Simultaneous determination of retinoic acid and hydroquinone in skin ointment using spectrophotometric technique (ratio difference method).

An innovative spectrophotometric method was developed for simultaneous determination of compounds with interfering spectra in binary mixtures without previous separation, showing significant advantages over the conventional methods regarding minimal data manipulation and applicability. The proposed method was applied for the determination of retinoic acid and hydroquinone in laboratory-prepared mixtures with mean percentage recoveries 100.13 ± 0.31 and 99.99 ± 0.04, respectively, and in their pharmaceutical formulation with mean percentage recoveries 100.13 ± 0.86 and 100.07 ± 0.58, respectively. The method was validated according to USP guidelines and can be applied for routine quality control testing.