ASTM D6971-22

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Designation: D6971 − 09 (Reapproved 2014) D6971 − 22
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 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 0.0075 % by 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
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. Terminology
3.1 Definitions:
3.1.1 See Terminology D4175 for a more extensive list of terms used in this test method.
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.09.0C on Oxidation of Turbine Oils.
Current edition approved May 1, 2014Nov. 1, 2022. Published July 2014November 2022. Originally approved in 2004. Last previous edition approved in 20092014 as
D6971 – 09.D6971 – 09 (2014). DOI: 10.1520/D6971-09R14.10.1520/D6971-22.
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.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
*A Summary of Changes section appears at the end of this standard
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D6971 − 22
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
3.1.2 electrolytic cell, n—an electrochemical cell in which chemical reactions are caused by applying an external potential
difference greater than, and opposite to, the galvanic electromotive force of the cell. IUPAC
3.1.3 linear sweep voltammetry, n—a technique applied to the monitoring of antioxidant additive content in lubricants, where the
current is detected as an applied potential is increased linearly over a period of time.
3.1.4 voltammogram, n—the plot of current versus voltage.
4. Summary of Test Method
4.1 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.
4.2 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.
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
Inczedy, J., Lengyel, T., and Ure, A. M., Orange Book: IUPAC Compendium on Analytical Nomenclature, Definitive Rules 1997, 3rd Edition, Blackwell Science, 1998.
D6971 − 22
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
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.
5. Significance and Use
5.1 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.
5.1.1 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.
5.2 The test is also suitable for manufacturing control and specification acceptance.
5.3 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 88 s to 12 s (or 0.80.8 V
to 1.2 V applied voltage) (see Note 1) for the aromatic amines, and an increase in the current of the produced voltammogram
between 1313 s and 16 s (or 1.31.3 V 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.
NOTE 1—Voltages listed with respect to reference electrode. The voltammograms shown in Figs. 1 and 2 were obtained with a platinum reference electrode
D6971 − 22
and a voltage scan rate of 0.1 V/s.
5.4 For turbine oil containing only aromatic amines as antioxidants, there will only be an increase in the current of the produced
voltammogram between 88 s to 12 seconds 12 s (or 0.80.8 V to 1.2 V 1.2 V applied voltage) (see Note 1) for the aromatic amines,
by using the neutral acetone test solution (first peak in Fig. 1).
5.5 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 33 s to 6 seconds
6 s (or 0.30.3 V to 0.6 V 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.
6. Apparatus
6.1 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(0 V to -1.8 V -1.8 V range with respect to the reference electrode) at a rate of 0.010.01 V ⁄s to 0.5 V ⁄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(0 V to 1 V 1 V full scale)
as shown in Figs. 1 and 2.
6.2 Vortex Mixer, with a 28002800 rpm to 3000 rpm motor and a pad suitable for mixing test tubes and vials.
6.3 Pipette, or equivalent, capable of delivering sample volumes required in the test method, from 0.100.10 mL to 0.50 mL.
6.4 Test Solution Dispenser, or equivalent, capable of delivering volumes of analysis test solution (see 6.37.3) required in the test
method, such as 3.03.0 mL and 5.0 mL.
6.5 Glass Vials, with caps, 44 mL or 7 mL capacity and containing 1 g of sand. White quartz suitable for chromatography, within
the size range of 200200 μm to 300 μm 6 100 μm.
7. Reagents
7.1 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
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.
7.2 Purity of Water—Unless otherwise specified, references to water that conforms to Specification D1193, Type II.
7.3 Analysis Materials:
7.3.1 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.)
7.3.2 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.)
Reagent Chemicals, American Chemical Society Specifications,ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference
Materials, American Chemica
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