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

A new simple and rapid reagent injection (FI) method is reported for the si multaneous determination of fluoride and monofluorophsphate (MFP) ions in t oothpastes, using a single calibration for both analytes. The method is bas ed on the inhibitory effect of fluoride ions upon the Fe(III)-catalyzed oxi dation of 2,4-diominophenol (DAP) by H2O2. Free fluoride ions form stable c omplexes with Fe(III), thus reducing its catalytic effect, while MFP does n ot inhibit the reaction under flow conditions. MFP ions are determined indi rectly as fluoride ions after quantitative acid hydrolysis. The decrease in the absorbance of the oxidation product is monitored at 500 nm. The variou s chemical and physical variables of the Fl system were optimized, and a st udy of interfering ions was also carried out. A linear calibration graph was obtained from 0 to 9 mg l(-1) for F-. The re lative standard deviation was very good (0.5% at 4.5 mg l(-1) F-, n = 12) a nd the 3 sigma detection limit was 0.09 mg l(-1)). The sampling rate was 12 0 injections h(-1). The method was successfully applied to the simultaneous determination of F- and MFP in toothpastes. (C) 2001 Elsevier Science B.V. All rights reserved.