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 to act as an indicator since a complex of iodine and starch will form and lead to a dark blue colour. This formation of complex is reversible so that when all the iodine is reacted,
The concentration of sodium thiosulphate.
Then we can know the mass of vitamin C
= No. of moles of vitamin C × Molar mass of vitamin C
= No. of moles of I2 reacted with vitamin C ×10 × Molar mass of vitamin C
= ( No. of moles of I2 produced - No. of moles of I2 reacted with Na2S2O3 ) ×10 × Molar mass of vitamin C
= ( No. of moles of KIO3 used ×3 - No. of moles Na2S2O3 ÷2) ×10 × Molar mass of vitamin C
Part A: Standardization of sodium thiosulphate solution
Mass of weighing bottle + solid potassium iodate(V) = 4.48g
Mass of weighing bottle = 3.96g...
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