Can you help me check my calculations for molarity and help me understand how to calculate the percentage of vitamin c in the juice? I included the instructions for comparison:
Goal
To determine the concentration of vitamin c in juices and freshly squeezed lemons juice. A redox titration involving an iodometric method will be used to do the analysis. The samples will be ranked by their vitamin c content.
- Vitamin C can be determined in food by use of an oxidation-reduction reaction.
- The redox reaction is preferable to an acid-base titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by iodine.
- The solubility of iodine is increased by complexation with iodide to form triiodide. The triiodide will then oxide vitamin C to yield dehydroascorbic acid.
- The endpoint is indicated by the reaction of iodine with starch suspension, which produces a blue-black product.
- As long as vitamin C is present, the triiodide is quickly converted to iodide ion, and no-blue-black iodine-starch product is observed.
- However, when all the vitamin C has been oxidized, the excess triiodide (in equilibrium with iodine) reacts with starch to form the expected blue-black color.
Preparation Instructions
Part 1 – Preparation of reagents
- Dissolve 5.00 gram potassium iodide (KI), and 0.268 gram potassium iodate (KIO3) in 200 mL of distilled water in a 400 mL beaker.
- Add 30 mL of 3M sulfuric acid.
- Pour the solution into a 500 mL volumetric flask, and dilute to a final volume of 500 mL with distilled water, and mix thoroughly.
- Dissolve 0.250 gram vitamin C in 100 mL water. Dilute in a 250 mL volumetric flask.
Part 2 – Standardization of the iodine solution with the vitamin C standard solution
- Add 25.00 mL of vitamin C solution into a 125 mL Erlenmeyer flask.
- Add 10 drops of 1% starch solution.
- Rinse the buret twice with 5-10 mL of iodine solution, and then fill it.
- Record your initial buret volume.
- Titrate the solution until the endpoint is reached. The first sign of blue color that remains after 20 seconds of swirling.
- Record the final volume.
- Repeat this titration three (3) times.
Part 3 – Determination of Vitamin C in your samples.
- Add 25.0 mL of your beverage sample into a 125 mL Erlenmeyer flask. Repeat titration from part 2.
- Repeat the iodometric titration until you have enough measurements to feel confident that your data is accurate. What does this mean?
- Add 10.0 mL freshly squeezed lemon juice into a 125 mL Erlenmeyer flask.
- Repeat titration from part 2.
- Repeat the iodometric titration until you have enough measurements to feel confident that your data is accurate. What does this mean?
| Compound | Measured Weight (g) |
| Potassium iodide | 5.0025 grams |
| Potassium iodate | 0.2685 grams |
| Vitamin C / Ascorbic acid | 0.2513 grams |
| Sample | # Drops of Starch Added | Initial Buret Volume mL | Volume of titrated solution used mL | Color Observed |
| Beaker #1 | 13 drops | 50 mL | 19.3 mL | Dark Red |
| Beaker #2 | 13 drops | 50 mL | 19.1 mL | Light Blue |
| Beaker #3 | 13 drops | 50 mL | 18.5 mL | Dark Purple |
| Sample with Orange Juice | # Drops of Starch Added | Initial Buret Volume mL | Volume of titrated solution used mL | Color Observed |
| Beaker #1 | 13 drops | 50 mL | 6.4 mL | Dirty yellow |
| Beaker #2 | 13 drops | 50 mL | 6.4 mL | Dirty yellow |
| Beaker #3 | 13 drops | 50 mL | 6.4 mL | Dirty yellow |
Known Values
Potassium iodide m.w. = 166.0028 grams/ mole
Potassium iodide weight measured = 5.0025 grams
Solution Volume: 500 mL / 0.500 Liters
Moles = (5.0025) g / (166.0028) g/mole = 0.03013 moles
Potassium iodide Molarity = moles / Liter
M = 0.03013 moles / 0.500 Liters = 0.06027 moles / Liter Potassium iodide
Known Values
Potassium iodate m.w. = 214.007 grams/ mole
Potassium iodate weight measured = 0.2685 grams
Solution Volume: 500 mL / 0.500 Liters
Moles = (0.2685) g / (214.007) g/mole = 0.0012546 moles
Potassium iodate Molarity = moles / Liter
M = 0.0012546 moles / 0.500 Liters = 0.002509 moles / Liter Potassium iodate
Known Values
Ascorbic acid m.w. = 176.12 grams/ mole
Ascorbic acid weight measured = 0.2513 grams
Solution Volume: 500 mL / 0.500 Liters
Moles = (0.2513) g / (176.12) g/mole = 0.001426 moles
Ascorbic acid Molarity = moles / Liter
M = 0.0014626 moles / 0.500 Liters = 0.00285 moles / Liter Ascorbic acid
Known Values
Sulfuric acid m.w. = 98.08 grams/ mole
Sulfuric acid weight measured = 30 mL
Solution Volume: 500 mL / 0.500 Liters
Moles = (3) g / (98.08) g/mole = 0.03058 moles
Sulfuric acid Molarity = 3 moles / Liter
M = 0.03058 moles / 0.500 Liters = 0.06117 moles / Liter Ascorbic acid
Calculate
- The percentage % of Vitamin C in your juices and freshly squeezes lemon juice samples with the standard deviation or confident limit.
