The effect of pH, buffer type and drug concentration on the photodegradation of ciprofloxacin

The photochemical behaviour of ciprofloxacin was investigated in the pH range 3.0-10.6 in solutions irradiated with a high-pressure mercury lamp at 313 nm. The reaction was followed by an isocratic reversed-phase high-performance liquid chromatographic method. Ciprofloxacin was most sensitive to photodegradation at slightly basic pH, where the drug is in zwitterionic form. The stability increased considerably when the pH was lowered towards 3-4. The reaction rate was inversely proportional to the initial drug concentration, but it was not affected by buffer type (acetate, citrate and phosphate at pH 5.0). Two major degradation products were formed in acidic solutions. In solutions at pH greater than or equal to 6.0, several additional degradation products were detected, which underwent secondary degradation on longer exposure to radiation.