Polarographic behavior of cephalexin and its determination in pharmaceuticals and human serum.

Cephalexin gives a reduction wave in 0.03 mol/l HCl medium at ca. -1.24 V. With cephalexin concentration higher than 2.5 x 10(-5) mol/l, another reduction wave is observed at ca. -0.90 V. These reduction waves are attributed to the reduction of ethylenic bond of a six-membered dihydrothiazine ring. When H2O2 is present, the reduction wave at ca. -0.90 V is catalyzed by H2O2 and its reduction intermediate hydroxyl radical *OH, producing a catalytic wave. However, the reduction wave at ca. -1.24 V remains nearly unchanged. A sensitive polarographic method for the determination of cephalexin is proposed based on the reduction wave of cephalexin. The second-order derivative peak current of the wave at ca. -1.24 V is rectilinear to the cephalexin concentration in the range 1.0 x 10(-7) to 2.5 x 10(-5) mol/l, and the detection limit is 5.0 x 10(-8) mol/l. The proposed method is applied to the individual tablet dosage form and human serum.