Passage II

In aqueous solutions, vitamin C (C₆H₈O₆) reacts with iodine (I₂) according to the balanced chemical equation

C₆H₈O₆ + I₂ → C₆H₆O₆ + 2HI

The amount of vitamin C in a solution can be determined by performing a titration. The titrant (a solution containing a known concentration of I₂) is slowly added to the analyte (a solution containing a particular concentration of vitamin C), and a chemical reaction occurs. This process is monitored by adding an indicator to the analyte. The indi­cator is a colorless starch solution that changes to a dark color by reacting with I₂ when no more vitamin C remains, signaling the end of the titration.

Students performed titrations to determine the amount of vitamin C in 4 different types of pulp-free juice.

Experiment 1

A 0.20 mg/mL vitamin C solution was prepared. A 1.0 mL sample of the solution was placed into a vial, and 1 drop of indicator was added to the vial. Drops of the I₂ solution were then added to the vial until the indicator changed to a dark color. The procedure was repeated for solutions of 0.40, 0.60, 0.80, and 1.0 mg/mL of vitamin C (see Figure 1) .

Experiment 2

A 1.0 mL sample of lime juice was placed into a vial, and 1 drop of indicator was added to the vial. Drops of the I₂ solution were added to the vial until the lime juice solu­tion changed to a dark color. This procedure was repeated for 3 other juices (see Table 1).

Figure and label adapted from R. Ballentine, "Determination of Ascorbic Acid in Citrus Fruit Juices." @ 1941 by the American Chemical Society.