When fluorine concentration was fixed to 100 mg L−1, optimum aluminum concentration was 3000 mg L−1, whereas for 500 mg L−1 F, the most suitable aluminum concentration was found as 5000 mg L−1.
Since the slurries of toothpaste samples were expected to contain around 20–30 mg L−1 of fluorine, which corresponds to around 1000–1500 g g−1 fluorine in toothpaste, 3000 mg L−1 aluminum could be safely used as the optimum amount in all quantifications.
After finding optimum aluminum concentration,in the presence of 3000 mg L−1 of aluminum,
a linear calibration curve was obtained up to 1000 mg L−1 of fluorine which was much higher than fluorine concentrations in the sample solutions prepared for analysis (Fig. 3).