Evaluation of the terbium(III)-sensitized luminescence with benzenepolycarboxylic acids: Determination of terephthalic acid in drink samples

Abstract

The reaction of terbium(III) with several benzenepolycarboxylic acids, viz. phthalic, isophthalic, terephthalic, hemimellitic, trimellitic and trimesic acids, was systematically studied in order to determine the effect of the geometry of these compounds on the luminescence intensity obtained. This study showed that terephthalic acid gave a very high luminescence signal with terbium(III) in the presence of trioctylphosphine oxide as synergistic agent and Triton X-100 as micellar medium, and by using the time-resolved mode. The system was used for the development of a very sensitive and fast method for the determination of terephthalic acid, which can be applied to migration studies of this compound released to drinks stored in plastic bottles of polyethylene terephthalate. Initial rate and equilibrium signal measurements were obtained in only 0.2 and 2 s, respectively, by using the stopped-flow mixing technique, although the features of the kinetic method were better than those of the equilibrium method. The calibration graph of the kinetic method was linear over the range 0.01–3.0 mg ml −1 and the detection limit was 8 ng ml −1. The relative standard deviation was close to 2%. The analytical recoveries obtained by applying the method to the analysis of drink samples ranged from 92.5 to 107.0%. The method can be extended to the determination of terephthalic acid esters, such as dimethyl terephthalate, after their alkaline hydrolysis.