Simultaneous spectrophotometric determination of fluoride and monofluorophosphate ions in toothpastes using a reversed flow injection manifold Original Research Article

A new simple and rapid reagent injection (FI) method is reported for the simultaneous determination of fluoride and monofluorophsphate (MFP) ions in toothpastes, using a single calibration for both analytes. The method is based on the inhibitory effect of fluoride ions upon the Fe(III)-catalyzed oxidation of 2,4-diominophenol (DAP) by H2O2. Free fluoride ions form stable complexes with Fe(III), thus reducing its catalytic effect, while MFP does not inhibit the reaction under flow conditions. MFP ions are determined indirectly as fluoride ions after quantitative acid hydrolysis. The decrease in the absorbance of the oxidation product is monitored at 500 nm. The various chemical and physical variables of the FI system were optimized, and a study of interfering ions was also carried out. A linear calibration graph was obtained from 0 to 9 mg l−1 for F−. The relative standard deviation was very good (0.5% at 4.5 mg l−1 F−, n=12) and the 3σ detection limit was 0.09 mg l−1). The sampling rate was 120 injections h−1. The method was successfully applied to the simultaneous determination of F− and MFP in toothpastes.