ASTM D7590-22

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7590 − 09 (Reapproved 2014) D7590 − 22
Standard Guide for
Measurement of Remaining Primary Antioxidant Content In
In-Service Industrial Lubricating Oils by Linear Sweep
Voltammetry
This standard is issued under the fixed designation D7590; 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.
INTRODUCTION
Under normal thermal and oxidative working conditions, which degrade the chemical composition
of the oil’s basestock and gradually deplete the oil’s additive package, good oil condition monitoring
procedures are necessary to determine and planning corrective actions before the oil properties
changes have passed their warning limits. Antioxidant monitoring practices are a vital part of modern
oil condition monitoring practices to achieve lubrication excellence. This guide addresses the correct
guidelines for voltammetric data interpretation.
1. Scope Scope*
1.1 This guide covers the voltammetric analysis for qualitative measurements of primary antioxidants in new or in-service type
industrial lubricants detectable in concentrations as low as 0.0075 0.0075 % by mass percent 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 guide can be used as a resource for a condition monitoring program to track the oxidative health of a range of industrial
lubricants which contain primary antioxidants. In order to avoid excessive degradation of the base-oil, these primary antioxidants
play a major role to protect the lubricants against thermal-oxidative degradation. This guide can help users with interpretation and
troubleshooting results obtained using linear sweep voltammetry (LSV).
1.3 When used as part of oil condition monitoring practices, it is important to apply trend analysis to monitor the antioxidant
depletion rate relative to a baseline sample rather than use voltammetry for an absolute measurement of the antioxidant
concentration. The trending pattern provides a proactive means to identify the level of oil degradation or abnormal changes in the
condition of the in-service lubricant.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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.
1.6 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.
This guide 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. 15, 2022. Published July 2014December 2022. Originally approved in 2009. Last previous edition approved in 20092014 as
ε1
D7590 – 09 (2014). . DOI: 10.1520/D7590-09R14.10.1520/D7590-22.
*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
D7590 − 22
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
D6304 Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl
Fischer Titration
D6810 Test Method for Measurement of Hindered Phenolic Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep
Voltammetry
D6971 Test Method for Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content in Non-zinc Turbine Oils
by Linear Sweep Voltammetry
D7214 Test Method for Determination of the Oxidation of Used Lubricants by FT-IR Using Peak Area Increase Calculation
2.2 ISO Standards:
ISO 4406.2 Hydraulic fluid power—Fluids—Method for coding the level of contamination by solid particles
2.3 Other Standards:
VGB Guideline VGB-M 416 M In-Service Monitoring of Turbine Oils
3. Terminology
3.1 Definitions:
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. Oil Condition Monitoring Programs
4.1 Most industrial lubricants consist of mineral or synthetic oils compounded with oxidation and rust inhibitors. Depending upon
their application and the performance level desired, specific required amounts of other additives such as metal deactivators, pour
depressants, extreme pressure additives, and foam suppressants can also be present.
4.2 With modern formulations of industrial lubricants, the antioxidants play a major role in protecting the base-oil against
excessive degradation. To prevent this base-oil degradation, resulting in the eventual build-up of deposits, varnish and sludge, the
monitoring of the antioxidants represents a proactive information on the remaining oxidative health of the in-service lubricant.
Oxidation is a chemical reaction between oxygen atoms with the base oil hydrocarbon molecules, which are converting the
hydrocarbon molecules into oxidation products and subsequently weak organic acids. The rate of oxidation depends on the
presence of antioxidant additives, which controls the speed of oxidation, but eventually the antioxidants are consumed.
Consequently as part of modern proactive maintenance strategies, it is vital to know at any time during the operating cycle of the
lubricants, its condition by assessing the remaining activity of antioxidants, to prevent the oxidative degradation of the base oil.
4.3 Antioxidant monitoring guidelines have been part of International Standards such as Practice D4378, Practice D6224, and
VGB Guideline VGB-M 416 M, as well International OEM Maintenance Specifications. This guide presents guidelines for the
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 International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http://www.iso.org.
Available from VGB PowerTech e.V., P. O. Box 10 39 32, D-45039 Essen, Klinkestraße 27 - 31, D-45136 Essen, http://www.vgb.org.
Inczedy, J., Lengyel, T., and Ure, A. M., Orange Book: IUPAC Compendium on Analytical Nomenclature, Definitive Rules 1997, 3rd Edition, Blackwell Science, 1998.
D7590 − 22
lubricant professionals using voltammetric techniques as part of their regular maintenance strategies, such as data interpretation,
oil analysis frequency, combination with other condition monitoring tests, etc.
5. Summary of Linear Sweep Voltammetric (LSV) Test Method
5.1 Linear Sweep Voltammetric (LSV) test can be performed on any type of industrial lubricant containing at least one type of
antioxidant. The voltammetric test is a comparative test method. By establishing a comparison between its reference oil (fresh oil
or standard) and its used oil, this guide can be used without the specific knowledge on the category to which the antioxidants
belong.
5.2 ASTM International has two standards, Test Method D6810 and D6971, that shall enable the measurement of the remaining
phenolic and aminic type of antioxidants. No standard test method has been developed for the detection of other type of
antioxidants by linear voltammetry, although LSV also has detection capabilities for these types of secondary antioxidants (such
as zinc dialkyl dithiophosphates).
5.3 A measured quantity of sample is dispensed into a vial containing a measured quantity of a selected test solution and
containing a layer of sand. When the vial is shaken, the antioxidants and other solution soluble oil components present in the
sample are extracted into the electrolytic 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 antioxidants dissolved in the test solution are quantified
by voltammetric analysis. The results are calculated and reported as mass 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 in-service oils.
5.4 Voltammetric analysis is a technique that applies electroanalytic methods wherein a sample to be analyzed is mixed with an
electrolyte and a solvent (acetone or ethanol based), 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, 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 electroactive 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
electroactive 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.
6. Significance and Use
6.1 The quantitative determination of remaining antioxidants for in-service industrial oils by measuring the amount of these
additives that have been added to the oil as protection against oxidation. Industrial lubricants, such as turbine oils, compressor oils,
gear oils, hydraulic oils, bearing lubricants and greases can be formulated with a wide variety of antioxidants types such as phenols
and amines (as primary antioxidants), which are working synergistically and therefore all important to be monitored individually.
For in-service oils, the LSV determines and compares the amount of original primary antioxidants remaining after oxidation have
reduced its initial concentration.
6.2 This guide covers procedures for primary antioxidants such as amines and phenols, as described by Test Method D6971 and
D6810.
6.3 LSV is not designed or intended to detect all of the antioxidant intermediates formed during the thermal and oxidative stressing
“Remaining Useful Life Measurements of Diesel Engine Oils, Automotive Engine Oils, Hydraulic Fluids, and Greases Using Cyclic Voltammetric Methods,” STLE,
Lubrication Engineering, Vol 51, 3, pp. 223–229.
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FIG. 1 Zinc Dialkyl Dithiophosphate (ZDDP) Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
of the oils, which are recognized as having some contribution to the remaining useful life of the used or in-service oil. In order
to measure the overall stability of an oil (including contribution of intermediates present), and 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 Practice D4378 and Practice D6224).
6.4 This guide is applicable to a wide range of industrial oils, both mineral or synthetic based, which can contain rust and oxidation
inhibitors, antiwear additives such as zinc dialkyl dithiophosphates on gear oils, circulating oils, transmission oils and other
industrial lubricating oils.
6.5 The test is also suitable for manufacturing control and specification acceptance.
6.6 When a voltammetric analysis is obtained for a industrial lubricant inhibited with at least one type of antioxidant, there is an
increase in the current of the produced voltammogram between 55 s to 8 s (or 0.50.5 V to 0.8 V 0.8 V applied voltage) (see Note
1) for the zinc dialkyl dithiophosphate type of antioxidant (Fig. 1), 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) (Fig. 2) (see Note 1) for the aromatic amines, and increase in the current
of the produced voltammogram between 1313 s and 16 s 16 s (or 1.31.3 V to 1.6 V 1.6 V applied voltage) (see Note 1) for the
hindered phenols or carbamates in the neutral acetone solution (Fig. 2: x-axis 1 s = 0.1 V), or both. 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. 1-6 were obtained with a platinum reference electrode
and a voltage scan rate of 0.1 V/s.
6.7 For industrial lubricants containing zinc dialkyl dithiophosphate type of antioxidants, there is an increase in the current of the
produced voltammogram between 55 s to 8 s (or 0.50.5 V to 0.8 V applied voltage) (see Note 1) by using the neutral acetone test
solution ( see (see Fig. 1). There is no corresponding ASTM International standard describing the test method procedures for
measuring zinc dialkyl dithiophosphates type of antioxidants in industrial lubricants.
6.8 For industrial lubricants containing only aromatic amines as 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, by using the
neutral acetone test solution (first peak in Fig. 2) as described in Test Method D6971.
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FIG. 2 Aromatic Amine and Hindered Phenol Voltammetric Response in the Neutral Test Solution with Blank Response Zeroed
FIG. 3 Hindered Phenol Voltammetric Response in Basic Test Solution with Blank Response Zeroed
6.9 For industrial lubricants containing only hindered phenolic antioxidants, it is preferable to use a basic alcohol solution rather
than the neutral acetone solutions, to achieve an increase in the current of the produced voltammogram between 33 s to 6 s (or
0.30.3 V to 0.6 V applied voltage) (see Note 1) in basic alcohol solution (Fig. 3: x-axis 1 s = 0.1 V) as described in Test Method
D6810.
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FIG. 4 Voltammetric Reading for an In-service Oil Sample Comparing Aromatic Amines (additive #1) and Hindered Phenols (additive #2)
Peaks (in the Neutral Test Solution)—Standard (top line) and Sample In-Service Oil (lower line)
FIG. 5 a Filming Problems Due to Oil Solubility
7. Voltammetric Test Apparatus
7.1 Voltammetric Analyzer —Specifically designed to perform antioxidant determinations of industrial oils. The instrument used
to quantify the hindered phenol and aromatic amine antioxidants is a voltammograph equipped with a three-electrode system
(referred further to as the probe) 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 and D6971. The voltammetric analyzer applies a linear voltage ramp
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FIG. 5 b Filming Due to Additive Concentration (continued)
FIG. 5 c Filming Problems Due to Oil Solubility (continued)
(0(0 V to –1.7 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.
7.2 Vortex Mixer—A vortex mixer with a 28002800 r ⁄min to 3000 r ⁄min motor and a pad suitable for mixing test tubes and vials.
Ultrasonic shakers may also be used to achieve a quick and efficient shaking of the prepared test solution.
7.3 Pipet—or equivalent, capable of delivering sample volumes required in this guide from 0.100.10 mL to 0.50 mL.
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FIG. 6 Shifting of Antioxidant Peaks Due to Oil Solubility
7.4 Solvent Dispenser—or equivalent, capable of delivering volumes of analytical test solution (see 6.37.3) required in this guide,
such as 3.03.0 mL and 5.0 mL.
7.5 Glass Vials with Caps—44 mL or 7 mL capacity, and containing 1 g of sand.
7.6 Sand—Required to be white quartz suitable for chromatography within the size range of 200200 μm to 300300 μm 6 100
microns.100 μm.
8. Sampling
8.1 Obtain the sample in accordance with Practice D4057.
9. Test Solutions – Reagents and Selection
9.1 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.
9.2 Purity of Water—Unless otherwise specified, references to water that conforms to Specification D1193, Type II water.
9.3 Analysis Materials:
9.3.1 Acetone–Based Test Solution (Neutral)—Proprietary Green Analytical Test Solution, acetone solvent (1:10 water/acetone
solution) containing dissolved neutral electrolytes.
Reagent Chemicals, American Chemical Society Specifications,ACS Reagent Chemicals, Specifications and Procedures for Reagents and Standard-Grade Reference
Materials, American Chemical Society, Washington, DC. For Suggestionssuggestions on the testing of reagents not listed by the American Chemical Society, see
AnnualAnalar 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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9.3.2 Warning—Corrosive, Poison, Flammable, and Skin Irritant. Harmful if inhaled.
9.3.3 Alcohol–Based Test Solution (Basic)—Proprietary Yellow Analytical Test Solution, ethanol solvent (1:10 water/ethanol
solution) containing dissolved base electrolytes.
9.3.4 Warning—Corrosive, Poison, Flammable, and Skin Irritant. Harmful if inhaled.
9.3.5 Alcohol Cleansing Pads—70%70 % isopropyl alcohol saturated cleansing pads (alcohol prepared skin cleansing pads, for
the preparation of the skin prior to injection (antiseptic).
10. Procedure
10.1 The voltammetric analyzer used in the LSV method gives linear results between 22 mmol to 50 mmol for all type of
antioxidants using an oil sample size of 0.40 mL and 5.0 mL of the analytical test solutions. The corresponding range of mass
percents depends on the molecular weight of the type of antioxidant, and the density of the base oil. For instance, the mass percent
range of 0.044 to 1.1 is equal to 22 mmol ⁄L 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 solutions remains at
5.0 mL.
10.2 General Voltammetric Test Procedure—The test procedure for voltammetric analysis consists of the blank reading
(calibration), followed by a standard reading and finally the sample (in-service oil) reading.
10.2.1 Blank Reading (0 mmol/L = 0 mass percent)—0 % by mass)—The blank reading (voltammetric number) is a measurement
of the analytical test solution by itself. The blank measurement gives a reference number with no antioxidants present (the zero
baseline).
10.2.2 Standard Reading (30(30 mmol ⁄L to 150 mmol/L – mass percent dependent on density of fresh oil and molecular weight
of antioxidant)—The standard reading is a measurement of a fresh, unused oil (containing at least one type of antioxidant) mixed
with an appropriate analytical test solution. This measurement gives you a voltammetric reading (standard reading) that
corresponds with the voltammetric response for the 100%100 % of antioxidant’s concentration (Additive RUL%RUL % =
100%)100 %) oil being tested or analyzed.
10.2.3 Sample (In-Service Oil), Reading—The samp
...