassium iodate to the solution and the back titration of iodine formed from the previous reaction between iodate and bromine. Hence, compare this data with the manufacturer's specification. Principle and Introduction: In this analysis, certain amounts of known molarity potassium bomate solid and dilute sulphuric acid are required to standardize first in a volumetric flask. By the addition of certain amount of ascorbic acid into the mixture, potassium bromide, which is known to excess is added to the mixed solution. By the reaction of potassium bromate, potassium bromide and sulphuric acid, bromine is formed. KBrO3 +5KBr+3H2SO4 3K2SO4+3Br2+3H2O After the formation of bromine, it then further react with ascorbic acid to form bromide ion. +Br2 +2H + +2Br - For the back titration, potassium iodide is added to the remaining bromine solution formed just now to allow the formation of iodine. Br2+2I - 2Br - + I2 The excess iodine is then back titrated by standard sodium thiosulphate solution, by using a burette and starch solution (used as end point indicator) Similarly, iodine can also be decolourized by thiosulphate ion: 2S2O32- + I2 S4O62- + 2I- Any iodine present will react with starch to form a blue-black complex. However, when all available iodine has been reacted, the blue-black complex becomes colourless which signals the end-point. ...read more.

Middle

To obtain the best results of the experiment, the tablet is dissolved in water only when titration is about to begin. Besides, the tablet (or the vitamin C solution) should avoid storage in direct sunlight, keep in a cool place and in an air-tight bottle. Moreover, only pour out enough amount of vitaminC from the volumetric flask just about to use and then stopper it immediately. -Starch solution should be added only when the solution becomes pale yellow. -The mixture of reacted vitamin C and excess I2 should be titrated with Na2S2O3 immediately to reduce I2 lost due to vapourization. -Handle chemicals with care, H2SO4 is an irritant which may irritate the skin especially if there is a wound. -Avoid exposure to I2 solution since the I2 vestige on skin is quite hard to remove, it will remain for a long time. Result: The table below shows the standardization of Na2S2O3 solution by titration. Trail 1 2 3 Initial reading of sodium thiosulphate/cm3 15.90 40.60 0.00 2.00 Final reading of sodium thiosulphate/cm3 40.60 50.00 13.50 28.70 Volume of sodium thiosulphate used /cm3 24.70 24.10 26.70 Average volume of Na2S2O3 added /cm3 25.17 Calculations: Involved equations: KBrO3 +5KBr+3H2SO4 3K2SO4+3Br2+3H2O +Br2 +2H + +2Br - Br2+2I - 2Br - + I2 2S2O32- + I2 S4O62- + 2I- Given: Molarity of diute sulphuric acid (H2SO4) =2M Molarity of Poatssium iodide (KI) =0.1M Molarity of potassium bromide (KBr) =0.3M Molarity of sodium thiosulpahte (Na2S203)=0.0249M Calculations on the number of mole of iodine reacted. ...read more.

Conclusion

In general, iodine solution is brown in color. While in this experiment, the resulting iodine mixture gives a color of dark brown. As iodine is only slightly dissolved in water , when it mixed with iodide ion, reddish color iodate ions are formed and contribute to a deeper brown color. I2(s) + I-(aq) I3-(aq) Discussion on errors Vitamin C is sensitive to heat and light, and will oxidize readily by reacting with the oxygen in the atmosphere. As a result, the amount of vitamin C is reduced when oxidation results. As iodine is lost to the atmosphere by vaporization when it is formed, so that the calculated value may not be so accurate. As the original vitamin c solution have already a similar color with the iodine solution, so the end point is more difficult to be exactly reach and hence a greater error results. Discussion on improvements. To obtain the results more accurately, the vitamin C tablets which contain no colorings should be used. The mixture containing potassium bromate, potassium iodate and sulphuric acid should be mixed well by even a longer time to make sure all the reactants are fully reacted. Discussion on the results and calculation In this experiment, the calculated mass of vitamin c is 1.04 g and the expected value is 1.00g. The 4 % error may due to the above errors mentioned. Conclusion: The amount of ascorbic acid(vitamin C) in a given tablet of commercial effervescent Vitamin C tablet is 1.04 g. ...read more.

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