Simple, rapid reagent-injection spectrophotometric determination of fluorides in pharmaceutical formulations.

A new simple and rapid reagent-injection method is reported for the determination of fluorides in pharmaceutical formulations. The method is based on the inhibitory effect of fluoride ions upon the Fe(III) catalytic oxidation of 2,4-diaminophenol (DAP) by H2O2. Free fluoride ions form stable complexes with Fe(III), thus reducing its catalytic effect. The decrease in the absorbance of the oxidation product is monitored spectrophotometrically 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 750 mg x l(-1) for F- ions. The precision was very good (sr=0.3%) and the 3sigma detection limit was satisfactory (cL=0.471 mg x l(-1)). The sampling rate was 90 injections x h(-1). The method has been successfully applied to the determination of F- ions in pharmaceutical formulations.