Ascorbic Acid: Acidity, Titration, and Ionic Concentrations

1. Calculate the molarity of the ascorbic acid solution:

(a) Mass of ascorbic acid used: 0.1
(b) Moles of ascorbic acid (MW=176.1 g/mol): not sure (0.000568g/moL)
(c) Volume of solution (mL): 100.01mL
(d) Ascorbic acid concentration (mol/L):

2. For each titration, record and calculate the following:

(a) Volume of iodine solution added (mL):
(b) Concentration of the iodine solution:

3. Calculate the iodine concentration, using the formula M1*V1 = M2*V2. Average your results and report one concentration for your standardized iodine solution.

Assignment 1 of Procedure 2

1. For each titration of the fresh orange juice, record and calculate the following:

(a) Volume of iodine solution added (mL): .1mL
(b) Concentration of the ascorbic acid in the juice

2. Calculate the average ascorbic acid concentration for the fresh orange juice, using the formula M1*V1 = M2*V2.

3. For each titration of the week-old orange juice, record and calculate the following:

(a) Volume of iodine solution added (mL):2 drops at a time from burette into a erlenmeyer flask
(b) Concentration of the ascorbic acid in the juice: new orange juice- 8mL of iodine from burette into flask......week old orange juice- 4mL

4. Calculate the average ascorbic acid concentration for the week-old orange juice, using the formula M1*V1 = M2*V2..

5. Report the average amount of ascorbic acid in the 2 samples of commercial orange juice in units of "mg per mL" of juice. The molecular weight of ascorbic acid is 176.12.

6. The minimum daily requirement for vitamin C is 60 mg per day. What percentage of this requirement is in one cup (200 mL) of fresh and week-old orange juice?

7. What happens to the ascorbic acid in orange juice over time? (hint: oxygen makes up 20% of our air.)

Assignment 2 of Procedure 2
Conclusion

Summarize the experiment in one or two paragraphs. Restate the overall purpose of the experiment. What were your results (from each procedure)? What are possible sources of error? What did you enjoy learning about this experiment?

DOES THIS HELP ANY WITH FIGURING OUT THE PROBLEMS?

PLEASE NOTE: Titration requires several steps in order to obtain exact results.

The prepared iodine solution on the Chemicals shelf with a stated concentration of 0.0015M is standardized (confirming the concentration) by performing the following titration:

1. Take a clean volumetric flask from the Glassware shelf and place it on the workbench.

2. Add 0.1g ascorbic acid to the volumetric flask.

3. Fill the volumetric flask with water. (This is done by adding the water normally and checking the "Fill to the Mark" option instead of entering an amount.) This is the most precise way of making a 100 mL solution. Record the amount of ascorbic acid used and the total volume prepared.

4. Take a 150 Erlenmeyer flask from the Glassware shelf and place it on the workbench.

5. Pour 20 mL of the ascorbic acid solution into the flask.

6. Add 1 mL of the starch indicator to the flask.

7. Take a burette from the Glassware shelf and place it on the workbench.

8. Fill the burette with 50 mL iodine solution, with an aproximately known concentration of 0.015M.

9. Drag the Erlenmeyer flask and drop it on the lower half of the burette.

10. Open the Properties window, click back on the burette, and then click the Pushpin icon in the blue bar of the Properties window to lock its functions to the burette.

11. Titrate the ascorbic acid by adding iodine until the solution in the Erlenmeyer flask turns dark blue.

12. You should do a rough titration by adding the iodine solution 1-2 mL at a time in order to quickly find the range in which the endpoint is reached. You can then do a fine titration one or two times in order to obtain exact results.

13. When standardizing a solution, you should repeat the titration so that your results are within a few drops of each other. The results are then averaged to determine the precise molarity.

Procedure 2 ( click to view assignments for this procedure )

1. Determine the ascorbic acid concentration in commercial orange juice, from a freshly opened container and from a container that was opened one week ago.

2. Prepare a sample of orange juice from the new container by adding 40 mL of the juice to a clean Erlenmeyer flask. Add 1 mL of starch indicator.

(Please excuse the very light color of the OJ - the pulp was REALLY filtered out)

3. Titrate the orange juice with the standardized iodine solution. First do a rough titration and then two accurate titrations. Record the volume of iodine delivered in each titration and then refill the burette.

4. Repeat steps 2-3 with the week-old orange juice.