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Chem Lab report. Objective: To determine the content of vitamin C in commercial tablets (redox titration and back titration)
The first 200 words of this essay...
Laboratory Report: Experiment 3
Analysis of commercial vitamin C tablets
Name: Cheung Chun Hin, Harry Class: 6L (12) Date: 14-10-2009
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
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