Chiang Mai Journal of Science

 This paper account a new technique for determination of tracing quantities of p-coumaric acid by its perturbation effects on a novel Briggs-Rauscher (BR) oscillator catalyzed by tetra-aza-macrocyclic Ni-complex [NiL](ClO₄)₂. Whereas, the ligand “L” in the complex is 5,7,7,12,14,14 -hexamethyl-1,4,8,11- tetraazacyclotetradeca-4,11-diene. In this novel-BR-system, not only the metal ion (Mn²⁺) catalyst was replaced by Ni-complex, but kept pH of the oscillator identical to the pH value of the human stomach (pH=2) as well. Experimental results indicated that the perturbation effect of p-coumaric acid is reflected in the quenching and regeneration of oscillations after a temporary interval called inhibition time (t_in). Such t_in of the system has directly proportional to the concentration of p-coumaric acid added. Thus linear regression curve over a concentration range from 6.50 × 10^-5 mol/L to 2.25 × 10^-4 mol/L of p-coumaric acid vs t_in with a correlation coefficient 0.99 was achieved. The observed limit of detection (LOD) is 1.50 × 10^-5 mol/L. The RSD calculated from 6 measurements of 1.0 × 10^-4 mol/L of p-coumaric acid is 2.24 %. Some factors influencing the determination were also examined. The reaction perturbation mechanism has been proposed from the well-known FCA oscillatory mechanism. A brief explanation is that the p-coumaric acid reacts with intermediate species HOO• (hydroperoxyl radical) and the system was temporarily ceased. Not only this method is applicable to determine p-coumaric acid but also utilized to determine the other antioxidants as well.