The bulk oil samples (20 µL) were weighed and dissolved in 2.8 mL of methanol/butanol solution (2:1, v/v).
A mixture of 15 µL of 3.94 M ammonium thiocyanate and 15 µL of 0.072 M ferrous solution was used as an indicator.
The ferrous solution was obtained from the supernatant of a mixture of one part of 0.144 M FeSO 4 and one part of 0.132 M BaCl2 in 0.4 M HCl.
After 20 min of incubation at room temperature, the absorbance of the samples was measured at 510 nm using a spectrophotometer (Genesys 20, Thermospectronic, Waltham, MA).
The concentration of hydroperoxides was calculated from a cumene hydroperoxide standard curve.