Team:Johns Hopkins/Notebook/VitProtocol

Vitamin C Concentration Assay

(By Anne Marie Helmenstine, Ph.D)

1. Add 25.00 ml of vitamin C standard solution to a 125 ml Erlenmeyer flask.
2. Add 10 drops of 1% starch solution.
3. Rinse your buret with a small volume of the iodine solution and then fill it. Record the initial volume.
4. Titrate the solution until the endpoint is reached. This will be when you see the first sign of blue color that persists after 20 seconds of swirling the solution.
5. Record the final volume of iodine solution. The volume that was required is the starting volume minus the final volume.
6. Repeat the titration at least twice more. The results should agree within 0.1 ml.
7. You titrate samples exactly the same as you did your standard. Record the initial and final volume of iodine solution required to produce the color change at the endpoint.

Vitamin C Assay Solutions

Starch Indicator Solution

1. Add 0.50 g soluble starch to 50 ml near-boiling distilled water.
2. Mix well and allow to cool before use. (doesn't have to be 1%; 0.5% is fine)

Iodine Solution

1. Dissolve 5.00 g potassium iodide (KI) and 0.268 g potassium iodate (KIO3) in 200 ml of distilled water.
2. Add 30 ml of 3 M sulfuric acid.
3. Pour this solution into a 500 ml graduted cylinder and dilute it to a final volume of 500 ml with distilled water.
4. Mix the solution.
5. Transfer the solution to a 600 ml beaker. Label the beaker as your iodine solution.

Vitamin C Standard Solution

1. Dissolve 0.250 g vitamin C (ascorbic acid) in 100 ml distilled water.
2. Dilute to 250 ml with distilled water in a volumetric flask. Label the flask as your vitamin C standard solution.

Dough Media Recipes

Optimized recipe per plate:

• 20ml H₂O
• 0.24g NaCl
• 0.15g bactoagar
• 0.75g flour