Three simple, selective and sensitive methods have been developed and validated for the determination of lamotrigine (LMT) in pure drug and in tablets. The first method (method A) is based on the formation of ion-pair complex between LMT and the dye, bromocresol purple (BCP) at pH 2.40 ± 0.01 which was extracted into dichloromethane (DCM) and the absorbance of yellow ion-pair complex was measured at 410 nm. In the second and third methods (method B and method C), the drug-dye ion-pair was
dissolved either in ethanol and the resulting acid form of the dye was measured at 410 nm or in ethanolic potassium hydroxide and the resulting base form of the dye was measured at 600 nm. Under the optimized conditions, Beer’s law was obeyed over 2.0-20.0 μg/mL, 150-1500 ng/mL and 50-600 ng/mL for method A, method B and method C, respectively, and the corresponding molar absorptivity values were 1.018 × 104, 1.43 × 105 and 4.21 × 105 L/mol/cm. The Sandell sensitivity values of 0.0252, 0.0018 and
0.0006 μg/cm2 for method A, method B and method C, respectively, and the corresponding values for limits of detection and quantification
were also reported for all three methods. The molar ratio of the formed ion-pair complex was found to be 1: 1 as deduced by Job’s method for method A, and the calculated stability constant was also reported. Over the linear ranges applicable, the accuracy and precision of the methods were evaluated on intra-day and inter-day basis; the reported mean accuracy values are 99.50 ± 1.09%, 99.99 ± 1.11% and 100.72 ± 0.62% for method A, method B, and method C, respectively; the relative error (RE) was ≤ 2.57% and the relative standard deviation (RSD) was ≤ 2.01%. Application of the proposed methods to bulk powder and commercial pharmaceutical tablets are also presented.