Development of a Flatbed Scanner-Based Colorimetric Method for the Indirect Determination of Fluoride Ions Using 96-Well Plates in Oral Hygiene Products

An indirect, novel, fast, and facile assay was developed for the colorimetric determination of fluoride anions using 96-well plates. The proposed method relies on the colorimetric degradation caused by fluoride ions after their reaction with the iron–thiocyanate complex in an acidic medium. The procedure required the addition of minimal amounts of ferric iron and thiocyanate anion solutions to form the corresponding complex with an intense blood-red color, after which, upon addition of fluoride ions, this complex would dissociate, and its color would gradually fade depending on the analyte concentration. The colorimetric differences were measured using a simple imaging device such as a flatbed scanner. Various parameters affecting the analytical performance of the proposed method were optimized, including solution concentrations, pH values, and reaction time for Fe(III)-SCN complex formation and its disintegration process. The proposed assay was successfully applied to the determination of F− in oral hygiene product samples. The method exhibited acceptable detection limits (3.2 mg L−1) with sufficient precision, good intra-day and inter-day reproducibility (ranging from 1.5 to 5.2%), and high selectivity against other anions and components of the samples under study.