Abstract
Two new, simple, rapid and reproducible spectrophotometric methods have been developed for the determination of lamotrigine (LMT) both in pure form and in its tablets. The first method (method A) is based on the formation of a colored ion-pair complex (1:1 drug/dye) of LMT with bromocresol green (BCG) at pH 5.02±0.01 and extraction of the complex into dichloromethane followed by the measurement of the yellow ion-pair complex at 410 nm. In the second (method B), the drug-dye ion-pair complex was dissolved in ethanolic potassium hydroxide and the resulting base form of the dye was measured at 620 nm. Beer’s law was obeyed in the concentration range of 1.5-15 μg mL-1 and 0.5-5.0 μg mL-1 for method A and method B, respectively, and the = corresponding molar absorptivity values are 1.6932 x 104 and 3.748 x 104 L mol-1cm-1. The Sandell sensitivity values are 0.0151 and 0.0068 μg cm-2 for method A and method B, respectively. The stoichiometry of the ion-pair complex formed between the dug and dye (1:1) was determined by Job’s continuous variations method and the stability constant of the complex was also calculated. The proposed methods were applied successfully for the determination of drug in commercial tablets.
Highlights
Absorption spectrum of the yellow colored LMT-bromocresol green (BCG) ion-pair complex is shown in Fig. 1 which has a maximum absorbance at 410 nm
This drug dye ion-pair complex was broken in ethanolic base to yield the base form of the blue dye which had maximum absorbance at 620 nm
Varying aliquots of LMT-BCG ion-pair complex (10 μg mL-1 in LMT; prepared by following the procedure described in method A) equivalent to 0.5 – 5.0 μg mL-1 with respect to LMT were transferred into a series of 10 mL standard flasks and the total volume was brought to 5 mL by adding dichloromethane
Summary
Adsorptive stripping voltammetric method [21] is highly complicated and is reported to be less precise (RSD ~10 %) Considering these drawbacks, there was a need to develop more advantageous spectrophotometric method for its determination in bulk powder and commercial dosage forms. We developed two higly sensitive, selective, reproducible and economical spectrophotometric methods for the determination of LMT in bulk powder and in tablets by exploiting its basic nature and its ability to form ion-pair complex with an anionic dye bromocresol green. Bromocresol green (0.4%): Dissolved 400 mg of the dye (S.D.Fine Chem Ltd, Mumbai, India) in 10 ml of ethanol and diluted to 100 ml with water. Standard drug solution (30 μg mL-1): LMT (pharmaceutical grade, 99.88 % pure) was procured from Cipla India Ltd, Mumbai, India, as a gift and was used as received. Lamosyn 100 and lamosyn 25 (both from Sun Pharmaceuticals Ltd, Mumbai, India) and Lametec 50-DT (Cipla India Ltd, Mumbai, India)all tablets were purchased from local market and were used in the investigation
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