ASTM D664-18e2

Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration

SIGNIFICANCE AND USE
5.1 New and used petroleum products, biodiesel, and blends of biodiesel may contain acidic constituents that are present as additives or as degradation products formed during service, such as oxidation products. The relative amount of these materials can be determined by titrating with bases. The acid number is a measure of this amount of acidic substance in the oil, always under the conditions of the test. The acid number is used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service. Any condemning limits must be empirically established.  
5.2 Since a variety of oxidation products contribute to the acid number and the organic acids vary widely in corrosion properties, the test method cannot be used to predict corrosiveness of oil or biodiesel and blends under service conditions. No general correlation is known between acid number and the corrosive tendency of biodiesel and blends or oils toward metals.
SCOPE
1.1 This test method covers procedures for the determination of acidic constituents in petroleum products, lubricants, biodiesel, and blends of biodiesel.  
1.1.1 Test Method A—For petroleum products and lubricants soluble or nearly soluble in mixtures of toluene and propan-2-ol. It is applicable for the determination of acids whose dissociation constants in water are larger than 10–9; extremely weak acids whose dissociation constants are smaller than 10–9 do not interfere. Salts react if their hydrolysis constants are larger than 10–9. The range of acid numbers included in the precision statement is 0.1 mg/g KOH to 150 mg/g KOH.  
1.1.2 Test Method B—Developed specifically for biodiesel and biodiesel blends with low acidity and slightly different solubility. This test method requires the use of an automatic titrator with automatic endpoint-seeking capability.
Note 1: In new and used oils, the constituents that may be considered to have acidic characteristics include organic and inorganic acids, esters, phenolic compounds, lactones, resins, salts of heavy metals, salts of ammonia and other weak bases, acid salts of polybasic acids, and addition agents such as inhibitors and detergents.  
1.2 The test method may be used to indicate relative changes that occur in oil during use under oxidizing conditions regardless of the color or other properties of the resulting oil. Although the titration is made under definite equilibrium conditions, the test method is not intended to measure an absolute acidic property that can be used to predict performance of oil under service conditions. No general relationship between bearing corrosion and acid number is known.
Note 2: The acid number obtained by this standard may or may not be numerically the same as that obtained in accordance with Test Methods D974 and D3339. There has not been any attempt to correlate this method with other non-titration methods.
Note 3: A few laboratories have made the observation that there is a difference in Test Method D664 results when aqueous versus nonaqueous buffers are used.  
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.4 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.  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Oct-2018
• ICS: 75.080 - Petroleum products in general
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.06 - Analysis of Liquid Fuels and Lubricants

Relations

• Refers: ASTM D974-14e1 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• Effective Date: 01-Dec-2014
• Refers: ASTM E177-14 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2014
• Refers: ASTM E177-13 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-May-2013
• Refers: ASTM D3339-12 - Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
• Effective Date: 15-Apr-2012
• Refers: ASTM D974-12 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• Effective Date: 15-Apr-2012
• Refers: ASTM D4057-06(2011) - Standard Practice for Manual Sampling of Petroleum and Petroleum Products
• Effective Date: 01-Jun-2011
• Refers: ASTM D3339-11 - Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
• Effective Date: 15-May-2011
• Refers: ASTM D974-11 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• Effective Date: 15-May-2011
• Refers: ASTM E177-10 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-Oct-2010
• Refers: ASTM E177-08 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-Oct-2008
• Refers: ASTM D3339-08 - Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
• Effective Date: 01-Jul-2008
• Refers: ASTM D3339-07 - Standard Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration
• Effective Date: 01-Nov-2007
• Refers: ASTM D974-07 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• Effective Date: 01-Nov-2007
• Refers: ASTM E177-06b - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 15-Nov-2006
• Refers: ASTM E177-06a - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
• Effective Date: 01-Nov-2006

Overview

ASTM D664-18e2: Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration is an internationally recognized method published by ASTM International. This standard outlines procedures for determining the acid number in a variety of petroleum products, lubricants, biodiesel, and their blends. The acid number indicates the amount of acidic constituents, such as additives or degradation products, present in oils and fuels. The primary technique involves potentiometric titration, which provides crucial data for quality control and monitoring of product degradation.

Key Topics

• Acid Number Determination: The method measures the milligrams of potassium hydroxide (mg KOH) needed to neutralize acidic compounds in a sample, offering a precise quantification of acidity.
• Test Methods A & B:
  • Method A: Designed for petroleum products and lubricants that dissolve or nearly dissolve in toluene and propan-2-ol mixtures. Applicable for acids with dissociation constants larger than 10⁻⁹.
  • Method B: Tailored for biodiesel and biodiesel blends with low acidity and distinct solubility characteristics. Mainly utilizes automatic titration systems.
• Quality Control Applications: The acid number is a vital parameter in the formulation and maintenance of lubricating oils. It serves as an indication of the presence of acidic degradation products, providing insights into lubricant service life and product quality.
• Limitations: The test does not infer the corrosive properties of the sample or predict performance in service conditions. There's no universal correlation between acid number and metallic corrosion rates in oils or biodiesel blends.
• Precision and Safety: Results are conducted in SI units under controlled test conditions. The standard emphasizes adherence to appropriate health, safety, and environmental protocols.

Applications

• Lubricant Quality Assurance: Regular monitoring of acid number assists manufacturers and users in maintaining the integrity of lubricating oils, ensuring they remain within acceptable specifications throughout their lifecycle.
• Degradation Monitoring: Tracking acid number in used oils or biodiesel enables timely detection of oxidation, additive depletion, and contamination. This helps schedule maintenance, oil changes, and prevents equipment wear.
• Biodiesel Industry: With the increasing use of biodiesel and blends, ASTM D664-18e2 offers a reliable measure to monitor acidity, ensuring fuel quality and compliance with regulatory standards.
• Research and Development: The method provides a standardized approach for testing formulations in product development, facilitating consistency in reporting and comparison across laboratories.

Related Standards

• ASTM D974: Test Method for Acid and Base Number by Color-Indicator Titration. An alternative colorimetric method for acid number determination.
• ASTM D3339: Test Method for Acid Number of Petroleum Products by Semi-Micro Color Indicator Titration.
• ASTM D4057 & D4177: Practices for manual and automatic sampling of petroleum and petroleum products.
• ASTM D1193: Specification for Reagent Water used in analytical procedures.
• ISO and International Guidelines: Developed in accordance with World Trade Organization Technical Barriers to Trade Committee principles, ensuring global relevance and acceptance.

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Keywords: ASTM D664-18e2, acid number, petroleum products, potentiometric titration, lubricants, biodiesel, blends, quality control, lubricant degradation, testing standard, oil analysis, industrial standards.

Standards Content (Sample)

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
Designation: D664 − 18 British Standard 4457
Designation 177/96
Standard Test Method for
Acid Number of Petroleum Products by Potentiometric
Titration
This standard is issued under the fixed designation D664; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—Subsection 13.3.1 and a statement in the Summary of Changes were corrected editorially in December 2018.
ε NOTE—Subsections 16.1.1.1 and 16.1.2.1 were corrected editorially in January 2019.
This standard has been approved for use by agencies of the U.S. Department of Defense.
NOTE2—Theacidnumberobtainedbythisstandardmayormaynotbe
1. Scope*
numerically the same as that obtained in accordance with Test Methods
1.1 This test method covers procedures for the determina-
D974andD3339.Therehasnotbeenanyattempttocorrelatethismethod
tion of acidic constituents in petroleum products, lubricants, with other non-titration methods.
NOTE 3—A few laboratories have made the observation that there is a
biodiesel, and blends of biodiesel.
difference in Test Method D664 results when aqueous versus nonaqueous
1.1.1 Test Method A—Forpetroleumproductsandlubricants
buffers are used.
soluble or nearly soluble in mixtures of toluene and propan-2-
1.3 The values stated in SI units are to be regarded as
ol. It is applicable for the determination of acids whose
–9
standard. No other units of measurement are included in this
dissociation constants in water are larger than 10 ; extremely
–9
standard.
weak acids whose dissociation constants are smaller than 10
do not interfere. Salts react if their hydrolysis constants are
1.4 This standard does not purport to address all of the
–9
larger than 10 . The range of acid numbers included in the safety concerns, if any, associated with its use. It is the
precision statement is 0.1mg⁄g KOH to 150mg⁄g KOH.
responsibility of the user of this standard to establish appro-
1.1.2 Test Method B—Developed specifically for biodiesel priate safety, health, and environmental practices and deter-
and biodiesel blends with low acidity and slightly different
mine the applicability of regulatory limitations prior to use.
solubility. This test method requires the use of an automatic 1.5 This international standard was developed in accor-
titrator with automatic endpoint-seeking capability. dance with internationally recognized principles on standard-
NOTE 1—In new and used oils, the constituents that may be considered
ization established in the Decision on Principles for the
to have acidic characteristics include organic and inorganic acids, esters,
Development of International Standards, Guides and Recom-
phenolic compounds, lactones, resins, salts of heavy metals, salts of
mendations issued by the World Trade Organization Technical
ammoniaandotherweakbases,acidsaltsofpolybasicacids,andaddition
Barriers to Trade (TBT) Committee.
agents such as inhibitors and detergents.
1.2 The test method may be used to indicate relative
2. Referenced Documents
changesthatoccurinoilduringuseunderoxidizingconditions
2.1 ASTM Standards:
regardless of the color or other properties of the resulting oil.
D974Test Method for Acid and Base Number by Color-
Although the titration is made under definite equilibrium
Indicator Titration
conditions, the test method is not intended to measure an
D1193Specification for Reagent Water
absolute acidic property that can be used to predict perfor-
D3339TestMethodforAcidNumberofPetroleumProducts
mance of oil under service conditions. No general relationship
by Semi-Micro Color Indicator Titration
between bearing corrosion and acid number is known.
D4057Practice for Manual Sampling of Petroleum and
Petroleum Products
This test method is under the jurisdiction of ASTM Committee D02 on
D4177Practice for Automatic Sampling of Petroleum and
Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Petroleum Products
Subcommittee D02.06 on Analysis of Liquid Fuels and Lubricants.
Current edition approved Nov. 1, 2018. Published November 2018. Originally
approved in 1942. Last previous edition approved in 2017 as D664 – 17a. DOI:
10.1520/D0664-18E02. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
This test method was adopted as a jointASTM-IPstandard in 1964.ASTM Test contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Method D4739 has been developed as an alternative to the base number portion of Standards volume information, refer to the standard’s Document Summary page on
D664. 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
´2
D664 − 18
E177Practice for Use of the Terms Precision and Bias in nessofoilorbiodieselandblendsunderserviceconditions.No
ASTM Test Methods general correlation is known between acid number and the
corrosive tendency of biodiesel and blends or oils toward
3. Terminology
metals.
3.1 Definitions:
6. Apparatus
3.1.1 acid number, n—the quantity of a specified base,
expressed in milligrams of potassium hydroxide per gram of
6.1 Manual Titration Apparatus:
sample, required to titrate a sample in a specified solvent to a
6.1.1 Meter, a voltmeter or a potentiometer that will operate
specified endpoint using a specified detection system.
with an accuracy of 60.005V and a sensitivity of 60.002V
3.1.1.1 Discussion—Thistestmethodexpressesthequantity
overarangeofatleast 60.5Vwhenthemeterisusedwiththe
of base as milligrams of potassium hydroxide per gram of
electrodes specified in 6.1.2 and 6.1.3 and when the resistance
sample, that is required to titrate a sample in a mixture of
between the electrodes falls within the range from 0.2MΩ to
toluene and propan-2-ol to which a small amount of water has
20MΩ. The meter shall be protected from stray electrostatic
been added from its initial meter reading in millivolts to a
fields so that no permanent change in the meter readings over
meter reading in millivolts corresponding to an aqueous basic
the entire operating range is produced by touching, with a
buffersolutionorawell-definedinflectionpointasspecifiedin
grounded lead, any part of the exposed surface of the glass
the test method.
electrode, the glass electrode lead, the titration stand, or the
3.1.1.2 Discussion—This test method provides additional
meter.
information. The quantity of base, expressed as milligrams of
NOTE 4—A suitable apparatus could consist of a continuous-reading
potassium hydroxide per gram of sample, required to titrate a
−12
electronicvoltmeterdesignedtooperateonaninputoflessthan5×10
sampleinthesolventfromitsinitialmeterreadinginmillivolts
A, when an electrode system having 1000MΩ resistance is connected
to a meter reading in millivolts corresponding to a freshly
across the meter terminals and provided with a metal shield connected to
prepared aqueous acidic buffer solution or a well-defined the ground, as well as a satisfactory terminal to connect the shielded
connectionwirefromtheglasselectrodetothemeterwithoutinterference
inflectionpointasspecifiedinthetestmethodshallbereported
from any external electrostatic field.
as the strong acid number.
3.1.1.3 Discussion—The causes and effects of the so-called 6.1.2 Sensing Electrode, standard pH, suitable for nonaque-
ous titrations.
strongacidsandthecausesandeffectsoftheotheracidscanbe
very significantly different. Therefore, the user of this test 6.1.3 Reference Electrode, silver/silver chloride (Ag/AgCl)
method shall differentiate and report the two, when they are reference electrode, filled with 1M to 3M LiCl in ethanol.
found. 6.1.3.1 Combination Electrodes—Sensing electrodes may
have the Ag/AgCl reference electrode built into the same
4. Summary of Test Method
electrode body, which offers the convenience of working with
andmaintainingonlyoneelectrode.Thecombinationelectrode
4.1 Thesampleisdissolvedinatitrationsolventandtitrated
shall have a sleeve junction on the reference compartment and
potentiometrically with alcoholic potassium hydroxide using a
shall use an inert ethanol electrolyte, for example, 1M–3M
glass indicating electrode and a reference electrode or a
LiCl in ethanol. These combination electrodes shall have the
combination electrode. The meter readings are plotted manu-
same response or better response than a dual electrode system.
allyorautomaticallyagainsttherespectivevolumesoftitrating
They shall have removable sleeves for easy rinsing and
solution and the end points are taken only at well-defined
addition of electrolyte.
inflections in the resulting curve. When no definite inflections
are obtained and for used oils, end points are taken at meter
NOTE 5—Athird electrode, such as a platinum electrode, may be used
readings corresponding to those found for aqueous acidic and
to increase the electrode stability in certain systems.
basic buffer solutions.
6.1.4 Variable-Speed Mechanical Stirrer, a suitable type,
equipped with a propeller-type stirring paddle. The rate of
5. Significance and Use
stirringshallbesufficienttoproducevigorousagitationwithout
5.1 Newandusedpetroleumproducts,biodiesel,andblends
spattering and without stirring air into the solution.Apropeller
of biodiesel may contain acidic constituents that are present as
with blades 6mm in radius and set at a pitch of 30° to 45° is
additives or as degradation products formed during service,
satisfactory. A magnetic stirrer is also satisfactory.
such as oxidation products. The relative amount of these
6.1.4.1 If an electrical stirring apparatus is used, it shall be
materials can be determined by titrating with bases. The acid
electrically correct and grounded so that connecting or discon-
number is a measure of this amount of acidic substance in the
necting the power to the motor will not produce a permanent
oil,alwaysundertheconditionsofthetest.Theacidnumberis
change in the meter reading during the course of the titration.
used as a guide in the quality control of lubricating oil
6.1.5 Burette, 10mL capacity, graduated in 0.05mL divi-
formulations. It is also sometimes used as a measure of
sions and calibrated with an accuracy of 60.02mL. The
lubricant degradation in service. Any condemning limits must
buretteshallhaveatipthatextends100mmto130mmbeyond
be empirically established.
thestopcockandshallbeabletodelivertitrantdirectlyintothe
5.2 Since a variety of oxidation products contribute to the titration vessel without exposure to the surrounding air or
acid number and the organic acids vary widely in corrosion vapors.TheburetteforKOHshallhaveaguardtubecontaining
properties,thetestmethodcannotbeusedtopredictcorrosive- soda lime or other CO -absorbing substance.
´2
D664 − 18
6.1.6 Titration Beaker, 250 mL, 125 mL, or suitable 7.7 Potassium Hydroxide. (Warning—Causes severe
capacity, made of borosilicate glass or other suitable material. burns.)
6.1.7 Titration Stand, suitable for supporting the electrodes,
7.8 Propan-2-ol, Anhydrous, (less than 0.1 % H O).
stirrer, and burette.
(Warning—Flammable.) If adequately dry reagent cannot be
NOTE6—Anarrangementthatallowstheremovalofthebeakerwithout procured,itcanbedriedbydistillationthroughamultipleplate
disturbing the electrodes and stirrer is desirable.
column, discarding the first 5% of material distilling overhead
and using the 95% remaining. Drying can also be accom-
6.2 Automatic Titration Apparatus:
plished using molecular sieves such as Linde Type 4A, by
6.2.1 Automatic titration systems shall be able to carry out
passing the solvent upward through a molecular sieve column
the necessary analyses as prescribed in the method. As a
using one part of molecular sieve per ten parts of solvent.
minimum, the automatic titration system shall meet the perfor-
NOTE 7—It has been reported that, if not originally inhibited against it,
mance and specification requirements listed in 6.1 as war-
propan-2-ol can contain peroxides. When this occurs, an explosion is
ranted.
possible when the storage of the vessel or other equipment such as a
6.2.2 A dynamic mode of titrant addition shall be used.
dispensing bottle, is near empty and approaching dryness.
Duringthetitration,thespeedandvolumeoftheadditionshall
7.9 Commercial Aqueous pH 4, pH 7, and pH 10 Buffer
vary depending on the rate of change of the system. The
Solutions—These solutions shall be replaced at regular inter-
recommended maximum volume increment is 0.5mL and the
vals consistent with their stability or when contamination is
recommended minimum volume increment is 0.05mL.
suspected. Information relating to their stability should be
6.2.3 Graduated Cylinder—50mL, or dispensing device
obtained from the manufacturer.
capable of delivering 50mL 6 0.5mL.
6.2.4 Pipette—2.0mL, Class A.
8. Electrode System
6.2.5 Titration Beaker—250 mL, 125 mL, or suitable
capacity, made of borosilicate glass or other suitable material.
8.1 Preparation of Electrodes—When a Ag/AgCl reference
electrode is used for the titration and it contains an electrolyte
7. Reagents
which is not 1M–3M LiCl in ethanol, replace the electrolyte.
Drain the electrolyte from the electrode, wash away all the salt
7.1 Purity of Reagents—Reagent-grade chemicals shall be
used in all tests. Unless otherwise indicated, it is intended that (if present) with water and then rinse with ethanol. Rinse
severaltimeswiththeLiClelectrolytesolution.Finally,replace
all reagents shall conform to the specifications of the commit-
the sleeve and fill the electrode with the LiCl electrolyte to the
tee onAnalytical Reagents of theAmerican Chemical Society,
filling hole.When refitting the sleeve, ensure that there will be
where such specifications are available. Other grades may be
a free flow of electrolyte into the system. A combination
used, provided it is first ascertained that the reagent is of
electrodeshallbepreparedinthesamemanner.Theelectrolyte
sufficiently high purity to permit its use without lessening the
in a combination electrode can be removed with the aid of a
accuracy of the determination.
vacuum suction.
7.1.1 Commercially available solutions may be used in
place of laboratory preparations, provided the solutions have
8.2 Testing of Electrodes—Test the meter-electrode combi-
been certified as being equivalent.
nation when first put into use, or when new electrodes are
7.1.2 Alternate volumes of the solutions may be prepared,
installed, and retest at intervals thereafter. Rinse the electrodes
provided the final solution concentration is equivalent.
withsolventthenwithwater,anddipthemintoapH4aqueous
7.2 Purity of Water—Unless otherwise indicated, reference
buffer solution. Read the mV value after stirring 1min.
to water shall be understood to mean reagent water that meets Removetheelectrodesandrinsewithwater.Diptheelectrodes
the requirements of either Type I, II, or III of Specification into a pH 7 aqueous buffer. Read the mV value after stirring
D1193.
1min. Calculate the mV difference. A good electrode system
will have a difference of at least 162mV (20°C to 25°C). If
7.3 Primary Standard—Where specified, these samples, or
the difference is less than 162mV, lift the sleeve of the
samplesofcommerciallyavailableprimarystandards,aretobe
electrode and ensure electrolyte flow. Repeat the measure-
used in standardizing the volumetric solutions.
ments. If the difference is still less than 162mV, clean or
7.4 Ethanol. (Warning—Flammable and toxic, especially
replace the electrode(s).
when denatured.)
8.2.1 When the sensing electrode and the reference elec-
7.5 Lithium Chloride, LiCl.
trodeareseparate,onepairofelectrodesshallbeconsideredas
one unit. If one or the other is changed, it shall be considered
7.6 Lithium Chloride Electrolyte, Prepare a 1M–3M solu-
as different pair and shall be retested.
tion of lithium chloride (LiCl) in ethanol.
8.3 Maintenance and Storage of Electrodes—Cleaning the
electrodes thoroughly, keeping the ground-glass joint free of
foreign materials, and regular testing of the electrodes are very
Reagent Chemicals, American Chemical Society Specifications, American
Chemical Society, Washington, DC. For Suggestions on the testing of reagents not
important in obtaining repeatable potentials, since contamina-
listed by the American Chemical Society, see Annual Standards for Laboratory
tion may introduce uncertain erratic and unnoticeable liquid
Chemicals, BDH Ltd., Poole, Dorset, U.K., and the United States Pharmacopeia
contact potentials.While this is of secondary importance when
and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville,
MD. end points are chosen from inflection points in the titration
´2
D664 − 18
curve, it may be quite serious when end points are chosen at 10.1.2 Agitate used oil samples thoroughly to ensure that
arbitrarily fixed cell potentials. any sediment present is homogeneously suspended before
analysis as the sediment can be acidic or basic or have
NOTE 8—See Appendix X1 for a possible procedure to check the
adsorbed acidic or basic material from the sample. When
electrode performance.
necessary, samples are warmed to aid mixing.
8.3.1 Cleantheglasselectrodeatfrequentintervalsbasedon
NOTE 10—As used oil can change appreciably in storage, samples
use and type of samples being analyzed (not less than once
should be tested as soon as possible after removal from the lubricating
every week during continual use) by immersing in non-
system and the dates of sampling and testing, if known, should be noted.
chromium containing, strongly oxidizing cleaning solution.
The reference electrode shall be cleaned periodically when in
Test Method A
use or when a new electrode is installed. Drain the reference
11. Reagents
electrode at least once each week and refill with the fresh LiCl
electrolyteasfarasthefillinghole.Ensurethattherearenoair
11.1 See Section 7.
bubbles in the electrode liquid. If air bubbles are observed,
11.2 Hydrochloric Acid (HCl)—Relative density 1.19.
hold the electrode in a vertical position and gently tap it to
(Warning—Corrosive, causes burns.)
release the bubbles. Maintain the electrolyte level in the
11.3 Toluene. (Warning—Flammable.)
reference electrode above that of the liquid in the titration
beaker or vessel at all times.
11.4 Hydrochloric Acid Solution, Standard Alcoholic, (0.1
8.3.2 Prior to each titration, soak the prepared electrodes in
mol/L). (Warning—See 11.2 and 7.8.) Mix 9mL of hydro-
water (pH 4.5 to 5.5) for at least 5min. Rinse the electrodes
chloric (HCl, relative density 1.19) acid with 1Lof anhydrous
with propan-2-ol immediately before use, and then with the
propan-2-ol. Standardize frequently enough to detect concen-
titration solvent.
tration changes of 0.0005 by potentiometric titration of ap-
8.3.3 When not in use, immerse the lower half of the
proximately 8mL (accurately measured) of the 0.1-mol/L
reference electrode in LiCl electrolyte. When the glass elec-
alcoholic KOH solution diluted with 125mL of CO -free
trode is used, store it in water that has been acidified with HCl
water.
to a pH of 4.5 to 5.5. Do not allow electrodes to remain
11.5 Potassium Hydroxide Solution, Standard Alcoholic,
immersedintitrationsolventforanyappreciableperiodoftime
(0.1 mol//L). (Warning—See 7.7 and 7.8.) Add6gof
between titrations. While the electrodes are not extremely
potassium hydroxide (KOH) to approximately 1L of propan-
fragile, handle them carefully at all times.
2-ol. Boil gently for 10min to effect solution. Allow the
8.3.3.1 Electrode Life—Typically, electrode usage is limited
solution to stand for two days and then filter the supernatant
to 3 to 6 months, depending upon usage. Electrodes have a
liquidthroughafinesintered-glassfunnel.Storethesolutionin
limited shelf life and shall be tested before use (see 8.2).
a chemically resistant bottle. Dispense in a manner such that
the solution is protected from atmospheric carbon dioxide
9. Standardization of Apparatus
(CO ) by means of a guard tube containing soda lime or soda
9.1 Determination of Meter Readings for the Aqueous
non-fibrous silicate absorbents and such that it does not come
Buffer Solutions—Toensurecomparableselectionofendpoints
into contact with cork, rubber, or saponifiable stopcock grease.
when definite inflection points are not obtained in the titration
Standardize frequently enough to detect concentration changes
curve, determine daily, for each electrode pair, the meter
of 0.0005 by potentiometric titration of weighed quantities of
readings obtained with aqueous acidic and basic buffer solu-
potassium acid phthalate dissolved in CO -free water.
tions.
11.6 Titration Solvent—Add 5mL 6 0.2mL of water to
NOTE 9—The response of different glass electrodes to hydrogen ion
495mL 6 5mL of anhydrous propan-2-ol and mix well.Add
activityisnotthesame.Therefore,itisnecessarytoestablishregularlyfor
500mL 6 5mL of toluene. (Warning—Flammable.) The
each electrode system the meter readings corresponding to the buffer
solutions arbitrarily selected to represent acidic or basic end points. titration solvent should be made up in large quantities, and its
blank value determined daily by titration prior to use.
9.2 Immerse the electrodes in the pH 4 and the pH 10
aqueousbuffersandstireachofthemforapproximately5min,
11.7 Chloroform.(Warning—Flammable.Hazardousmate-
maintaining the temperature of the buffer solution at a tem-
rial.)
perature within 2°C of that at which the titrations are to be
12. Procedure for Acid Number and Strong Acid
made. Read the cell voltage for each of them. The readings so
Number
obtained are taken as the end points in titration curves having
no inflection points.
12.1 Into a 250mL beaker or a suitable titration vessel,
introduce a weighed quantity of sample as recommended in
10. Preparation of Sample
Table 1(a) (see Note 11) and add 125mL of titration solvent
10.1 When applicable, refer to Practice D4057 (manual (seeNote12).Alternatively,intoa125mLbeakerorasuitable
sampling) or Practice D4177 (automatic sampling) for proper titration vessel, introduce a weighed quantity of sample as
sampling techniques. recommendedinTable1(b)andadd60mLoftitrationsolvent.
10.1.1 When sampling used lubricants, the specimen shall Prepare the electrodes as directed in 8.1. Place the beaker or
be representative of the system sampled and shall be free of titration vessel on the titration stand and adjust its position so
contamination from external sources. that the electrodes are about half immersed. Start the stirrer,
´2
D664 − 18
TABLE 1 Recommended Size of Test Portion
(a) 125 mL Solvent (b) 60 mL Solvent
Mass of Test Portion, Accuracy of Weighing, Mass of Test Portion, Accuracy of Weighing,
Acid Number Acid Number
g g g g
0.05 to <1.0 20.0 ± 2.0 0.10 0.05 to <1.0 10.0 ± 1.0 0.10
1.0 to <5.0 5.0 ± 0.5 0.02 1.0 to <5.0 2.5 ± 0.25 0.02
5 to <20 1.0 ± 0.1 0.005 5 to <20 0.5 ± 0.05 0.005
20 to <100 0.25 ± 0.02 0.001 20 to <100 0.25 ± 0.02 0.001
100 to <260 0.1 ± 0.01 0.0005 100 to <260 0.1 ± 0.01 0.0005
and stir throughout the determination at a rate sufficient to presence of such acids. Record the volume of KOH added to
produce vigorous agitation without spattering and without reach the mVof the pH 4 aqueous buffer.This value is used to
stirring air into the solution. calculate the strong acid number. Proceed with the automatic
NOTE 11—If it suspected that the recommended sample size will foul
titration and record potentiometric curves or derivative curves
the electrodes, a smaller sample size can be taken. Results using smaller
as the case may be.
sample size may not be equivalent to results obtained with the recom-
12.4.3 Titrate with the 0.1mol⁄L alcoholic KOH solution.
mended sample size. The precision statement does not include results
The apparatus shall be adjusted or programmed such that,
when using a smaller sample size.
NOTE 12—A titration solvent that contains chloroform (Warning— when an inflection point, suitable for use in the calculation is
May be fatal if swallowed. Harmful if inhaled. May produce toxic vapors
approached, the rate of addition of titrant and volume of titrant
if burned) can be used in place of toluene to completely dissolve certain
added are based on the change in slope of the titration curve.
heavy residues of asphaltic materials. Results using chloroform may not
The titrant shall be added in increments of a suitable size to
be equivalent to results obtained using toluene. The precision statement
achieveapotentialdifferenceof5mVto15mVperincrement.
does not include results when using chloroform.
Increment volume shall vary between 0.05mL and 0.5mL.
12.2 Select the right burette, fill with the 0.1mol⁄L alco-
Thenextincrementshallbeaddedifthesignaldoesnotchange
holic KOH solution, and place the burette in position on the
more than 10mV in 10s. The maximum waiting time in
titration assembly, ensuring that the tip is immersed about
between increments shall not exceed 60s.
25mm in titration vessel liquid. Record the initial burette and
12.4.4 The titration can be terminated when the signal
meter (cell potential) readings.
reaches the pH 10 buffer potential past 200mV. An equiva-
12.3 Manual Titration Method:
lence point is recognizable if the first derivative of the titration
12.3.1 Add suitable small portions of 0.1mol⁄L alcoholic
curve produces a maximum, which is significantly higher than
KOH solution and wait until a constant potential has been
the noise produced by electrostatic effects. See also 13.1.1.
established, record the burette and meter readings.
12.4.5 The goal of cleaning is to rinse the residue from the
12.3.2 At the start of the titration and in any subsequent
previoussampleandtorehydratetheelectrode.Oncompletion
regions (inflections) where 0.1mL of the 0.1mol⁄L KOH
ofthetitration,rinsetheelectrodesandburettetipwithtitration
solution consistently produces a total change of more than
solvent. If clean, then rinse with 2-propanol and then with
30mV in the cell potential, add 0.05mL portions.
water. Immerse the electrodes in pH 4.5–5.5 water for at least
12.3.3 Intheintermediateregions(plateau)where0.1mLof
3min to 5min to rehydrate the aqueous gel layer of the glass
0.1mol⁄L alcoholic KOH changes the cell potential less than
electrode. Rinse with 2-propanol prior to beginning the next
30mV, add larger portions sufficient to produce a total poten-
sampletoremovethewater.Ifsampleresidueremainsafterthe
tialchangeapproximatelyequalto,butnotgreaterthan30mV.
rinse with titration solvent, another solvent such as toluene,
12.3.4 Titrate in this manner until the potential changes less
xylene,heptane,orchloroformmaybeusedforrin
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