Calibration of Squalene, p-Cymene, and Sunflower Oil as Standard Oxidizable Substrates for Quantitative Antioxidant Testing

The autoxidation kinetics of stripped sunflower oil (SSO), squalene (SQ), and p-cymene (p-C) initiated by 2,2′-azobis(isobutyronitrile) at 303 K were investigated under controlled conditions by differential oximetry in order to build reference model systems that are representative of the natural variability of oxidizable materials, for quantitative antioxidant testing. Rate constants for oxidative chain propagation (kp) and chain termination (2kt) and the oxidizability (kp/√2kt) were measured using 2,6-di-tert-butyl-4-methoxyphenol, 2,2,5,7,8-pentamethyl-6-chromanol, BHT, and 4-methoxyphenol as reference antioxidants. Measured values of kp (M–1 s–1)/2kt (M–1 s–1)/oxidizability (M–1/2 s–1/2) at 303 K in chlorobenzene were 66.9/3.45 × 106/3.6 × 10–2, 68.0/7.40 × 106/2.5 × 10–2, and 0.83/2.87 × 106/4.9 × 10–4, respectively, for SSO, SQ, and p-C. Quercetin, magnolol, caffeic acid phenethyl ester, and 2,4,6-trimethylphenol were investigated to validate calibrations. The distinctive usefulness of the three substrates in testing antioxidants is discussed.