Titrimetric determination of fluoride in some pharmaceutical products used for fluoridation

Two titrimetric methods were developed for the determination of fluoride contents in some pharmaceutical preparations used for fluoridation. One of the methods is catalytic controlled-current potentiometry involving two identical platinum indicator electrodes and thorium nitrate as titrant. The reaction between hydrogen peroxide and potassium iodide in the presence of acetate buffer (pH 3.6), which is catalysed by the excess of thorium nitrate, served for the end-point indication. The other method is the automatic potentiometric titration involving a fluoride-selective electrode and lanthanum nitrate as titrating agent. In both procedures, special attention was paid to sample pretreatment and to determination of optimal experimental conditions. Fluoride contents in the range 16-32 microg/ml are determined with a relative standard deviation less than 1.34%. The results are compared to those obtained by standard methods described in the United States Pharmacopeia XXI and recommended by the manufacturer of the preparations.