ASTM D4739-23

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Designation: D4739 − 17 D4739 − 23
Standard Test Method for
Base Number Determination by Potentiometric Hydrochloric
Acid Titration
This standard is issued under the fixed designation D4739; 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
Currently, there are two ASTM test methods under the jurisdiction of Committee D02 used for
determining base number by potentiometric titration: Test Method D2896 and Test Method D4739.
They are both used throughout the petroleum industry. Test Method D2896 is for new oils and is used
in setting specifications since it is more accurate than Test Method D4739. Test Method D4739 shall
be used exclusively for the purpose of tracking base number loss as an oil proceeds in service. In many
cases, the test methods will provide different results. Some comparisons are given in the Scope and
the Significance and Use sections of this test method to distinguish between the two standards and to
provide guidance to users.
1. Scope*
1.1 This test method covers a procedure for the determination of basic constituents in petroleum products and new and used
lubricants. This test method resolves these constituents into groups having weak-base and strong-base ionization properties,
provided the dissociation constants of the more strongly basic compounds are at least 1000 times than that of the next weaker
groups. This test method covers base numbers up to 250.
1.2 In new and used lubricants, the constituents that can be considered to have basic properties are primarily organic and inorganic
bases, including amino compounds. This test method uses hydrochloric acid as the titrant, whereas Test Method D2896 uses
perchloric acid as the titrant. This test method may or may not titrate these weak bases and, if so, it will titrate them to a lesser
degree of completion; some additives such as inhibitors or detergents may show basic characteristics.
1.3 When testing used engine lubricants, it should be recognized that certain weak bases are the result of the service rather than
having been built into the oil. This test method can be used to indicate relative changes that occur in oil during use under oxidizing
or other service conditions regardless of the color or other properties of the resulting oil. The values obtained, however, are
intended to be compared with the other values obtained by this test method only; base numbers obtained by this test method are
not intended to be equal to values by other test methods. Although the analysis is made under closely specified conditions, this
test method is not intended to, and does not, result in reported basic properties that can be used under all service conditions to
predict performance of an oil; for example, no overall relationship is known between bearing corrosion or the control of corrosive
wear in the engine and base number.
1.4 This test method was developed as an alternative for the former base number portion of Test Method D664 (last published in
Test Method D664 – 81).
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
D02.06 on Analysis of Liquid Fuels and Lubricants.
Current edition approved Dec. 1, 2017July 1, 2023. Published February 2018August 2023. Originally approved in 1987. Last previous edition approved in 2017 as
D4739 – 11 (2017).D4739 – 17. DOI: 10.1520/D4739-17.10.1520/D4739-23.
*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
D4739 − 23
1.4.1 Colorimetric test methods for base number are Test Method D974, IP 139, and 5102.1 on acid and base number by extraction
(color-indicator titration) of Federal Test Method Standard No. 791b. Test results by these methods may or may not be numerically
equivalent to this test method.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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.
1.7 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:
D664 Test Method for Acid Number of Petroleum Products by Potentiometric Titration
D974 Test Method for Acid and Base Number by Color-Indicator Titration
D1193 Specification for Reagent Water
D2896 Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
2.2 IP Standard:
IP 139 Petroleum products and lubricants—Determination of acid and base number—Colour-indicator titration method
2.3 U.S. Federal Test Method:
Federal Test Method Standard No. 791b Lubricants Liquid Fuels and Related Products; Methods of Testing
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer to Terminology D4175.
3.2 Definitions:Definitions of Terms Specific to This Standard:
3.2.1 base number, n—the quantity of a specified acid, expressed in terms of the equivalent number of milligrams of potassium
hydroxide per gram of sample, required to titrate a sample in a specified solvent to a specified endpoint using a specified detection
system.
3.2.1.1 Discussion—
In this test method, the sample is titrated to a meter reading corresponding to aqueous acidic buffer solution or appropriate
inflection point.
3.2.2 strong base number, n—the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium
hydroxide per gram of sample, that is required to titrate a sample dissolved in the specified solvent from the initial meter reading
to a meter reading corresponding to a basic buffer solution.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 strong base number, n—the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium
hydroxide per gram of sample, that is required to titrate a sample dissolved in the specified solvent from the initial meter reading
to a meter reading corresponding to a basic buffer solution.
4. Summary of Test Method
4.1 The sample is dissolved in a mixture of toluene, propan-2-ol (isopropyl alcohol), chloroform, and a small amount of water and
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D4739 − 23
titrated potentiometrically with alcoholic hydrochloric acid solution. The test results of this procedure are obtained by titration
mode of fixed increment and fixed time additions of the titrant. An endpoint is selected from a titration curve according to the
criteria given in Section 12 and used to calculate a base number.
5. Significance and Use
5.1 New and used petroleum products can contain basic constituents that are present as additives. The relative amount of these
materials can be determined by titration with acids. The base number is a measure of the amount of basic substances in the oil
always under the conditions of the test. It is sometimes used as a measure of lubricant degradation in service. However, any
condemning limit shall be empirically established.
5.2 As stated in 1.2, this test method uses a weaker acid to titrate the base than Test Method D2896, and the titration solvents are
also different. Test Method D2896 uses a stronger acid and a more polar solvent system than Test Method D4739. As a result, Test
Method D2896 will titrate salts of weak acids (soaps), basic salts of polyacidic bases, and weak alkaline salts of some metals. They
do not protect the oil from acidic components due to the degradation of the oil. This test method may produce a falsely exaggerated
base number. Test Method D4739 will probably not titrate these weak bases but, if so, will titrate them to a lesser degree of
completion. It measures only the basic components of the additive package that neutralizes acids. On the other hand, if the additive
package contains weak basic components that do not play a role in neutralizing the acidic components of the degrading oil, then
the Test Method D4739 result may be falsely understated.
5.3 Particular care is required in the interpretation of the base number of new and used lubricants.
5.3.1 When the base number of the new oil is required as an expression of its manufactured quality, Test Method D2896 is
preferred, since it is known to titrate weak bases that this test method may or may not titrate reliably.
5.3.2 When the base number of in-service or at-term oil is required, this test method is preferred because in many cases, especially
for internal combustion engine oils, weakly basic degradation products are possible. Test Method D2896 will titrate these, thus
giving a false value of essential basicity. This test method may or may not titrate these weak acids.
5.3.3 When the loss of base number value, as the oils proceed in service, is the consideration, this test method is to be preferred
and all values including the unused oil shall be determined by this test method. Base numbers obtained by this test method shall
not be related to base numbers obtained by another test method such as Test Method D2896.
5.3.4 In ASTM Interlaboratory Crosscheck Programs for both new and used lubricants, historically Test Method D2896 gives a
higher value for base number.
6. Apparatus
6.1 Potentiometric Titration, automatic or manual, with capability of adding fixed increments of titrant at fixed time intervals (see
Annex A1).
6.1.1 The titrimeter must automatically (or manually) control the rate of addition of titrant as follows: Delivery of titrant will be
incremental; after delivery of precisely a 0.100 mL increment (see 6.1.2), the delivery is stopped and a fixed time period of 90 s
is allowed to pass before another 0.100 mL increment of titrant is delivered. This procedure is repeated until the titration is
completed.
6.1.2 The precision of addition of the 0.100 mL increments of titrant must be 60.001 mL for automatic titrators. For manual buret,
it should be 60.005 mL. A higher incremental precision is required for an automatic buret, because the total volume to the end
point is summed from the individual increments; it is read from a scale with a manual buret.
6.2 Sensing Electrode, standard pH with glass membrane, suitable for non-aqueous titrations.
6.3 Reference Electrode, Silver/Silver Chloride (Ag/AgCl) reference electrode with sleeve junction, filled with 1 M to 3 M LiCl
in ethanol.
6.3.1 Combination Electrode—Sensing electrodes may have the Ag/AgCl reference electrode built into the same electrode body,
which offers the convenience of working with and maintaining only one electrode. The combination electrode shall have a sleeve
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junction type of reference and shall use an inert ethanol electrolyte, for example, 1 M to 3 M LiCl in ethanol. In the reference
compartment, the sensing electrode part shall use a glass membrane designed for non-aqueous titrations. These combination
electrodes shall have the same response or better response than a dual electrode system. They shall have removable sleeves for
easy rinsing and addition of electrolyte. (Warning—When a movable sleeve is part of the electrode system, ensure that the sleeve
is unimpaired before every titration.)
6.4 Stirrer, Buret, Stand, Titration Vessel, as specified in Annex A1, are required.
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 is of sufficiently high
purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water—Unless otherwise indicated, references to water shall be understood to mean reagent water that meets the
requirement of either Type I, II, or III of Specification D1193.
7.3 Buffer, Aqueous Acid—Commercial pH 3 buffer solution with a tolerance of 60.02 pH units at 25 °C. This solution shall be
replaced at regular intervals consistent with its stability or when contamination is suspected. Information related to the stability
should be obtained from the manufacturer.
7.4 Buffer, Aqueous Basic—Commercial pH 10 buffer solution with a tolerance of 60.02 pH units at 25 °C. This solution shall be
replaced at regular intervals consistent with its stability or when contamination is suspected. Information related to the stability
should be obtained from the manufacturer.
7.5 Chloroform—Reagent grade. (Warning—Toxic and suspected carcinogen.)
7.6 Hydrochloric Acid Solution, Standard Alcoholic (0.1 M)—Mix 9 mL of reagent grade hydrochloric acid (HCl, sp gr 1.19)
(Warning—Toxic and corrosive), with 1 L of anhydrous isopropyl alcohol. Standardize frequently enough to detect normality
changes of 0.0005 by potentiometric titration of approximately 8 mL (accurately measured) of the 0.1 M alcoholic KOH solution
diluted with 125 mL CO -free water.
7.7 Ethanol—reagent grade. (Warning—Flammable and toxic, especially when denatured.)
7.8 Lithium Chloride Electrolyte—Prepare a solution of 1 M to 3 M LiCl in ethanol.
7.9 Potassium Hydroxide—(Warning—Causes severe burns.)
7.10 Potassium Hydroxide Solution, Standard Alcoholic (0.1 M)—Add 6 g of reagent grade potassium hydroxide (KOH)
(Warning—Toxic and corrosive), to approximately 1 L of anhydrous isopropyl alcohol. Boil gently for 10 min to effect solution.
Allow the solution to stand for 2 days, and then filter the supernatant liquid through a fine sintered-glass funnel. Store the solution
in a chemically resistant bottle. Dispense in a manner such that the solution is protected from atmospheric carbon dioxide (CO )
by means of a guard tube containing soda lime or soda non-fibrous silicate absorbent, and such that it does not come into contact
with cork, rubber, or saponifiable stopcock grease. Standardize frequently enough to detect normality changes of 0.0005 by
potentiometric titration of weighed quantities of potassium acid phthalate dissolved in CO -free water.
7.11 Propan-2-ol (Isopropyl Alcohol)—Anhydrous, (less than 0.1 % H O). (Warning—Flammable.) If dry reagent cannot be
procured, dry it by distillation through a multiple plate column, discarding the first 5 % of material distilling over and using the
95 % remaining. Also, drying can be accomplished using molecular sieves by passing the solvent upward through a molecular sieve
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.
D4739 − 23
column using one part of molecular sieve per ten parts of solvent. (Warning—It has been reported that, if not inhibited against
it, propan-2-ol can contain peroxides. When this occurs, an explosive mixture is possible when the storage vessel or other
equipment such as a dispensing bottle, are near empty and approaching dryness.
7.12 Commercially available solutions may be used in place of laboratory preparations provided the solutions have been certified
as equivalent.
7.13 Alternate volumes of solutions may be prepared provided the final solution concentration is equivalent.
7.14 Toluene—Reagent grade. (Warning—Extremely flammable.)
7.15 Titration Solvent—In a brown reagent bottle, add 30 mL of water to 1 L of isopropyl alcohol, and mix thoroughly. Add 1 L
each of toluene and chloroform, and mix thoroughly.
8. Preparation of Electrode System
8.1 Maintenance and Storage of Electrodes—Cleaning the electrode thoroughly, keeping the ground-glass joint free of foreign
materials, and regular testing of the electrodes are very important in obtaining repeatable potentials, since the contamination may
introduce uncertain erratic and unnoticeable liquid contact potentials. While this is of secondary importance when end points are
chosen from inflection points in the titration curve, it may be very serious when end points are chosen at experimentally determined
cell potentials as outlined in the procedure.
8.1.1 Clean the pH indicating electrode or the pH indicating part of the combination electrode at frequent intervals based on use
and type of samples being analyzed by immersing in non-chromium containing, strongly oxidizing cleaning solution. The electrode
shall be cleaned periodically when in use or when a new electrode is installed. Drain the LiCl electrolyte from the electrode at least
once each week, and refill with fresh LiCl electrolyte as far as the filling hole. Ensure that there are no air bubbles in the electrode
liquid. If air bubbles are observed, hold the electrode in a vertical position and gently tap it to release the bubbles. Maintain the
electrolyte level in the electrode above that of the liquid in the titration beaker at all times.
8.1.2 When not in use, immerse the lower halves of the electrodes in either water (sensing) or the LiCl in isopropyl alcohol
electrolyte (reference). Do not allow them to remain immersed in titration solvent for any appreciable period of time between
titrations. While the electrodes are not extremely fragile, handle them carefully at all times.
8.1.3 Electrode Life—Typically, electrode usage is limited to 3 to 6 months, depending upon usage. Electrodes have a limited shelf
life and shall be tested before use (8.3).
8.2 Preparation of Electrodes:
8.2.1 When Ag/AgCl reference electrode is used for the titration and it contains an electrolyte that is not 1 M to 3 M LiCl in
ethanol, replace the electrolyte. Drain the electrolyte from the electrode, wash away all the salt (if present) with water and then
rinse with ethanol. Rinse seve
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