CBSE Class 11-science Answered

Vitanin C titration using iodine solution.

The reaction:

shows how the iodine "consumes" the ascorbic acid (vit C). Note the products are all colorless, but when all of the vitamin C gets used up, the next drop of I₂ reacts with a starch indicator (added separately) to produce a dark blue color. This is known as the titration endpoint. Obviously, if more vitamin C is in the sample, more drops of iodine are required to consume it all and reach the dark-blue end-point.

 Procedure:

1.Use the graduated pipet to deliver 1.00 mL (carefully measured) of 0.200 mg/mL vit. C solution to the first well of a 24-well plate. Repeat for the 0.400, 0.600, 0.800 and 1.000 mg/mL solutions in successive wells.

2.Add 1-2 drops of starch solution indicator to each of these wells.

3. Place the plate on a white background. Then, using a thin-stem pipet (held vertically), add the iodine solution drop by drop to the 0.200 mg/L vit. C solution (carefully counting each drop! ) until the solution turns a permanent dark color. COUNT EACH DROP.

Note: It helps to mix the solution with a micro-stirring rod as you add the drops, but do not inadvertently knock the drops off the pipet with the rod; the drops must fall on their own to ensure consistent drop size.

Repeat this technique for the remaining vit. C solutions. Record the number of drops required to titrate each solution in Data table

4. Plot drops iodine vs. vitamin C conc. on the graph, then draw the best fit straight line (use a straight edge) for the plotted points.

5. Now repeat steps 1-3 with each of the six beverages. Be sure to rinse the pipet between samples. Place water in one of the wells. Rinse the pipet with water, and discard the water. Then rinse with a small portion of the test beverage, and discard the rinse. Carefully measure each sample of beverage. Record the number of drops in Data Table #2, then use the graph to determine the specific vitamin C concentration for each of beverage.

Note the colors observed in the samples are slightly different than the colors in the standard because many extra chemical compounds are present. But, there should still be a definite end-point for which a single drop causes a discrete, irreversible color change.

Vitamin C can also be detected by dye-titration and microfluorometric methods, ion exclusion chromatography and HPLC.