Evaluation of free and total fluoride concentration in mouthwashes via measurement with ion-selective electrode

Background: The aim of this study was to compare free fluoride concentration and total fluoride concentration in mouthwashes. Methods: Fluorine-containing mouthwashes from various companies and manufacturers (Colgate Total Plax Classic Mint (R), Colgate-Palmolive, New York, USA; Colgate Total Plax Gentle Mint (R), Colgate-Palmolive, New York, USA; Colgate Total Plax Fresh Mint (R), Colgate-Palmolive, New York, USA; Oral B Advantage (R), Procter&Gamble, Cincinnati, USA; Reach Fresh Mint (R), Johnson&Johnson, New Brunswick, USA; Foramen (R), Laboratorios Foramen, Guarnizo, Spain; Lacalut Sensitive (R), Dr. THEISS, Homburg, Germany; Sensodyne (R), GlaxoSmithKline, London, UK; Vesna F (R), Vita, Saint Petersburg, Russia; Lacalut Fresh (R), Dr. THEISS, Homburg, Germany) were selected as study objects. Fluoride measurements were carried out using the fluoride selective electrode. Results: Free fluoride:total fluoride ratio was more than 80% for six samples (Colgate Total Plax Gentle Mint (R) - 88%, Colgate Total Plax Fresh Mint (R) - 99%, Oral B Advantage (R) - 92%, Reach Fresh Mint (R) - 92 and 89% for the mouthwash of another batch, Lacalut Sensitive (R) - 94%) and less than 63% for three samples (Colgate Total Plax Classic Mint (R) - 56%, Foramen (R) - 62%, Vesna F (R) - 61%). Two samples had more than 70% and less than 80% of unbound fluoride, respectively (Sensodyne (R) - 77%, another batch of Oral B Advantage (R) mouthwash - 74%). Rinse containing sodium monofluorophosphate (Na2PO3F) (Vesna F (R)) had more than 50% of free fluoride, while the rinse containing amine fluoride (AmF) (Lacalut Sensitive (R)) had 94%. The difference in the free fluoride:total fluoride ratio can be explained by binding of fluoride ions by components contained in mouthwashes, such as coloring agents and polymeric compounds. The lowest concentration of free fluoride ions (0.000093 mol/L) was observed for aluminum fluoride (AlF3) rinse (Lacalut Fresh (R)), while the total fluoride amount was not determined due to possible generation of strong fluoride complexes. This implies that fluoride ions will not be uptaken by tooth tissue and may even be washed away from it, compromising the efficacy of mouthwashes. Conclusions: The differences in free fluoride: total fluoride ratio between analyzed mouthwashes reveal a need to develop a method for evaluation of free fluorides in mouthwashes for proper updating of national and international guidelines.