Analysis of Vitamin C

In this experiment you will analyze a tablet of Vitamin C and calculate the percentage of ascorbic acid present, H2C6H6O6, in the tablets. Vitamin C tablets contain ascorbic acid as the active ingredient, however, it is mixed with such fillers as starch which will tend to obscure the endpoint. Ascorbic acid has two ionizable hydrogens but since the second one comes off only at a pH of 10 or 11 the reaction of ascorbic acid with sodium hydroxide will produce the acid salt, sodium hydrogen ascorbate, and not the normal salt when titrated using an indicator like phenolphthalein or brom thymol blue. Since this is a quantitative lab exercise all measurements to be used in calculations must be recorded to the proper number of significant digits.

Mass out a tablet of Vitamin C and record. Place the tablet in an erlenmeyer flask, add about 50 mL of warm distilled water, and crush the tablet with a glass stirring rod. Add two or three drops of indicator to the flask and titrate with sodium hydroxide solution until the endpoint is reached. (Your instructor will provide you with the concentration of the sodium hydroxide.) If time and materials permit analyze a second tablet using a different indicator.

1. Write a balanced equation for the reaction of sodium hydroxide with ascorbic acid to form sodium hydrogen ascorbate.
2. Calculate the number of mols of sodium hydroxide that reacted with the ascorbic acid.
3. Calculate the number of mols of ascorbic acid that was present in the tablet.
4. Calculate the mass (in milligrams) of ascorbic acid present in the the tablet.
5. Determine the percentage of ascorbic acid (vitamin C) present in the tablet.
6. If you did the titration with a different indicator repeat the calculations above using the same balanced equation and compare the results.
7. How would the percentage of ascorbic acid in the vitamin C tablet be affected if you forgot to fill the tip of the buret with sodium hydroxide solution before you started the titration.
8. Assume that your lab partner "overshot" the endpoint in the second titration. How could you salvage the lab without starting over?

Teacher Comments
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This is a nice quantitative experiment that allows students to apply the chemistry they have learned to analyze a commercial product. In my unit on acids and bases I do two titrations. The first is a standard titration of vinegar, which gives a nice "clean" endpoint. The endpoint in this experiment is not as "clean" because of the fillers that are used in the construction of the Vitamin C tablet. Therefore, this experiment is the last titration I do in my acid-base unit.

The concentration of the sodium hydroxide is about 0.150 Molar. I generally standardize my bases against potassium hydrogen phthalate.

Questions? Comments??