Vitamin C estimation by back titration

Vitamin C estimation by back titration

Vitamin C (ascorbic acid) is a reducing agent and reacts with iodine, I2, according to the following equation.

+ I2

+ H+ + I- (This is a line formula of an organic molecule in which each corner where lines meet represents a carbon atom).

The difficulties of handling iodine solution makes it preferable to use a technique in which a known excess of I2 is generated by reacting iodate with iodide to produce a solution of iodine.

When a known amount of Vitamin C is added to this solution the ascorbic acid (Vitamin C) present reduces the iodine. The excess iodine remaining in the solution is then determined by titration with sodium thiosulfate. As the thiosulfate is added the colour of the iodine fades and when the solution is a pale yellow colour a few drops of starch indicator solution is added. This makes the solution go a dark blue colour and means that a clearer endpoint will be obtained with further addition of sodium thiosulfate. Note that the stated colors may vary if the Vitamin C solution is colored and initial titrations will have to be carried out to determine the colour change that will occur at the endpoint of the titration.

Theory behind the analytical method

This analysis is an example of a back titration. The amount of iodine initially produced in the solution is determined by titration with sodium thiosulfate. This is then compared with the amount of iodine found to be present after a sample of juice containing Vitamin C has been added. The difference between these 2 amounts gives the amount of Vitamin C added. Note however that the presence of other resultants interfere with the results

It is often difficult to be sure whether the endpoint of the titration has in fact been reached. To check, it is possible to add another drop of starch and see if the solution goes dark blue at the point where the starch mixes with the solution. If it does then further thiosulfate needs to be added.

Part A: Standardization 1. Pipette out 10cm3 of 0.01M KIO3 into a 100cm3 conical flask

2. Add 5 cm3 1.0 M H2SO4 with measuring cylinder and there after add 15 cm3 of 0.2M KI(excess). The solution will appear a dark yellow-brown colour due to the formation of iodine.

3. Titrate this solution with the standardize 0.1M sodium thiosulfate solution, until the yellow brown colour of the iodine changes to pale yellow.

4. Add about 2 cm3 of 1% starch indicator, then continue adding the sodium thiosulfate drop wise until the blue / dark brown colour of the starch-iodine complex has just disappeared.

5. Repeat procedure at least 3 times till difference in end point is 0.l cm3.

Part B: Determination of Vitamin C in tablet:

1. Weigh accurately given Vitamin C tablet (0.01gm). Dilute the given tablet to 100 cm3 in volumetric flask. 2. Pipette out 10 cm3 diluted Vitamin C into a 100 cm3 conical flask. Add 10cm3 of 0.01M KIO3.

3. Add 5 cm3 1.0 M H2SO4 and there after add 15 cm3 of 0.2M KI. The solution will appear a dark yellow-brown colour due to the formation of iodine.

4. Titrate this solution with the standard 0.1M sodium thiosulfate solution, until the yellow brown colour of the iodine changes to pale yellow.

5. Add about 2 cm3 of 1% starch indicator, then continue adding the sodium thiosulfate drop wise until the blue / dark brown colour of the starch-iodine complex has just disappeared. 6. Repeat procedure at least 3 times till difference in end point is 0.l cm3.

Determine amount and % yield. of Vitamin C in given tabletYou will be assessed for DCP and CE.

ONE POSSIBLE REACTION

IO3- + I- + H+ ------> I3- + H2O

I3- + S2O32- -----------> I- + S4O62-C6H8O6 + I3- + H2O -----> C6H8O6 + I- + H+http://www.studymode.com/essays/Chem-Lab-Report-Objective-To-Determine-1052885.html EMBED ISISServer

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