Spectrophotometric analysis based on charge-transfer complex method for metformin hydrochloride in pure and pharmaceutical preparations

The proposed method is based on a charge-transfer complexation reaction (CTCR) between metformin hydrochloride, acting as an electron donor, and bromanil (2,3,5,6-tetrabromo-1,4-benzoquinone) as a π-electron acceptor. The reaction was carried out in a basic medium (pH 13.6) to give an orange-brown, stable complex that shows maximum absorbance at 434 nm. Experimental conditions affecting the reaction were systematically optimized. Under the optimized conditions (30 °C, 5 min development time), the calibration curve was linear over the concentration range of 15–40 µg mL⁻¹, with a correlation coefficient (R²) of 0.9982. The limits of detection (LOD) and quantification (LOQ) were 1.66 and 5.04 µg mL⁻¹, respectively. The method demonstrated excellent precision, as reflected by a low relative standard deviation (RSD) of 0.45% and a relative error (RE) of −0.723%. Stoichiometric investigation revealed a 1:1 drug–reagent complex with a favorable stability constant. In summary, the validated method was successfully applied to determine metformin hydrochloride in commercial tablet formulations, yielding acceptable recoveries and showing no interference from common pharmaceutical excipients. Owing to its simplicity, accuracy, and cost-effectiveness, the proposed method is suitable for routine quality control analysis of metformin hydrochloride in pharmaceutical preparations.