Automatic potentiometric two-phase titration in pharmaceutical analysis : Part 3. Titrimetric Identification of Drugs

The conditional acidity constant in a liquid-liquid two-phase system (pK^*_HA + value) can be used as an identity parameter for drug compounds. The pK^*_HA + value can be calculated if the phase ratio and the acidity and distribution constants are known. Selectivity is obtained mainly by the choice of the organic phase. An excess of hydrochloric acid is first added to the sample in the two-phase system. The hydrochloric acid and the sample are then titrated sequentially with sodium hydroxide. The proposed titration procedure makes it possible to calibrat the electrodes, determine the pK^*_HA + value(s), quantify one or more compounds, and check the performance of the electrodes in one and the same experimental run. Lidocaine was “identified” and assayed in different dosage forms such as ointment and solution for injection. The results were in agreement with those obtained by liquid chromatography.