BS EN 14112 Fat and oil derivatives - Fatty Acid Methyl Esters (FAME) - Determination of oxidation stability (accelerated oxidation test)
8 Procedure
8.1 Set up the apparatus shown in Figure 1. If the apparatus is available commercially, follow the manufacturer's instructions.
8.2 Attach the gas diaphragm pump (5.1.2) and adjust the flow to exactly 10 l/h. Then switch the pump off again. Commercially available apparatus may control the set flow automatically.
8.3 Bring the heating block (5.1.8) up to the desired temperature (usually 110 °C, but see 7.2.2) using the thyristor and contact thermometer (5.1.7) or by using an electronic controller. The temperature shall be maintained constant to within +/- 0.1 °C during the test period.
Pour some glycerol (4.4) into the holes of the heating block (5.1.8) in order to promote heat transfer if necessary.
If a heating bath (5.1.8) is used, bring it to the desired temperature and check in the manner described.
8.4 Fill the measurement cells (5.1.4) with 50 ml of distilled or demineralized water using a measuring pipette (5.3).
NOTE At temperatures above 20 °C, volatile carboxylic acids may evaporate from the water in the measurement cell. This can lead to a decrease in the conductivity of the aqueous solution. The rapidly rising part of the conductivity curve will therefore produce a deviant shape so it becomes impossible to determine the tangent on this part of the curve (see reference [2]).
8.5 Check the electrodes (5.1.5) and adjust their signals using a calibration potentiometer so that they are on the zero axis of the recorder paper.
Set the paper rate at 10 mm/h and the measuring frequency at one measuring point per 20 s. Set the measuring value of 200 µS/cm at the maximum result of 100 %.
If it is not possible to adjust the paper rate to 10 mm/h, but 20 mm/h, this should be reported on the recorder paper.
NOTE Commercially available apparatuses may acquire the data via PC.
8.6 Using a pipette (5.3), weigh to the nearest 0.01 g, 3.0 g of the conditioned sample (see clause 7.2) into an reaction vessel (5.1.3).
8.7 Switch on the gas diaphragm pump (5.1.2) and set the flow again at exactly 10 l/h. Connect the air inlet tube and outlet tube with the reaction vessels and the measurement cells, using the connecting hoses (5.5).
8.8 Place the reaction vessel with the sealing cap (5.1.3) into the hole intended for it in the heating block or into the heating bath (5.1.8), both of which shall have reached the required temperature.
The preparation steps 8.7 and 8.8 should be done as quickly as possible. Then immediately start the automatically data recording or note the time that measurements were begun on the recorder paper.
8.9 Finish the measurements when the signal has reached 100 % of the recorder scale, usually 200 µS.
8.10 During the determination, carry out the following precautions:
a) check the setting of the flow meter and adjust where necessary in order to ensure a constant flow;
b) check the colour of the molecular sieve (4.1) of the air filter and repeat measurements when the molecular sieve turns colour during the test. It is recommended to exchange the molecular sieve prior to each run.
9 Calculation (Evaluation)
9.1 Manual Calculation (Evaluation)
Draw the optimum tangent along the first, moderate increasing part of the curve. Draw the optimum tangent along the upper part of the rapidly increasing part of the curve (fore more details, see Figure A.1 in annex A). Carry out the determination again if it is not possible to draw the optimum tangent.
Determine the oxidation stability by reading off the time at the point where the two lines intersect (the induction time).
9.2 Automatic Calculation (Evaluation)
Commercially equipment allows an automatic calculation of the induction period, by using the maximum of second derivative. (For more details, see Figure A.1c) in annex A).
Express the oxidation stability in hours rounded to the nearest 0.1 h.
NOTE Examples of conductivity curves are shown in Figure A.1. A curve, which rises very rapidly, can be the result of the temperature of the solution in the measurement cell being too high, causing volatile carboxylic acid to evaporate from the solution (see reference [3]).
