Sensitive, selective and extraction-free spectrophotometric determination of lamotrigine in pharmaceuticals using two sulphonthalein dyes

Two direct, simple, sensitive and rapid extraction-free spectrophotometric methods have been developed for the determination of lamotrigine (LMT) in pure form and in its dosage forms. The methods are based on formation of ion-pair complex between the drug and two sulphonthalein acidic dyes namely, bromothymol blue (method A) and bromocresol green (method B), followed by the measurement of absorbance at 410 nm in both the methods. Conformity to Beerûs law enabled the assay of the drug in the range 1-12 μg mL-1 in both the methods with apparent molar absorptivities of 2.06 x 104 and 1.787 x 104 L mol-1cm-1 for method A and method B, respectively. The Sandell sensitivity values, limits of detection (LOD) and quantification (LOQ) values have also been reported for both the methods. The stoichiometry of the ion-pair complexes as evaluated by Jobûs continuous variations method was 1:1 and the conditional stability constant (log Kf) were calculated to be 7.05 and 8.24 for method A and method B, respectively. The accuracy and precision of the methods were evaluated on intra-day and inter-day basis; the relative error (%RE) was < 2.0% and the relative standard deviation (RSD) was < 1.60%. The methods were successfully applied to the determination of drug in tablets without interference by the common coformulated substances. Statistical comparison of the results with the reference method showed good agreement and indicated no significant difference in accuracy and precision.