2.3. Acidity, peroxide, iodine and saponification values, unsaponifiable matter, Oil Stability Index (OSI), water content and trace metals
Physicochemical characteristics were determined according to the Official Methods and Recommended Practices of the AOCS (AOCS, 2009): Cd 3d-63, Cd 8b-90, Cd 1c-85, Cd 3b-76, Ca 6b-53, Ca 2e-84, Cd 12b-92, for acidity value, peroxide value, iodine value, saponification value, unsaponifiable matter, water content, and Oil Stability Index (OSI), respectively. OSI was determined at 130 °C, air flow rate at 20 L/h using an OSI analysis system (893 Professional Biodiesel Rancimat, Metrohm, Switzerland). Induction time was determined by the OSI inflection point at a maximum of the second derivative of the conductivity curve. Acidity was determined by using an automatic titrator (848 Titrino plus, Metrhom, Switzerland). Water content was determined by using an automatic Karl Fisher titrator (870 KF Titrino plus, Metrohm, Switzerland).
Trace metals were determined according to ASTM method D5185 – 13 (ASTM, 2013). Inorganic elements were quantified by axial measurements in a dichroic spectral combiner (ICP OES 5100 SVDV, Agilent Technologies, Japan). Operating conditions were: 1.5 kW plasma power; argon flow at 15 L/min; auxiliary argon flow 0.4 L/min; nebulization flow at 0.45 L/min; stabilization time set at 15 s. Wavelengths of 317.93 nm, 259.94 nm, 280.27 nm, and 213.62 nm were employed for calcium, iron, magnesium, and phosphorus, respectively.