A kinetic potentiometric method is described for the determination of hydrazines (hydrazine, phenylhydrazine, hydralazine and procarbazine), isoniazid and sodium azide, based on monitoring their reactions at 25 degrees and pH 9.0 with 1-fluoro-2,4-dinitrobenzene by means of a fluoride-selective electrode. Initial-rate and fixed-time methods were used to construct calibration graphs, generally over the range 1 x 10(-4)-1 x 10(-2)M. Hydralazine, procarbazine and isoniazid were determined in commercial formulations with a precision and error of 2-3% and the results were comparable with those of the official methods. The presence of common excipients and concomitant drugs in combination products do not interfere and the method can be used for coloured and cloudy sample solutions. A kinetic study of the reactions was made and the overall second-order rate constants are given. Base catalysis was observed. The fluoride-selective electrode is shown to be a valuable tool for monitoring fluoride-liberating organic reactions in kinetic studies and kinetic analysis.