Determination of Vitamin C by Redox Titration (Iodometric Titration of Ascorbic Acid) Lab

The full procedure of the redox titration of vitamin C is outlined in four parts.
a) Transferring 25.00 mL of ascorbic acid.
b) Filling a buret with iodine.
c) Standardizing an iodine solution.
d) Titrating a sample of juice.

Ascorbic acid (should be prepared fresh each semester)

3 L of ~0.005 M ascorbic acid solution.

To prepare 1 L of this solution, weigh about 0.88 g of ascorbic acid (MW = 176.12 g/mol) in a 100 mL beaker. Measure carefully 1 L of DI water into a 1 L graduated cylinder. Use a powder funnel to transfer the bulk of the ascorbic acid to a 1 L plastic screw top bottle. Rinse in the remaining solid in the beaker using small portions of water from the graduated cylinder, washing the solid off the powder funnel into the bottle during the process. Add about half of the remaining water to the bottle, tightly screw the cap on the bottle and shake it well to dissolve the bulk of the solid. Add the remaining water to the bottle.

Use the following formula to calculate the molarity of the ascorbic acid AA solution:

Exact mass AA used x (1 mol AA/176.12 g AA) x (1 / 1 L)

Write this value (in M) clearly on the bottle, to two significant figures.

Iodine Solution (should be prepared fresh each semester) OR buy it pre-made, ~0.02 M solution (Fisher SI92-1) diluted to ~0.005 M.

3 L of ~0.005 M iodine solution.

Weigh 4.00 g of potassium iodide in a 100 mL beaker. Measure carefully 1 L of DI water into a 1 L graduated cylinder. Use a powder funnel to transfer the bulk of the potassium iodide solid to a 1 L glass amber screw top bottle. Use about 100 mL of the water from the graduated cylinder to help transfer the residue in the beaker into the bottle. Tightly cap the bottle and shake it well to dissolve the solid. Weigh about 1.27 g of iodine solid (MW = 253.81 g/mol) in a 100 mL beaker. Pre-crush the iodine in a mortar and pestle to help it dissolve. Transfer the iodine to the bottle using a powder funnel. Rinse in the remaining solid in the beaker using small portions of water from the graduated cylinder, washing the solid off the powder funnel during the process. Add about half of the remaining water to the bottle, tightly screw the cap on the bottle and shake it well to dissolve the bulk of the solid. Add the remaining water to the bottle. After all of the solid has dissolved, add 2 drops of concentrated hydrochloric acid.

Starch Indicator Solution (should be prepared fresh each semester)

100 mL of starch indicator solution

Add about 1 gram of starch (either corn or potato) into 10 mL of distilled water, shake well, and pour into 100 mL of boiling, distilled water. Stir thoroughly and boil for a 1 minute. Leave to cool. If the precipitate forms, decant the supernatant (carefully pour off the liquid without the solid) and use as the indicator solution.

Acetic Acid Solution

100 mL of 0.2 M acetic acid solution

To prepare 1 L of this solution, measure 11 mL of glacial acetic acid (density = 1.049 g/mL; MW = 60.05 g/mol) in a 25 mL graduated cylinder. Measure 990. mL of DI water into a 1 L graduated cylinder. Add the acetic acid to a 1 L screw top plastic bottle. Rinse the 25 mL graduated cylinder with small portions of water from the 1 L graduated cylinder into the bottle. Add about half of the remaining water to the bottle, tightly screw the cap on the bottle and shake it well. Add the remaining water to the bottle.

Commercial Apple Juice

A commercial sample of apple juice (1 L) will need to be purchased each semester. As long as it is refrigerated, it will keep for several weeks. Tropicana 100% Apple Juice is recommended.