Passage IV

Calcium (Ca) supplements often contain magnesium in the form Mg²⁺. In aqueous solution, Mg²⁺ does not absorb visible light. However, when α-naphthylamine (α-naph) is added, Mg²⁺ ions react with α-naph to form a purple-colored complex, which absorbs visible light at a wavelength of 595 nm.

Students measured the absorbance at 595 nm—the A₅₉₅ —of 6 solutions having known Mg²⁺ concentrations and then determined the Mg²⁺ content of 4 brands of calcium supplements (Brands W–Z).

Experiment 1
Steps 1–5 were performed 6 times:

1. A certain volume of an aqueous stock solution having a concentration of 0.08 mg Mg²⁺/mL was placed in a 100.0 mL flask.
2. Three mL of sodium citrate solution was added to the flask to buffer the pH.
3. Two mL of α-naphthylamine 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 A₅₉₅ 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 Mg²⁺ concentration, and the A₅₉₅

Experiment 2
Each of the Brands W–Z was analyzed by crushing 1 tablet, dissolving the powder in H₂O to form a 100.0 mL solution, placing 2 mL of the solution into a flask, and then performing Steps 2–5. From the results, each brand’s magnesium (Mg²⁺) content, in mg/tablet, was calculated. Table 2 compares, for each brand, the calculated content with the magnesium (Mg²⁺) content stated on the label.