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Chem Lab report. Objective: To determine the content of vitamin C in commercial tablets (redox titration and back titration)

Extracts from this essay...

Introduction

Laboratory Report: Experiment 3 Analysis of commercial vitamin C tablets Name: Cheung Chun Hin, Harry Class: 6L (12) Date: 14-10-2009 Objective: To determine the content of vitamin C in commercial tablets (redox titration and back titration) Principle of method: Vitamin C, which is ascorbic acid, reacts with iodine rapidly in an acidic medium. C6H8O6 (aq) + I2 (aq) C6H6O6 (aq) + 2H+(aq) + 2I- (aq) Since iodine dissolves slightly in water, ascorbic acid cannot be titrated directly by a standard solution of iodine. Therefore, back titration technique is employed in this experiment. In an acidic medium, potassium iodate(V) reacts with potassium iodide to produce iodine in situ. KIO3 (aq) + 5KI (aq) + 3H2SO4 (aq) 3I2 (aq) + 3H2O (l) + 3K2SO4 (aq) The iodine produced reacts with ascorbic acid immediately. Then, the iodine left is titrated by a standard solution of sodium thiosulphate. 2Na2S2O3 (aq) + I2 (aq) Na2S4O6 (aq) + 2NaI (aq) Since the concentration of standard sodium thiosulphate solution is unknown, sodium thiosulphate solution is first titrated against iodine. Since the end-point of the titration is not observable, starch solution is added to act as an indicator since a complex of iodine and starch will form and lead to a dark blue colour.

Middle

2 3 Final burette reading 12.90 16.75 23.00 Initial burette reading 7.20 12.90 16.75 Volume of Na2S2O3 used (cm3) 5.70 3.85 6.25 Calculation: The first trial was not involved in calculation since it is not accurate. In Part A, Average volume of sodium thiosulphate used = 29.625 cm3 In Part B, Average volume of sodium thiosulphate used = 5.05 cm3 Discussion: It was found that the mass of vitamin C in one commercial tablet is 1.06g. The titration results are not reliable because they are not consistent. There are some errors in this experiment. First, potassium iodate(V) is a strong oxidizing agent which will oxidize ascorbic acid once it is added to the acid. Therefore, the vitamin C was not all oxidized by iodine and so the sodium thiosulphate used was greater. Thus, the mass of ascorbic acid calculated was smaller. To reduce this error, potassium iodide should be added before potassium iodate(V) is added so that potassium iodate(V) will immediately react with potassium iodide but not ascorbic acid. Second, Vitamin C decomposes in air or heated especially dissolved in water.

Conclusion

It is because iodine may vaporize and thus leads to an error. This also explains why iodine cannot be placed in the burette but a conical flask when standardizing sodium thiosulphate. When more and more sodium thiosulphate solution was added, the reaction mixture turns from dark brown to yellow. Starch solution should be added when the mixture just turns pale yellow. It is because if it is added in darker colour, which means the concentration of iodine in the mixture is high, the complex formed between starch and iodine will be irreversible. Thus, the dark blue colour will remain forever and so no end-point can be observed. Moreover, although iodine is just slightly soluble in water, the presence of iodide ion will highly increase the solubility of iodine because of the formation of reddish triiodide ion. This can explain why there is red colour appeared in the reaction mixture. Conclusion: The mass of vitamin C in one commercial tablet was deduced to be 1.06g. The titration result was not consistent and there were some errors in the experiment. Therefore the result was not acceptable and thus the objective of this experiment was not fulfilled. - End of Report - ?? ?? ?? ?? 1

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