Simultaneous determination of paracetamol and codeine by first order derivative spectrophotometry

A simple first derivative spectrophotometric method has been developed for the simultaneous determination of paracetamol and codeine in pharmaceutical formulations. Dimethylformamide was used as solvent for extracting the drugs from the formulations and subsequently the samples were evaluated directly by first order derivative spectrophotometry. Simultaneous determination of the both drugs can be carried out using the graphical and zero-crossing methods for paracetamol and codeine, respectively. The method does not require simultaneous equations to be solved as it is commonly necessary in zero-order spectrophotometry. The calibration graphs were linear between the ranges 4.3 . 10(-5) M to 1.0 . 10(-3) M for codeine and 6.1 . 10(-5) M to 1.6 . 10(-3) M for paracetamol. The simultaneous determination is reliable only when the molar ratio of paracetamol to codeine is lower than 4/1. Commonly the pharmaceutical formulations contain both drugs in a mass ratio between 5/1 to 33/1, therefore a previous enrichment of the samples were carried out with pure codeine in order to reach the above mentioned ratio. The ingredients commonly found in commercial pharmaceutical formulations do not interfere, hence the proposed method can be applied to the determination of these drugs in tablets.