Photodegradation of norfloxacin in aqueous and organic solvents: A kinetic study

Abstract The kinetics of photodegradation of norfloxacin (NF) on UV irradiation in aqueous solution (pH 2–12) and in organic solvents has been studied using a validated HPLC method. The apparent first-order rate constants ( k obs ) for the reaction in aqueous solution range from 0.28 (pH 2) to 5.19 × 10 −3  min −1 (pH 10) and in organic solvents from 0.34 (1-butanol) to 0.80 × 10 −3  min −1 (acetonitrile). The rate–pH profile for the photodegradation of NF shows an initial slow increase in the rate up to pH 6, followed by a rise in the rate to form a bell-shaped curve with pH max around 10. It represents the involvement of cationic, zwitterionic and anionic species of NF undergoing photodegradation at variable rates in acid, neutral and alkaline regions. The increase in rate in the pH range 6–10 is due to the participation of zwitterionic and anionic species and may largely result from the hydrolytic degradation of piperazine side chain in the higher pH range. The decrease in rate above pH 10 is due to low reactivity of anionic species which are less susceptible to photodegradation. The role of excited triplet states of NF species to influence the rate of photodegradation has been highlighted. The photodegradation of NF in organic solvents is a linear function of the dielectric constant and an inverse function of the viscosity of the medium. Five photodegraded products of NF have been identified by LC–MS/MS analysis and schemes for the photodegradation pathways of NF in acid, neutral and alkaline solutions are presented.