ASTM D6810-22

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Designation: D6810 − 21 D6810 − 22
Standard Test Method for
Measurement of Hindered Phenolic Antioxidant Content in
Non-Zinc Turbine Oils by Linear Sweep Voltammetry
This standard is issued under the fixed designation D6810; 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 antioxidants in new or in-service non-zinc turbine
oils in concentrations from 0.0075 % by weight up to concentrations found in new oils by measuring the amount of current flow
at a specified voltage in the produced voltammogram.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.4 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
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
D6447 Test Method for Hydroperoxide Number of Aviation Turbine Fuels by Voltammetric Analysis
D6971 Test Method for Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content in Non-zinc Turbine Oils
by Linear Sweep Voltammetry
3. Terminology
3.1 For definitions of terms used in this method, refer to Terminology D4175.
3.1 Definitions:
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 July 1, 2021July 1, 2022. Published August 2021August 2022. Originally approved in 2002. Last previous edition approved in 20132021 as
D6810 – 13.D6810 – 21. DOI: 10.1520/D6810-21.10.1520/D6810-22.
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.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6810 − 22
3.1.1 See Terminology D4175 for a more extensive list of terms used in this test method.
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 alcohol-based electrolyte solution
and containing a layer of sand. When the vial is shaken, the hindered phenol antioxidants and other solution soluble oil components
present in the sample are extracted into the solution and the remaining droplets suspended in the solution are agglomerated by the
sand. The sand/droplet suspension is allowed to settle out and the hindered phenol antioxidants dissolved in the solution are
quantified by voltammetric analysis. The results are calculated and reported as weight percent of antioxidant or as millimoles
(mmol) of antioxidant per litre of sample for prepared and fresh oils and as a percent remaining antioxidant for used oils.
4.2 Voltammetric analysis is a technique that applies electro-analytic methods when a sample to be analyzed is mixed with an
electrolyte and a solvent 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 nonpolarizable
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, auxiliary electrodes, can be added to the electrode system to eliminate the effects of resistive drop for high resistance
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.
5. Significance and Use
5.1 The quantitative determination of hindered phenol antioxidants in a new turbine oil measures the amount of this material 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 used oil, the determination measures the amount of original (phenolic) antioxidant
remaining after oxidation have 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 used or 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 used
oil, which might result in the replacement of the oil reservoir, it is advised to perform additional analytical techniques (in
accordance with Practices D6224 and D4378), having the capability of measuring remaining oxidative life of the used oil.
5.1.1 This test method is applicable to non-zinc turbine oils. These are refined mineral oils containing rust and oxidation inhibitors,
but not antiwear additives. This test method has not yet been established with sufficient precision for antiwear oils.
5.2 This test method is also suitable for manufacturing control and specification acceptance.
Inczedy, J., Lengyel, T., and Ure, A. M., Orange Book: IUPAC Compendium on Analytical Nomenclature, Definitive Rules 1997, 3rd Edition, Blackwell Science, 1998.
D6810 − 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
indicator before and after standard.
FIG. 1 Hindered Phenol Voltammetric Response in Basic Test Solution with Blank Response Zeroed
5.3 When a voltammetric analysis is obtained for a turbine oil inhibited with a typical hindered phenol antioxidant, there is an
increase in the current of the produced voltammogram between 3 s to 5 s (or 0.3 V to 0.6 V applied voltage) (see Note 1) in the
basic test solution (Fig. 1—x-axis 1 second = 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).
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.
5.4 For non-zinc turbine oils containing aromatic (aryl) amine compounds (antioxidants and corrosion inhibitors), there is an
increase in the current of the produced voltammogram between 7 s to 11 s (0.7 V to 1.1 V applied voltage in Fig. 2) (see Note 1)
which does not interfere with the hindered phenol measurement in the basic test solution. For the measurement of these aromatic
amine antioxidants, refer to Test Method D6971, where the neutral test solution shall be used.
6. Apparatus
6.1 Voltammetric Analyzer—The instrument used to quantify the hindered phenol 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 D6447. The voltammetric analyzer applies a linear voltage ramp (0 V to -1.8 V
range with respect to the reference electrode) at a rate of 0.01 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 V to 1 V full scale) as shown in Figs. 1 and 2.
6.2 Vortex Mixer, with a 2800 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 this test method, from 0.10 mL to 0.50 mL.
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NOTE 1—x-axis = time (seconds) and y-axis is current (arbitrary units). Top line in Fig. 2 is fresh oil, and lower line is used oil.
FIG. 2 Amine and Hindered Phenols Peaks in the Basic Test Solution with Blank Response Zeroed
6.4 Solvent Dispenser, or equivalent, capable of delivering volumes of analysis solution (see 8.3) required in this test method, such
as 3.0 mL and 5.0 mL.
6.5 Glass Vials, with caps, 4 mL or 7 mL capacity; and containing 1 g of sand white quartz suitable for chromatography, within
the size range of 200 μm to 300 μm 6 100 μm.
7. Sampling
7.1 Obtain the sample in accordance with Practice D4057.
8. Reagents
8.1 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 is sufficiently pure to
permit its use without lessening the accuracy of the determination.
8.2 Purity of Water—Unless otherwise specified, references to water that conforms to Specification D1193, Type II.
8.3 Analysis Materials:
ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference Materials, American Chemical Society, Washington, DC. For
suggestions on the testing of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and
the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.
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8.3.1 Alcohol Test Solution (Basic
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