Utility of the ion-pair formation for spectrophotometric determination of terfenadine in pure form and in some pharmaceutical formulations

A spectrophotometric procedure for the determination of terfenadine and a number of its pharmaceutical preparations has been developed that offers advantages of simplicity, rapidity, sensitivity and stability indication over the official USP (1995) method. The proposed method is based on the formation of ion-pairs by the reaction of terfenadine with some chromotropic acid mono- and bis-azo dyes. Different variables affecting the ion-pair formation were studied and optimized. At the maximum absorption of 557, 521, 592 and 543 nm, Beer's law is obeyed in the range 0.2–18.6, 0.2–16.4, 0.2–25.0 and 0.2–22.2 g ml^–1 on using reagents I, II, III and IV, respectively. The stoichiometric ratio and stability of each ion-pair were estimated and the mechanism of the reaction is discussed. The molar absorptivity and Sandell sensitivity of the produced ion-pairs were calculated in addition to Ringbom optimum concentration ranges. Statistical treatment of the experimental results indicates that the procedures are precise and accurate. Excipients used as additives in pharmaceutical formulations did not interfere in the proposed procedures. The reliability of the methods was established by parallel determination against the official USP method. The procedures described were successfully applied to the determination of the bulk drug and its pharmaceutical formulations by applying the standard addition technique.