ASTM D6971-09(2014)

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Designation: D6971 − 09 D6971 − 09 (Reapproved 2014)
Standard Test Method for
Measurement of Hindered Phenolic and Aromatic Amine
Antioxidant Content in Non-zinc Turbine Oils by Linear
Sweep Voltammetry
This standard is issued under the fixed designation D6971; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method covers the voltammetric determination of hindered phenol and aromatic amine antioxidants in new or
in-service type non-zinc turbine oils in concentrations from 0.0075 mass % up to concentrations found in new oils by measuring
the amount of current flow at a specified voltage in the produced voltammogram.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D2272 Test Method for Oxidation Stability of Steam Turbine Oils by Rotating Pressure Vessel
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4378 Practice for In-Service Monitoring of Mineral Turbine Oils for Steam, Gas, and Combined Cycle Turbines
D6224 Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment
D6810 Test Method for Measurement of Hindered Phenolic Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep
Voltammetry
2.2 ISO Standards:
ISO 6743 Part 4, Lubricants, Industrial Oils, and Related Products
3. Summary of Test Method
3.1 A measured quantity of sample is dispensed into a vial containing a measured quantity of acetone based electrolyte test
solution and a layer of sand. When the vial is shaken, the hindered phenol and aromatic amine antioxidants and other test solution
soluble oil components present in the sample are extracted into the test solution and the remaining droplets suspended in the test
solution are agglomerated by the sand. The sand/droplet suspension is allowed to settle out and the hindered phenol and aromatic
amine antioxidants dissolved in the test solution are quantified by voltammetric analysis. The results are calculated and reported
as mass % of antioxidant or as millimoles (mmol) of antioxidant per litre of sample for prepared and fresh oils and as a percent
remaining antioxidant for in-service oils.
3.2 Voltammetric analysis is a technique that applies electro-analytic methods wherein a sample to be analyzed is mixed with
an electrolyte and a test solution, and placed within an electrolytic cell. Data is obtained by measuring the current passing through
the cell as a function of the potential applied, and test results are based upon current, voltage, and time relationships at the cell
electrodes. The cell consists of a fluid container into which is mounted a small, easily polarized, working electrode, and a large,
non-polarizable, reference electrode. The reference electrode should be massive relative to the working electrode so that its
behavior remains essentially constant with the passage of small current; that is, it remains unpolarized during the analysis period.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.09.0C on Oxidation of Turbine Oils.
Current edition approved June 1, 2009May 1, 2014. Published June 2009July 2014. Originally approved in 2004. Last previous edition approved in 20042009 as
D6971D6971 – 09.–04. DOI: 10.1520/D6971-09.10.1520/D6971-09R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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D6971 − 09 (2014)
NOTE 1—x-axis = time (seconds) and y-axis is current (arbitrary units). Top line in Fig. 1 is voltammogram of a fresh R&O turbine oil showing valley
indicators before and after antioxidant valleys.
FIG. 1 Aromatic Amine and Hindered Phenol Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
Additional electrodes, such as auxiliary electrodes, can be added to the electrode system to eliminate the effects of resistive drop
for high resistance test solutions. In performing a voltammetric analysis, the potential across the electrodes is varied linearly with
time, and the resulting current is recorded as a function of the potential. As the increasing voltage is applied to the prepared sample
within the cell, the various additive species under investigation within the oil are caused to electrochemically oxidize. The data
recorded during this oxidation reaction can then be used to determine the remaining useful life of the oil type. A typical
current-potential curve produced during the practice of the voltammetric test can be seen by reference to Fig. 1. Initially the applied
potential produces an electrochemical reaction having a rate so slow that virtually no current flows through the cell. As the voltage
is increased, as shown in Fig. 1, the electro-active species (for example, substituted phenols) begin to oxidize at the working
electrode surface, producing an anodic rise in the current. As the potential is further increased, the decrease in the electro-active
species concentration at the electrode surface and the exponential increase of the oxidation rate lead to a maximum in the
current-potential curve shown in Fig. 1.
4. Significance and Use
4.1 The quantitative determination of hindered phenol and aromatic amine antioxidants in a new turbine oil measures the
amount of these compounds that has been added to the oil as protection against oxidation. Beside phenols, turbine oils can be
formulated with other antioxidants such as amines which can extend the oil life. In in-service oil, the determination measures the
amount of original (hindered phenol and aromatic amine) antioxidants remaining after oxidation has reduced its initial
concentration. This test method is not designed or intended to detect all of the antioxidant intermediates formed during the thermal
and oxidative stressing of the oils, which are recognized as having some contribution to the remaining useful life of the in-service
oil. Nor does it measure the overall stability of an oil, which is determined by the total contribution of all species present. Before
making final judgment on the remaining useful life of the in-service oil, which might result in the replacement of the oil reservoir,
it is advised to perform additional analytical techniques (as in accordance with Test Methods D6224 and D4378; see also Test
Method D2272), having the capability of measuring remaining oxidative life of the in-service oil.
4.1.1 This test method is applicable to non-zinc type of turbine oils as defined by ISO 6743 Part 4, Table 1. These are refined
mineral oils containing rust and oxidation inhibitors, but not antiwear additives.
4.2 The test is also suitable for manufacturing control and specification acceptance.
4.3 When a voltammetric analysis is obtained for a turbine oil inhibited with a typical synergistic mixture of hindered phenol
and aromatic amine antioxidants, there is an increase in the current of the produced voltammogram between 8 to 12 s (or 0.8 to
1.2 V applied voltage) (see Note 1) for the aromatic amines, and an increase in the current of the produced voltammogram between
13 and 16 s (or 1.3 to 1.6 V applied voltage) (see Note 1) for the hindered phenols in the neutral acetone test solution (Fig. 1: x-axis
1 s = 0.1 V). Hindered phenol antioxidants detected by voltammetric analysis include, but are not limited to, 2,6-di-tert-butyl-
4-methylphenol; 2,6-di-tert-butylphenol; and 4,4’-Methylenebis (2,6-di-tert-butylphenol). Aromatic amine antioxidants detected
by voltammetric analysis include, but are not limited to, phenyl alpha naphthylamines, and alkylated diphenylamines.
D6971 − 09 (2014)
NOTE 1—x-axis = time (seconds) and y-axis is current (arbitrary units) with top line in Fig. 2 showing the fresh oil.
FIG. 2 Hindered Phenol Voltammetric Response in Basic Test Solution with Blank Response Zeroed
NOTE 1—Voltages listed with respect to reference electrode. The voltammograms shown in Figs. 1 and 2 were obtained with a platinum reference
electrode and a voltage scan rate of 0.1 V/s.
4.4 For turbine oil containing only aromatic amines as antioxidants, there will only be an increase in the current of the produced
voltammogram between 8 to 12 seconds (or 0.8 to 1.2 V applied voltage) (see Note 1) for the aromatic amines, by using the neutral
acetone test solution (first peak in Fig. 1).
4.5 For turbine oils containing only hindered phenolic antioxidants, it is preferable to use a basic alcohol test solution rather
than the neutral acetone test solutions, as there is an increase in the current of the produced voltammogram between 3 to 6 seconds
(or 0.3 to 0.6 V applied voltage) (see Note 1) in basic alcohol test solution (Fig. 2: x-axis 1 second = 0.1 V) in accordance with
Test Method D6810.
5. Apparatus
5.1 Voltammetric Analyzer—The instrument used to quantify the hindered phenol and aromatic amine antioxidants is a
voltammograph equipped with a three-electrode system and a digital or analog output. The combination electrode system consists
of a glassy carbon disc (3 mm diameter) working electrode, a platinum wire (0.5 mm diameter) auxiliary electrode, and a platinum
wire (0.5 mm diameter) reference electrode, as described in Test Method D6810. The voltammetric analyzer applies a linear
voltage ramp (0 to -1.8 V range with respect to the reference electrode) at a rate of 0.01 to 0.50.5 V V/s ⁄s (0.1 optimum) to the
auxiliary electrode. The current output of the working electrode is converted to voltage by the voltammetric analyzer, using the
gain ratio of 1 V/20 μA, and is outputted to an analog or digital recording device (0 to 1 V full scale) as shown in Figs. 1 and 2.
5.2 Vortex Mixer, with a 2800 to 3000 rpm motor and a pad suitable for mixing test tubes and vials.
5.3 Pipette, or equivalent, capable of delivering sample volumes required in the test method, from 0.10 to 0.50 mL.
5.4 Test Solution Dispenser, or equivalent, capable of delivering volumes of analysis test solution (see 6.3) required in the test
method, such as 3.0 and 5.0 mL.
5.5 Glass Vials, with caps, 4 or 7 mL capacity and containing 1 g of sand. White quartz suitable for chromatography, within
the size range of 200 to 300 μm 6 100 μm.
6. Reagents
6.1 Purity of Reagents—Reagent-grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all
reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where
D6971 − 09 (2014)
such specifications are available. Other grades may be used, provided it is first ascertained that the reagent’s purity suffices to
permit its use without lessening the accuracy of the determination.
6.2 Purity of Water—Unless otherwise specified, references to water that conforms to Specification D1193, Type II.
6.3 Analysis Materials:
6.3.1 Acetone Test Solution (Neutral)—Proprietary Green Test Solution, Acetone test solution (1:10 distilled water/acetone test
solution) containing a dissolved neutral electrolyte. (Warning—Corrosive, poisonous, flammable, and a skin irritant. Harmful if
inhaled.)
6.3.2 Alcohol Test Solution (Basic)—Proprietary Yellow Test Solution, Ethanol test solution (1:10 distilled water/ethanol test
solution) containing a dissolved base electrolyte. (Warning—Corrosive, poisonous, flammable, and a skin irritant. Harmful if
inhaled.)
6.3.3 Alcohol Cleansing Pads—70 % isopropyl alcohol saturated cleansing pads (alcohol prepared skin cleansing pads, for the
preparation of the skin prior to injection (antiseptic)).
7. Sampling
7.1 Obtain the sample in accordance with Practice D4057.
8. Procedure
8.1 The voltammetric analyzer used in this test method gives linear results between 2 to 50 mmol 50 mmol for hindered phenols
and aromatic amines using an oil sample size of 0.40 mL and 5.0 mL of the analysis test solution. The corresponding range of mass
% depends on the molecular weight of the hindered phenol and aromatic amine, and the density of the base oil. For instance, the
mass % range of 0.044 to 1.1 is equal to 2 to 5050 mmol mmol/L ⁄L for a hindered phenol containing one hydroxyl group and
with a molecular weight of 220 g/mol (2,6-di-tert-butyl-4-methylphenol) and an oil density of 1 g/mL. Below 2 mmol, the noise
to signal ratio becomes large, decreasing the accuracy of the measurements. For measurements below 2 mmol or for fresh oils with
high noise to signal ratios, the sample size should be increased to 0.60 mL and the volume of analysis test solution decreased to
3.0 mL.
8.2 General Voltammetric Test Procedure—The test procedure for voltammetric analysis will consist of the blank reading
(calibration), followed by a standard reading, and finally the sample (in-service oil) reading.
8.2.1 Blank Reading—(0 mmol/L = 0 mass %).
8.2.1.1 The blank reading (voltammetric number) is a measurement of the analysis test solution by itself. The blank
measurement gives a reference number with no antioxidant present (the zero baseline).
8.2.2 Standard Reading—(30 to 150 mmol/L—mass % dependent on density of fresh oil and molecular weight of antioxidant).
8.2.2.1 The standard reading is a measurement of a fresh, unused oil (containing hindered phenol and amines antioxidants)
mixed with an appropriate analysis test solution. This measurement gives you a voltammetric reading (standard reading) that
indicates the voltammetric response for the concentration hindered phenol and aromatic amines antioxidants
...