Indirect spectrophotometric determination of p-aminobenzoic acid in sunscreen formulations by sequential injection analysis

A simple indirect sequential injection spectrophotometric method for the determination of the UV-filter p-aminobenzoic acid (PABA) in sunscreen formulations is proposed. The determination is based on the reaction of PABA with hypochlorite in acidic medium and the subsequent measurement of the residual chlorine by using the well-known reaction with o-tolidine. The experimental variables involved in the sequential injection analysis (SIA) system (sandwich arrangement, volumes of sample and reagents, propulsion flow rate, reaction coil length) and adequate concentrations of reagents were studied. The SIA method has a linear range up to 20 mug ml(-1) (3s(y/x)/b detection limit) of 1.0 mug ml(-1) and a measurement throughput of 55 injections h(-1). Sunscreen cosmetics containing PABA were analyzed by the proposed method and the results were validated by comparison with those obtained by a liquid chromatographic procedure used as reference. The analytical features of the proposed method make it suitable for quality control of final products. (C) 2003 Elsevier B.V. All rights reserved.