Passage V

Multivitamin (MV) tablets often contain iron in the form of Fe²⁺. In aqueous solution, Fe²⁺ does not absorb visi­ble light. However, when o-phenanthroline (o-phen) is present, it interacts with Fe²⁺ to form an orange-colored complex, which absorbs visible light at a wavelength of 508 nanometers (nm).

Students measured the absorbance at 508 nm—the A₅₀₈—of 6 solutions having known Fe²⁺ concentrations and then determined the Fe²⁺ content of 4 brands of MV tablets (Brands A-D).

Experiment 1

Steps 1-5 were performed 6 times:

1. A certain volume of an aqueous stock solution having a concentration of 0.04 mg Fe²⁺/mL was placed in a 100.0 mL flask.

2. Two mL of hydroquinone solution was added to the flask to stabilize Fe²⁺.

3. Three mL of o-phen solution was added to the flask.

4. The contents of the flask were diluted with H₂O to form a solution of 100.0 mL.

5. The A₅₀₈ of the solution formed was measured with a colorimeter.

Table 1 shows, for each of the 6 solutions formed (Solutions 1-6), the volume of stock solution used, the Fe²⁺ concentration, and the A₅₀₈.

Experiment 2

Each of Brands A-D was analyzed by crushing 1 tablet, dissolving the powder in H₂O to form a 100.0 mL solution, placing 1 mL of the solution into a flask, and then performing Steps 2-5. From the results, each brand's iron (Fe²⁺) content, in mg/tablet, was calculated. Table 2 compares, for each brand, the calculated content with the iron (Fe²⁺) content stated on the label.