Development of Diffusion Method for the Determination of Fluoride

Objective: Complete hydrolysis is considered to be an important step in determination of fluoride content in biological samples. The purpose of this study was to investigate the influence of acid types and concentrations on the validity of fluoride determination using a diffusion analysis method. Methods: Fluoride samples ranged 0.2-80 ppmF were used in this study. Microdiffusion of fluoride was carried out using a diffusion vessel. The apparatus was made of Teflon. It is composed of two compartments with a screw cap. The inner compartment of the vessel was filled with a trapping solution (0.1 N NaOH). Fluoride sample was placed in the outer compartment and dissociated by incubation with hexamethyldisiloxane (HMDS)-saturated 3N H₂SO₄, 6N H₂SO₄ or 5M HClO₄ at 90^°C for 1h. After cooling at room temperature, TISAB III was added to the trapping solution and fluoride concentrations were directly measured using a combination fluoride electrode. Data were analyzed using repeated measure ANOVA. Results: Diffusion with 5M HClO₄ showed significant better percent recovery (95%CI: 90.55-99.77) compared to that of 3N H₂SO₄ (95%CI: 48.15-79.40) and 6N H₂SO₄ (95%CI: 53.88-101.41), p<0.05. Conclusion: Our results suggest that microdiffusion with HMDS-saturated 5M HClO₄ is a good candidate for fluoride determination.