Introduction

Vitamin C, also known as ascorbic acid, is an antioxidant that is essential for humans. The main source of vitamin C for most people is raw citrus fruits and their juices. There are several benefits of vitamin C including boosting your immunity, improving iron absorption, and reducing blood pressure. According to the FDA, the recommended daily allowance (RDA) for adults is 90 mg/day1 Dietary Supplement Label Database https://www.dsld.nlm.nih.gov/dsld/dailyvalue.jsp.

One way to determine the amount of vitamin C in juice is to use a redox titration. In a redox titration, one compound donates electrons to another compound. The compound losing electrons is oxidized and the compound gaining electrons is reduced. A redox titration is a better choice than an acid-base titration because the reaction is selective for vitamin C and the titrant doesn’t react with other acids in the juice.

In this redox reaction, vitamin C is oxidized to dehydroascorbic acid while iodine is reduced to iodide:

C6H8O6  + I2   →  C6H6O6 + 2I + 2H+

Iodine is relatively insoluble, but this can be improved by complexing the iodine with iodide to form triiodide. Triiodide also oxidizes vitamin C to form dehydroascorbic acid:

C6H8O6 + I3  → C6H6O6 + 3I  + 2H+

The triiodide ion is formed using iodate and iodide in an acidic solution according to the following equation:

IO3 + 8I + 6H+ → 3I3 + 3H2O

In this experiment, you will use an Oxidation-Reduction Potential (ORP) Sensor to monitor the redox titration of vitamin C from a juice sample and determine its concentration.

Objectives

  • Titrate a vitamin C solution.
  • Determine the concentration of vitamin C in the solution.