Determination of 4-aminophenol impurities in selected pharmaceutical preparations by HPLC method with amperometric detection.

4-Aminophenol (4-APh) is the main impurity present in preparations containing paracetamol. Using the cyclic voltammetry and differential pulse voltammetry methods, the electrode behaviour of 4-APh has been studied in various non-buffered and buffered solutions at glassy carbon and golden electrodes. By means of the high-performance liquid chromatography with amperometric detection (HPLC-EC), the dependency of current intensities of the 4-APh peaks on the potential in the range (0-600 mV in the pH range 2-5, the ionic strength of mobile phase ranging from 0.01 mol (-1) to 0.20 mol l(-1) LiCl, has been studied. While employing the HPLC-EC method in this work, a glassy carbon electrode was used as the amperometric detector. The optimal potential selected in this research was +325 mV. It was found that a 0.05 mol l(-1) LiCl solution, containing 18% of methanol, at pH 4.0 adjusted with orthophosphoric acid, is suitable for the separation of 4-APh and paracetamol from each other and from other pharmaceutical excipients present in tablets. By using the elaborated HPLC-EC method, the content of 4-APh (at concentrations from 4 ng ml(-1)) in the paracetamol from Aldrich and in tablets from different producers, containing 500 mg of paracetamol in a tablet was found. Statistical evaluation of the obtained results has shown that the proposed HPLC-EC method for the determination of 4-APh is characterized by a good accuracy and precision (RSD about 6%) and can be applied to routine investigations of pharmaceutical preparations in the form of tablets.