Four simple, sensitive, and accurate spectrophotometric methods (A-D) have been developed for the determination of domperidone (I) and doxycycline (II) in bulk and in pharmaceutical formulations. The first two methods (A and B) are based on the formation of yellow ion-pair complexes between the examined drugs with bromocresol green (BCG) and methyl orange (MO) as chromophoric reagents in Britton-Robinson (B-R) universal buffer of pH 3.0 and 2.2, respectively. The formed complexes were extracted with chloroform and measured at 420 nm and 424 nm for I and II using BCG, respectively and 480 nm for the studied drugs using MO. The last two methods (C and D) are based on charge transfer complex formation between the studied drugs and tetracyanoethylene (TCNE); and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Different variables affecting the reactions were studied and optimized. Under the optimum reaction conditions, linear relationships with good correlation coefficients (0.9992 - 0.9998) were found between the absorbance and the concentration of I in the range of 1.4-22.4μg mL−1, whereas that for II in the range 1.4-25.5μg mL−1. For more accurate analysis, Ringbom optimum concentration range was found to be between 3.0-21.5 and 3.0-23.5μg mL−1, for I, and II, respectively. Sandell’s sensitivity, correlation coefficient, and limits of detection and quantification were calculated for each method. A Job’s plot of the absorbance versus the molar ratio of drug to each of formed complex under consideration indicated (1:1) ratio. No interference was observed from common pharmaceutical excipients and additives. Statistical comparison of the results with those obtained by the official method shows excellent agreement and indicates no significant difference in accuracy and precision.
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