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An accessible method for the evaluation of the thermo-oxidative stability of organic substrates based on vegetable oils
Thermochimica Acta 632 (2016) 91–93
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short communication
An accessible method for the evaluation of the thermo-oxidative stability of organic substrates based on vegetable oils Valeriy I. Dzyuba, Lyudmila I. Koval ∗ , Vasyl I. Pekhnyo V.I. Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences, Prospect Palladina 32-34, Kyiv 142, 03680, Ukraine
a r t i c l e
i n f o
Article history: Received 22 September 2015 Received in revised form 3 March 2016 Accepted 1 April 2016 Available online 4 April 2016 Keywords: Computerized method Oxidation induction period Oxidation stability Vegetable oils
a b s t r a c t A novel cost-effective computerized method is proposed for the determination of the oxidation induction period (OIP) of organic substrates. All component parts of the device are factory-made (interchangeable) details, which are widely used in laboratory practice. A sample is charged into the reactor, the closed reactor-condenser-stopcock system is evacuated. Then the device is filled with oxygen. The reactor is warmed up to balance with atmospheric pressure. The device is sealed and stirring of the heating medium, substrate and digital data transfer from the pressure sensor to the computer is started. The monitoring of the autooxidation of the sample is terminated after the system pressure has decreased by 300 hPa. A test report is formed with the aid of a computer using Microsoft Excel; OIP is determined as the point of intersection of two tangent lines to the curve. The proposed method is on a par with standardized methods in terms of the repeatability of measurement results. © 2016 Published by Elsevier B.V.
1. Introduction Vegetable oils and a number of their synthetic derivatives, such as biodiesel, biobased lubricants, etc., contain fragments of molecules with double bonds and are oxidized therefore quickly, losing necessary consumer properties. With the increase in the degree of unsaturation of molecules the autooxidation process occurs at a faster rate. A number of analytical instrumental methods are used for the routine evaluation of the oxidation degree of above substrates [1]. However, there is no universal method which would comprehensively bring to light all changes that take place during the autooxidation of substrate. Therefore, the necessity arises to choose every time a convenient and adequate method which is suitable for a particular case [2]. The overwhelming majority of organic substrates are oxidized by a free-radical chain mechanism to form peroxide compounds as primary oxidation products [3]. The time interval before the accumulation of a noticeable amount of peroxide compounds or their derivatives (secondary oxidation products) is commonly called an oxidation induction period (OIP), which is in fact a measure of the oxidation stability of substrate [4]. Among the numerous types of testing, those are most frequently used which are a basis of valid standards and are carried out on modern devices with high
∗ Corresponding author. E-mail addresses: l [email protected], [email protected] (L.I. Koval). http://dx.doi.org/10.1016/j.tca.2016.04.002 0040-6031/© 2016 Published by Elsevier B.V.
level of automation and computerization of the determination of substrate OIP. The above methods can be arbitrarily divided according to the design of the pneumatic schemes, viz into flow- and hermetic measuring systems. The first type includes standardized test methods – ISO 6886 (oils and fats) [5], EN 14112 (fatty acid methyl esters – biodiesel) [6], in which the purified air stream is passed through the sample preheated to 100–110 ◦ C (Rancimat method). The gaseous substances formed during oxidation are entered together with air in a flask, filled with distilled water, with an electrode placed in it for the measurement of electrical conductivity. The device registers the end of an induction period when conductivity begins to increase rapidly. This type also includes the determination of substrate OIP by differential scanning calorimetry (DSC) and pressurized differential scanning calorimetry (PDSC) in accordance with the requirements of the ASTM D6186 standard [7]. A small sample of substrate (∼3.5 mg) is placed in a test cell, which is heated to a definite temperature and filled with oxygen. A preset temperature (130–210 ◦ C) and pressure (3.5 ± 0.2 MPa) are automatically maintained in the system with fixed oxygen purge rate (100 ± 10 mL/min) over the specimen spot surface. The time interval between the beginning of oxygen passing and the beginning of exothermal reaction is defined as sample OIP. The data obtained by the DSC method correlate with the OIP values obtained by the Rancimat method [8] and also by its analogue – Oxidative Stability Index (OSI) method [9]. A hermetic measuring system is used in the method “rotating bomb oxidation test” (RBOT) in accordance with the ASTM
V.I. Dzyuba et al. / Thermochimica Acta 632 (2016) 91–93
D2272 standard [10]. A glass reactor with a 50 g sample is placed in a metallic container (autoclave), the system is filled with oxygen up to 6.2 bar and is sealed. The kinetic regime of oxidation is achieved by stirring the substrate by rotating the autoclave with reactor about the axis and thermostating at 150 ± 0.1 ◦ C. The time interval between the moment of reaching the maximum pressure (∼13.5 bar) and the moment of sharp pressure drop at least 1.75 bar is defined as OIP. A good correlation between RBOT and PDSC techniques was shown [11]. Another example of devices with hermetic pneumatic scheme is a device with the trade mark Oxidograph [12], which consists of a magnetic stirrer with digital heat controller and a thermostated reactor, connected by a pneumatic line to a digital pressure sensor for the monitoring of pressure parameters and their transfer to the computer, where the obtained information is processed, and an OIP protocol is formed. 5.0 g of sample is placed in the reactor, the system blown through with oxygen; the device is sealed after filling with an oxidant, the sample is heated to the preset temperature, a valve system equalizes the apparatus pressure with the atmospheric one. The substrate is stirred with a PTFE-coated magnetic rod. The time interval between the moment of reaching the preset temperature by the specimen and the beginning of sharp pressure drop in the system is defined as OIP [12]. The data obtained by the Oxidograph method linearly correlate with the OIP values obtained by the Rancimat method [13]. The standardized instrumental methods for the determination of OIP have passed validation procedures; therefore, the use of them is always preferable to that of other methods, especially under the conditions of commercial production of oils (substrates). However, for sporadic measurement, e.g. in research laboratories, the equipment for the above methods is too expensive. Therefore, the aim of this work was to develop a cost-effective instrumental method for the determination of OIP substrates, which would ensure a high level of repeatability of obtained results.
Fig. 1. Scheme of the device for determining oxidation stability (1) – hot-plate magnetic stirrer; (2) – PTFE-coated cylindrical magnetic bar; (3) – heating medium bath; (4) – PTFE-coated oval magnetic bar; (5) – glass reactor, joint 14/20; (6) – Graham condenser, joint 14/20; (7) – adapter, joint 14/20; (8) – vacuum tubing; (9) – T-bore vacuum stopcock; (10) – digital pressure sensor; (11)–computer).
400 Pressure drop (hPa)
92
350 300 250 200 150 100 50
193
0
2. Experimental
0
50
100
150
200
250
Time (min)
Different vegetable oils: sunflower oil, olive oil, linseed oil, purchased in local market, were used in the work. 2.5 g of vegetable oil is charged into the reactor, an oval magnetic bar is introduced, and the device is assembled according to the schematic shown in Fig. 1. The vacuum stopcock is connected to the vacuum line. The closed reactor-condenser-stopcock system is evacuated with stirring for 10 min at a pressure of about 15 Pa, whereby the tightness of the system is checked, and the volatiles, which accumulate in a trap immersed in liquid nitrogen, are removed from the substrate. Then the device is filled with oxygen at a low excess pressure (∼1050 hPa). The reactor is warmed up several minutes to the preset temperature without stirring the heating medium and substrate with equalization of the system pressure with the atmospheric one, which is indicated by the cessation of oxygen flow in the bubbler. The device is sealed, and stirring of the heating medium and substrate and digital data transfer from the pressure sensor to the computer is started. The monitoring (two measurements per minute) of the autooxidation of the sample is terminated after the system pressure has decreased by 300 hPa. A test report, a typical example of which is shown in Fig. 2, is formed with the aid of a computer using Microsoft Excel; OIP is determined as the point of intersection of two tangent lines to the curve (manual calculation [6]). Two sequential measurements are made under the same conditions. The check of the precision of results obtained by this method is identical to that used in the standardized method-analog [5]. For the repeatability limit obtained in the test, a probability of 95% holds.
Fig. 2. Autooxidation of sunflower oil at 110 ◦ C.
3. Results and discussion Table 1 lists OIP values for sunflower oil at a fixed temperature (110 ± 0.2 ◦ C) with samples of different mass and with the use of reactors of different capacity. The best repeatability of results was observed for the samples with a mass of ∼2.5 g at a nominal reactor volume of 10 mL (optimal sample). The above data indicate a high reproducibility of OIP values in spite of the considerable difference in the mass of samples and in the capacity of reactors. This indicates that a reserve of compensating ability of the pneumatic scheme (Fig. 1) is sufficient for the stable operation of the device. All component parts of the device are factory-made (interchangeable) details, which are widely used in laboratory practice. Thus, the above device can be assembled from improvised means practically in any chemical laboratory. The use of an efficient magnetic stirrer with heater and digital heat controller and a cylindrical magnetic bar makes it possible to reliably (±0.2 ◦ C) control the temperature of the heating medium (silicone oil) and to vary the experimental conditions (stirring rate and temperature) over a wide range. It has been found experimentally that the required excess of bath temperatures above the nominal substrate temperature values is 1.5 ◦ C, which corresponds with the data for the Rancimat method [14]. This makes it possible to use this device for testing substrates as heterophase mixtures, ensuring the
V.I. Dzyuba et al. / Thermochimica Acta 632 (2016) 91–93
93
Table 1 Oxidation induction period of sunflower oil at 110 ◦ C. #
Sample mass (g) 2.5 1.5 3.5 9.3
1 2 3 4
Reactor capacity (mL)
Induction period (min)
Discrepancy in comparison with the optimal sample (%)
10 10 10 50
192 188 198 188
– 2.1 3.1 2.1
Table 2 Oxidation induction period of sunflower, olive and linseed oil at different temperatures.
1 2 3 4 5
#
Substrate
Test temperature (±0.2 ◦ C)
Induction period (min)
Discrepancy between two parallel tests (%)
1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.2 5.1 5.2
Sunflower oil
100
0.2
Sunflower oil
110
Sunflower oil
120
Olive oil
120
Linseed oil
110
421 420 192 193 75 76 333 337 103 102
kinetic regime of oxidation. The use of a condenser reliably isolates the volatile substrate oxidation products (ideal shape of oxidation curves [5]), making it possible to use an unprotected pressure sensor (household barometer, resolution ±0.01 hPa, absolute measurement error ±1.0 hPa), greatly reducing thereby the total cost of the device. The spiral condenser (Graham) was selected based on the principle of the maximum surface area-to-volume ratio. The temperature of the cooling liquid (water) in the condenser can be varied between 5 and 25 ◦ C, but must be stable in time. Since the daily water temperature variation in the water-supply system is not over 0.5 ◦ C, this cooling medium is quite suitable for the realization of the method, which does not rule out the use of a thermostat for the above purpose. Table 2 presents results of oxidation stability tests of sunflower oil at different temperatures and the optimum sample mass (2.5 g) and reactor capacity (10 mL); it also lists OIP values for vegetable oils with a different degree of polyunsaturation of fatty acid substituents, olive and linseed oils. These data indicate that the developed method compares very favorably on the required values for the method [5] in the context of repeatability of the results. 4. Conclusion A cost-effective computerized method for the determination of the oxidation induction period of organic substrates has been developed, which is on a par with standardized methods in terms of the repeatability of measurement results. The use of standardized goods of mass production as component parts of the device and an accessible computer program can attract attention of a large circle of potential users to the developed method. References [1] N.J. Fox, G.W. Stachowiak, Vegetable oil-based lubricants—a review of oxidation, Tribol. Int. 40 (2007) 1035–1046.
0.5 1.3 1.1 1.0
[2] F. Shahidi, Y. Zhong, Lipid oxidation: measurement methods, in: F. Shahidi (Ed.), Bailey’s Industrial Oil and Fat Products, sixth ed., John Wiley & Sons Inc Hoboken, NJ, 2005, pp. 357–385. [3] A. Adhvaryu, S.Z. Erhan, Z.S. Liu, J.M. Perez, Oxidation kinetic studies of oils derived from unmodified and genetically modified vegetables using pressurized differential scanning calorimetry and nuclear magnetic resonance spectroscopy, Thermochim. Acta 364 (2000) 87–97. [4] K. Tiang, P.K. Dasgupta, Determination of oxidative stability of oils and fats, Anal. Chem. 71 (1999) 1692–1698. [5] Animal and vegetable fats and oils – Determination of oxidative stability (Accelerated oxidation test), International Standard, ISO 6886, 1996(E). [6] Fat and oil derivatives – Fatty acid methyl esters (FAME) – Determination of oxidation stability (accelerated oxidation test), European Standard, EN 14112, 2003 E. [7] Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry (PDSC), ASTM International, USA ASTM D6186-98. [8] S. Arain, S.T.H. Sherazi, M.I. Bhanger, Farah N. Talpur, S.A. Mahesar, Oxidative stability assessment of Bauhinia purpurea seed oil in comparison to two conventional vegetable oils by differential scanning calorimetry and Rancimat methods, Thermochim. Acta 484 (2009) 1–3. [9] C.P. Tan, Y.B. Che Man, J. Selamat, M.S.A. Yusoff, Comparative studies of oxidative stability of edible oils by differential scanning calorimetry and oxidative stability index methods, Food Chem. 76 (2002) 385–389. [10] Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel, ASTM, International, USA, ASTM D 2272-02. [11] Y. Wu, W. Li, M. Zhang, X. Wang, Improvement of oxidative stability of trimethylolpropane trioleate lubricant, Thermochim. Acta 569 (2013) 112–118. [12] M. Nogala-Kalucka, J. Korczak, M. Dratwia, E. Lampart-Szczapa, A. Siger, M. Buchowski, Changes in antioxidant activity and free radical scavenging potential of rosemary extract and tocopherols in isolated rapeseed oil TAGs during accelerated tests, Food Chem. 93 (2005) 227–235. [13] I. Hradkova, R. Merkl, J. Smidrkal, J. Kyselka, V. Filip, Antioxidant effect of mono- and dihydroxyphenols in sunflower oil with different levels of naturally present tocopherols, Eur. J. Lipid Sci. Technol. 115 (2013) 747–755. [14] Y.C. Liang, C.Y. May, C.S. Foon, M.A. Ngan, C.C. Hock, Y. Basiron, The effect of natural and synthetic antioxidants on the oxidative stability of palm diesel, Fuel 85 (2006) 867–870.
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short communication
An accessible method for the evaluation of the thermo-oxidative stability of organic substrates based on vegetable oils Valeriy I. Dzyuba, Lyudmila I. Koval ∗ , Vasyl I. Pekhnyo V.I. Vernadskii Institute of General and Inorganic Chemistry, Ukrainian National Academy of Sciences, Prospect Palladina 32-34, Kyiv 142, 03680, Ukraine
a r t i c l e
i n f o
Article history: Received 22 September 2015 Received in revised form 3 March 2016 Accepted 1 April 2016 Available online 4 April 2016 Keywords: Computerized method Oxidation induction period Oxidation stability Vegetable oils
a b s t r a c t A novel cost-effective computerized method is proposed for the determination of the oxidation induction period (OIP) of organic substrates. All component parts of the device are factory-made (interchangeable) details, which are widely used in laboratory practice. A sample is charged into the reactor, the closed reactor-condenser-stopcock system is evacuated. Then the device is filled with oxygen. The reactor is warmed up to balance with atmospheric pressure. The device is sealed and stirring of the heating medium, substrate and digital data transfer from the pressure sensor to the computer is started. The monitoring of the autooxidation of the sample is terminated after the system pressure has decreased by 300 hPa. A test report is formed with the aid of a computer using Microsoft Excel; OIP is determined as the point of intersection of two tangent lines to the curve. The proposed method is on a par with standardized methods in terms of the repeatability of measurement results. © 2016 Published by Elsevier B.V.
1. Introduction Vegetable oils and a number of their synthetic derivatives, such as biodiesel, biobased lubricants, etc., contain fragments of molecules with double bonds and are oxidized therefore quickly, losing necessary consumer properties. With the increase in the degree of unsaturation of molecules the autooxidation process occurs at a faster rate. A number of analytical instrumental methods are used for the routine evaluation of the oxidation degree of above substrates [1]. However, there is no universal method which would comprehensively bring to light all changes that take place during the autooxidation of substrate. Therefore, the necessity arises to choose every time a convenient and adequate method which is suitable for a particular case [2]. The overwhelming majority of organic substrates are oxidized by a free-radical chain mechanism to form peroxide compounds as primary oxidation products [3]. The time interval before the accumulation of a noticeable amount of peroxide compounds or their derivatives (secondary oxidation products) is commonly called an oxidation induction period (OIP), which is in fact a measure of the oxidation stability of substrate [4]. Among the numerous types of testing, those are most frequently used which are a basis of valid standards and are carried out on modern devices with high
∗ Corresponding author. E-mail addresses: l [email protected], [email protected] (L.I. Koval). http://dx.doi.org/10.1016/j.tca.2016.04.002 0040-6031/© 2016 Published by Elsevier B.V.
level of automation and computerization of the determination of substrate OIP. The above methods can be arbitrarily divided according to the design of the pneumatic schemes, viz into flow- and hermetic measuring systems. The first type includes standardized test methods – ISO 6886 (oils and fats) [5], EN 14112 (fatty acid methyl esters – biodiesel) [6], in which the purified air stream is passed through the sample preheated to 100–110 ◦ C (Rancimat method). The gaseous substances formed during oxidation are entered together with air in a flask, filled with distilled water, with an electrode placed in it for the measurement of electrical conductivity. The device registers the end of an induction period when conductivity begins to increase rapidly. This type also includes the determination of substrate OIP by differential scanning calorimetry (DSC) and pressurized differential scanning calorimetry (PDSC) in accordance with the requirements of the ASTM D6186 standard [7]. A small sample of substrate (∼3.5 mg) is placed in a test cell, which is heated to a definite temperature and filled with oxygen. A preset temperature (130–210 ◦ C) and pressure (3.5 ± 0.2 MPa) are automatically maintained in the system with fixed oxygen purge rate (100 ± 10 mL/min) over the specimen spot surface. The time interval between the beginning of oxygen passing and the beginning of exothermal reaction is defined as sample OIP. The data obtained by the DSC method correlate with the OIP values obtained by the Rancimat method [8] and also by its analogue – Oxidative Stability Index (OSI) method [9]. A hermetic measuring system is used in the method “rotating bomb oxidation test” (RBOT) in accordance with the ASTM
V.I. Dzyuba et al. / Thermochimica Acta 632 (2016) 91–93
D2272 standard [10]. A glass reactor with a 50 g sample is placed in a metallic container (autoclave), the system is filled with oxygen up to 6.2 bar and is sealed. The kinetic regime of oxidation is achieved by stirring the substrate by rotating the autoclave with reactor about the axis and thermostating at 150 ± 0.1 ◦ C. The time interval between the moment of reaching the maximum pressure (∼13.5 bar) and the moment of sharp pressure drop at least 1.75 bar is defined as OIP. A good correlation between RBOT and PDSC techniques was shown [11]. Another example of devices with hermetic pneumatic scheme is a device with the trade mark Oxidograph [12], which consists of a magnetic stirrer with digital heat controller and a thermostated reactor, connected by a pneumatic line to a digital pressure sensor for the monitoring of pressure parameters and their transfer to the computer, where the obtained information is processed, and an OIP protocol is formed. 5.0 g of sample is placed in the reactor, the system blown through with oxygen; the device is sealed after filling with an oxidant, the sample is heated to the preset temperature, a valve system equalizes the apparatus pressure with the atmospheric one. The substrate is stirred with a PTFE-coated magnetic rod. The time interval between the moment of reaching the preset temperature by the specimen and the beginning of sharp pressure drop in the system is defined as OIP [12]. The data obtained by the Oxidograph method linearly correlate with the OIP values obtained by the Rancimat method [13]. The standardized instrumental methods for the determination of OIP have passed validation procedures; therefore, the use of them is always preferable to that of other methods, especially under the conditions of commercial production of oils (substrates). However, for sporadic measurement, e.g. in research laboratories, the equipment for the above methods is too expensive. Therefore, the aim of this work was to develop a cost-effective instrumental method for the determination of OIP substrates, which would ensure a high level of repeatability of obtained results.
Fig. 1. Scheme of the device for determining oxidation stability (1) – hot-plate magnetic stirrer; (2) – PTFE-coated cylindrical magnetic bar; (3) – heating medium bath; (4) – PTFE-coated oval magnetic bar; (5) – glass reactor, joint 14/20; (6) – Graham condenser, joint 14/20; (7) – adapter, joint 14/20; (8) – vacuum tubing; (9) – T-bore vacuum stopcock; (10) – digital pressure sensor; (11)–computer).
400 Pressure drop (hPa)
92
350 300 250 200 150 100 50
193
0
2. Experimental
0
50
100
150
200
250
Time (min)
Different vegetable oils: sunflower oil, olive oil, linseed oil, purchased in local market, were used in the work. 2.5 g of vegetable oil is charged into the reactor, an oval magnetic bar is introduced, and the device is assembled according to the schematic shown in Fig. 1. The vacuum stopcock is connected to the vacuum line. The closed reactor-condenser-stopcock system is evacuated with stirring for 10 min at a pressure of about 15 Pa, whereby the tightness of the system is checked, and the volatiles, which accumulate in a trap immersed in liquid nitrogen, are removed from the substrate. Then the device is filled with oxygen at a low excess pressure (∼1050 hPa). The reactor is warmed up several minutes to the preset temperature without stirring the heating medium and substrate with equalization of the system pressure with the atmospheric one, which is indicated by the cessation of oxygen flow in the bubbler. The device is sealed, and stirring of the heating medium and substrate and digital data transfer from the pressure sensor to the computer is started. The monitoring (two measurements per minute) of the autooxidation of the sample is terminated after the system pressure has decreased by 300 hPa. A test report, a typical example of which is shown in Fig. 2, is formed with the aid of a computer using Microsoft Excel; OIP is determined as the point of intersection of two tangent lines to the curve (manual calculation [6]). Two sequential measurements are made under the same conditions. The check of the precision of results obtained by this method is identical to that used in the standardized method-analog [5]. For the repeatability limit obtained in the test, a probability of 95% holds.
Fig. 2. Autooxidation of sunflower oil at 110 ◦ C.
3. Results and discussion Table 1 lists OIP values for sunflower oil at a fixed temperature (110 ± 0.2 ◦ C) with samples of different mass and with the use of reactors of different capacity. The best repeatability of results was observed for the samples with a mass of ∼2.5 g at a nominal reactor volume of 10 mL (optimal sample). The above data indicate a high reproducibility of OIP values in spite of the considerable difference in the mass of samples and in the capacity of reactors. This indicates that a reserve of compensating ability of the pneumatic scheme (Fig. 1) is sufficient for the stable operation of the device. All component parts of the device are factory-made (interchangeable) details, which are widely used in laboratory practice. Thus, the above device can be assembled from improvised means practically in any chemical laboratory. The use of an efficient magnetic stirrer with heater and digital heat controller and a cylindrical magnetic bar makes it possible to reliably (±0.2 ◦ C) control the temperature of the heating medium (silicone oil) and to vary the experimental conditions (stirring rate and temperature) over a wide range. It has been found experimentally that the required excess of bath temperatures above the nominal substrate temperature values is 1.5 ◦ C, which corresponds with the data for the Rancimat method [14]. This makes it possible to use this device for testing substrates as heterophase mixtures, ensuring the
V.I. Dzyuba et al. / Thermochimica Acta 632 (2016) 91–93
93
Table 1 Oxidation induction period of sunflower oil at 110 ◦ C. #
Sample mass (g) 2.5 1.5 3.5 9.3
1 2 3 4
Reactor capacity (mL)
Induction period (min)
Discrepancy in comparison with the optimal sample (%)
10 10 10 50
192 188 198 188
– 2.1 3.1 2.1
Table 2 Oxidation induction period of sunflower, olive and linseed oil at different temperatures.
1 2 3 4 5
#
Substrate
Test temperature (±0.2 ◦ C)
Induction period (min)
Discrepancy between two parallel tests (%)
1.1 1.2 2.1 2.2 3.1 3.2 4.1 4.2 5.1 5.2
Sunflower oil
100
0.2
Sunflower oil
110
Sunflower oil
120
Olive oil
120
Linseed oil
110
421 420 192 193 75 76 333 337 103 102
kinetic regime of oxidation. The use of a condenser reliably isolates the volatile substrate oxidation products (ideal shape of oxidation curves [5]), making it possible to use an unprotected pressure sensor (household barometer, resolution ±0.01 hPa, absolute measurement error ±1.0 hPa), greatly reducing thereby the total cost of the device. The spiral condenser (Graham) was selected based on the principle of the maximum surface area-to-volume ratio. The temperature of the cooling liquid (water) in the condenser can be varied between 5 and 25 ◦ C, but must be stable in time. Since the daily water temperature variation in the water-supply system is not over 0.5 ◦ C, this cooling medium is quite suitable for the realization of the method, which does not rule out the use of a thermostat for the above purpose. Table 2 presents results of oxidation stability tests of sunflower oil at different temperatures and the optimum sample mass (2.5 g) and reactor capacity (10 mL); it also lists OIP values for vegetable oils with a different degree of polyunsaturation of fatty acid substituents, olive and linseed oils. These data indicate that the developed method compares very favorably on the required values for the method [5] in the context of repeatability of the results. 4. Conclusion A cost-effective computerized method for the determination of the oxidation induction period of organic substrates has been developed, which is on a par with standardized methods in terms of the repeatability of measurement results. The use of standardized goods of mass production as component parts of the device and an accessible computer program can attract attention of a large circle of potential users to the developed method. References [1] N.J. Fox, G.W. Stachowiak, Vegetable oil-based lubricants—a review of oxidation, Tribol. Int. 40 (2007) 1035–1046.
0.5 1.3 1.1 1.0
[2] F. Shahidi, Y. Zhong, Lipid oxidation: measurement methods, in: F. Shahidi (Ed.), Bailey’s Industrial Oil and Fat Products, sixth ed., John Wiley & Sons Inc Hoboken, NJ, 2005, pp. 357–385. [3] A. Adhvaryu, S.Z. Erhan, Z.S. Liu, J.M. Perez, Oxidation kinetic studies of oils derived from unmodified and genetically modified vegetables using pressurized differential scanning calorimetry and nuclear magnetic resonance spectroscopy, Thermochim. Acta 364 (2000) 87–97. [4] K. Tiang, P.K. Dasgupta, Determination of oxidative stability of oils and fats, Anal. Chem. 71 (1999) 1692–1698. [5] Animal and vegetable fats and oils – Determination of oxidative stability (Accelerated oxidation test), International Standard, ISO 6886, 1996(E). [6] Fat and oil derivatives – Fatty acid methyl esters (FAME) – Determination of oxidation stability (accelerated oxidation test), European Standard, EN 14112, 2003 E. [7] Standard Test Method for Oxidation Induction Time of Lubricating Oils by Pressure Differential Scanning Calorimetry (PDSC), ASTM International, USA ASTM D6186-98. [8] S. Arain, S.T.H. Sherazi, M.I. Bhanger, Farah N. Talpur, S.A. Mahesar, Oxidative stability assessment of Bauhinia purpurea seed oil in comparison to two conventional vegetable oils by differential scanning calorimetry and Rancimat methods, Thermochim. Acta 484 (2009) 1–3. [9] C.P. Tan, Y.B. Che Man, J. Selamat, M.S.A. Yusoff, Comparative studies of oxidative stability of edible oils by differential scanning calorimetry and oxidative stability index methods, Food Chem. 76 (2002) 385–389. [10] Standard Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel, ASTM, International, USA, ASTM D 2272-02. [11] Y. Wu, W. Li, M. Zhang, X. Wang, Improvement of oxidative stability of trimethylolpropane trioleate lubricant, Thermochim. Acta 569 (2013) 112–118. [12] M. Nogala-Kalucka, J. Korczak, M. Dratwia, E. Lampart-Szczapa, A. Siger, M. Buchowski, Changes in antioxidant activity and free radical scavenging potential of rosemary extract and tocopherols in isolated rapeseed oil TAGs during accelerated tests, Food Chem. 93 (2005) 227–235. [13] I. Hradkova, R. Merkl, J. Smidrkal, J. Kyselka, V. Filip, Antioxidant effect of mono- and dihydroxyphenols in sunflower oil with different levels of naturally present tocopherols, Eur. J. Lipid Sci. Technol. 115 (2013) 747–755. [14] Y.C. Liang, C.Y. May, C.S. Foon, M.A. Ngan, C.C. Hock, Y. Basiron, The effect of natural and synthetic antioxidants on the oxidative stability of palm diesel, Fuel 85 (2006) 867–870.