4.6. Chemical Determination of Oxidative Stability

Measurements of p-anisidine value (pAV) for oil samples, as a measure of the level of secondary oxidative products, were carried out according to the ISO standard [34]. Spectrophotometric measurements were taken with a quartz cuvette with the 10 mm optical path length. A sample of 3 ± 0.001 g grams was weighed for measurement. The values obtained were calculated by means of the following equation:

where Ao is the absorbance of the non-reacting sample, A1 is the absorbance of the reacting sample, A2 is an absorbance of the blank sample and m is the mass of the sample [g].

Peroxide value was determined by following the ISO 3960 standard [35]. A sample of 5 ± 0.001 g grams was weighed for measurement.

Calculations were performed using the following equation:

where PV is peroxide value [meq O₂/kg], V—volume of titrant in test portion [ml], V0—volume of titrant in blank [ml], C_thio—molar concentration of the sodium thiosulfate solution in mol/l, F—exact concentration of the 0.01 N thiosulfate solution, m—weighed portion of test substance [g].

In accordance with the ISO 3960 standard [35], the total oxidation value (TOTOX) parameter was calculated, based on the pAV and PV values, by means of the following formula:

TOTOX = pAV + 2PV, expressing the overall rate of oil oxidation.

Acid value (AV), as an indicator of the degree of hydrolytic changes, was measured in five oils according to the AOCS official method [36]. A sample of 10 ± 0.001 g was weighed for measurement. The resulting values were calculated using the following equation:

where: AV—acid value [mg KOH/g of test portion], A—volume of standard alkali used in the titration [ml], B—volume of standard alkali used in titrating the blank [ml], M—molarity of standard alkali, W—mass of test portion [g].