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.

Normal and differential demasking flow-injection manifold for the direct spectrophotometric determination of zinc(II) in biological materials and pharmaceutical formulations.

A normal and a differential demasking flow-injection (FI) manifold were developed and optimized for the spectrophotometric determination of microamounts of Zn(II) in biological materials and pharmaceutical formulations. The reported method is very sensitive, rapid, simple and it is based upon the reaction of Zn(II) with 2,2'-dipyridyl-2-pyridylhydrazone (DPPH) in a strongly basic medium to form a yellow-coloured complex (lambda max = 448 nm). By using the differential demasking manifold, the tolerance of the method to many cations was enhanced by a mean factor of 25. The obtained calibration graphs were linear in the range of 0-10 mg l-1 Zn(II), at a sampling rate of 120 injections h-1 in both cases. The precisions of both manifolds were very good (RSD = 0.6 and 0.8%, respectively) and the 3 sigma detection limits were quite satisfactory (cL = 4 and 6 ng ml-1 respectively). The method has been successfully applied to the determination of Zn(II) in serum, human hair and pharmaceutical formulations with recoveries ranging between 98.0 and 101.6%. The obtained results were also in excellent agreement with flame atomic absorption spectrometry (FAAS), since the mean relative error was er = 0.9%.