Isoxazoles. 9. Degradation kinetics of 4-(isoxazolylamino)-1,2- naphthoquinone in acidic aqueous solution

Article abstract of DOI:10.1002/jps.2600831018

The degradation kinetics of N-(5-methyl-4-isoxazolyl)-4-amino-1,2- naphthoquinone (1) was studied in aqueous solution over a pH range of 0.65- 7.50, at 35 °C and at a constant ionic strength of 0.5. The degradation rates were determined by high-pressure liquid chromatography and were observed to follow pseudo-first-order kinetics with respect to the concentration of 1. The pH-rate profile was linear with slope -1 under acidic pH, becoming pH independent from 3.50 to 7.50. Good agreement between the theoretical pH-rate profile and the experimental data supports the proposed degradation process. The catalytic rate constants for hydrogen ion and water were k(H) = 0.901 M^-1 h^-1 and k(o) = 1.34 x 10^-3 h^-1, respectively. These data are appropriate to develop a stable dosage form of 1. The accuracy, peak sharpness, and asymmetry factor for the analytical method were determined.