Application of Hansen Solubility Parameters for Green Solvent Extraction of Flavonoids from Oroxylum indicum (L.) Kurz and Chromatographic Profiling Across Plant Organs

Oroxylum indicum (L.) Kurz is an edible medicinal plant whose pharmacological properties are largely attributed to four flavonoids: baicalein, oroxin A, oroxin B, and chrysin. This study developed an ultrasound-assisted extraction method using environmentally benign ethanol–water mixtures selected with Hansen solubility parameters (HSPs) to recover these compounds from six plant tissues: seeds, leaves, roots, bark, pod husks, and flowers. A high-performance liquid chromatography method was developed and validated for their quantitative determination. Comparative tissue profiling showed that seeds contained the highest concentrations of all four flavonoids. Baicalein and chrysin exhibited maximum recoveries with 60% ethanol, yielding 53.47 ± 1.07 and 28.11 ± 0.68 mg/g dry weight, respectively, only baicalein values being consistent with HSP-based predictions. In contrast, oroxin B and oroxin A were extracted most efficiently with 80% ethanol, reaching 65.18 ± 1.09 and 128.38 ± 1.88 mg/g dry weight, respectively, contrary to predictions for the glycosylated flavonoids. These results demonstrate that HSPs can effectively support green-solvent selection for baicalein extraction, although their predictive performance may be limited for flavonoid glycosides and chrysin. The method’s sustainability was confirmed using AGREEprep™, which produced a score of 0.67, compared with 0.53 for a petroleum ether–methanol system. This work provides a practical foundation for sustainable extraction and standardization of O. indicum for pharmaceutical applications.