Spectrophotometric study of the photodecomposition kinetics of nifedipine.

Nifedipine is a photosensitive compound. Irradiation for 4 h under a fluorescent lamp placed 30 cm from a solution of nifedipine in 95% ethanol leads to complete photo-oxidation as determined spectrophotometrically. The disappearance of the reduced form and appearance of the oxidized form is best described by zero-order kinetics at concentrations higher than 4 x 10(-4) M. At lower concentrations pseudo-first order kinetics are followed. Monochromatic irradiation of nifedipine at wavelengths 400 to 700 nm in 25 nm increments showed no change in the absorbance at 280 nm, and, except for a hyperchromic effect at 237 nm, no other spectral changes were observed. Its photo-oxidation was dependent on the intensity of light and increased exponentially as solutions were irradiated progressively closer to a fluorescent light source. The pH studies showed that aqueous solutions of nifedipine photo-oxidized fastest at pH 2.