Simultaneous Determination of Acetaminophen with Orphenadrine Citrate, Ibuprofen or Chlorzoxazone in Combined Dosage Forms by Zero-Crossing Derivative Spectrophotometry

A derivative spectrophotmetric procedure for the simultaneous determination of acetaminophen-orphenadrine citrate, acetaminophen-ibuprofen and acetaminophen-chlorzoxazone, binary mixtures is described. The procedure minimises the mutual interference between these drugs in mixtures and allows the determination of these compounds without a previous extraction step. The precision of the method, expressed as the relative standard deviation, is better than 4%. The method has been successfully applied to laboratory mixtures and commercial tablets containing these drugs.