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

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  • Essay length: 1005 words
  • Submitted: 28/01/2010
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AS and A Level Organic Chemistry

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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

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