Flow injection spectrophotometric determination of paracetamol in pharmaceuticals by means of on-line microwave-assisted hydrolysis and reaction with 8-hydroxyquinoline (8-quinolinol)

A new procedure is proposed for the spectrophotometric determination of paracetamol in pharmaceuticals. The method is based on the on-line alkaline hydrolysis of paracetamol to p-aminophenol (PAP) and the reaction with 8-hydroxyquinoline (8-quinolinol) in the presence of potassium periodate as oxidant to form a blue indophenol dye which absorbs at 608 nm. The method has a 30-limit of detection of 0.2 μg ml−1 and provides a sampling frequency of 70 h−1. Previous batch studies using closed PTFE reactors demonstrate that paracetamol can be hydrolysed quantitatively to PAP in only 2 min using an irradiation power of 990 W and 5 M NaOH or in 3 min using 1100 W and 3 M HCl. The on-line alkaline hydrolysis, carried out in a 6 m reaction coil located inside the cavity of a Microdigest 301 system provides an hydrolysis yield of 54.1% for a reaction time of 94 s and allows the complete automation of the analytical procedure. The method developed has been applied to the analysis of real pharmaceutical formulations using alkaline solutions of paracetamol as standards. In the flow conditions, the recovery percentage values obtained for added paracetamol concentrations, from 1.06 to 3.17 × 10−5 M, range from 96.3%, in syrup samples, to 98–100% in tablets and capsules, and 102% in suppositories. The relative standard deviation found, for three independent analyses of each sample, varies from 0.8 to 4% as a function of the paracetamol concentration and the type of sample, the less precise values being obtained or syrup analysis.