ASTM D664-95(2001)e1
(Test Method)Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
SCOPE
1.1 This test method covers procedures for the determination of acidic constituents in petroleum products and lubricants soluble or nearly soluble in mixtures of toluene and propan-2-ol (Note 1). 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.
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 an 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 an 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.
1.3 The values stated in acceptable SI units are to be regarded as the 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn. Contact ASTM
International (www.astm.org) for the latest information.
An American National Standard
e1
Designation:D664–95 (Reapproved 2001) 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 (e) indicates an editorial change since the last revision or reapproval.
This test method was adopted as a joint ASTM-IP standard in 1964.
This test method has been adopted for use by government agencies to replace Method 5106 of Federal Test Method Standard No. 791b.
ASTM Test Method D 4739 has been developed as an alternative to the base number portion of D 664.
e NOTE—Warning notes were placed in the text editorially in January 2001.
1. Scope 1.3 The values stated in acceptable SI units are to be
regarded as the standard.
1.1 This test method covers procedures for the determina-
1.4 This standard does not purport to address all of the
tionofacidicconstituentsinpetroleumproductsandlubricants
safety concerns, if any, associated with its use. It is the
soluble or nearly soluble in mixtures of toluene and propan-
responsibility of the user of this standard to establish appro-
2-ol (Note 1). It is applicable for the determination of acids
−9
priate safety and health practices and determine the applica-
whose dissociation constants in water are larger than 10 ;
bility of regulatory limitations prior to use.
extremely weak acids whose dissociation constants are smaller
−9
than 10 do not interfere. Salts react if their hydrolysis
2. Referenced Documents
−9
constants are larger than 10 .
2.1 ASTM Standards:
NOTE 1—In new and used oils, the constituents that may be considered
D974 Test Method for Acid and Base Number by Color-
to have acidic characteristics include organic and inorganic acids, esters,
Indicator Titration
phenolic compounds, lactones, resins, salts of heavy metals, salts of
D1193 Specification for Reagent Water
ammoniaandotherweakbases,acidsaltsofpolybasicacids,andaddition
D3339 Test Method for Acid Number of Petroleum Prod-
agents such as inhibitors and detergents.
ucts by Semi-Micro Color Indicator Titration
1.2 The test method may be used to indicate relative
changes that occur in an oil during use under oxidizing
3. Terminology
conditions regardless of the color or other properties of the
3.1 Definitions:
resulting oil. Although the titration is made under definite
3.1.1 acid number, n—the quantity of base, expressed as
equilibrium conditions, the test method is not intended to
milligrams of potassium hydroxide per gram of sample, re-
measure an absolute acidic property that can be used to predict
quired to titrate a sample to a specified end point.
performance of an oil under service conditions. No general
3.1.1.1 Discussion—Thistestmethodexpressesthequantity
relationship between bearing corrosion and acid number is
of base as milligrams of potassium hydroxide per gram of
known.
sample, that is required to titrate a sample in the solvent from
NOTE 2—Theacidnumberobtainedbythisstandardmayormaynotbe its initial meter reading in millivolts to a meter reading in
numerically the same as that obtained in accordance with Test Methods
millivolts corresponding to a freshly prepared non-aqueous
D974 and D3339.
basic buffer solution or a well-defined inflection point as
specified in the test method.
This test method is under the jurisdiction of ASTM Committee D02 on
Petroleum Products and Lubricants and is the direct responsibility of Subcommittee
D02.06 on Analysis of Lubricants. Annual Book of ASTM Standards, Vol 05.01.
Current edition approved Oct. 10, 1995. Published December 1995. Originally Annual Book of ASTM Standards, Vol 11.01.
published as D664–42T. Last previous edition D664–89. Annual Book of ASTM Standards, Vol 05.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn. Contact ASTM
International (www.astm.org) for the latest information.
e1
D664–95 (2001)
3.1.1.2 Discussion—This test method provides additional
information. The quantity of base, expressed as milligrams of
potassium hydroxide per gram of sample, required to titrate a
sampleinthesolventfromitsinitialmeterreadinginmillivolts
to a meter reading in millivolts corresponding to a freshly
prepared nonaqueous acidic buffer solution or a well-defined
inflectionpointasspecifiedinthetestmethodshallbereported
as the strong acid number.
3.1.1.3 Discussion—The causes and effects of the so-called
strongacidsandthecausesandeffectsoftheotheracidscanbe
very significantly different. Therefore, the user of this test
method shall differentiate and report the two, when they are
found.
FIG. 1 Cell for Potentiometric Titration
4. Summary of Test Method
4.1 The sample is dissolved in a mixture of toluene and
grounded lead, any part of the exposed surface of the glass
propan-2-ol containing a small amount of water and titrated
electrode, the glass electrode lead, the titration stand, or the
potentiometrically with alcoholic potassium hydroxide using a
meter.
glass indicating electrode and a calomel reference electrode.
NOTE 3—A suitable apparatus could consist of a continuous-reading
The meter readings are plotted manually or automatically
−12
electronicvoltmeterdesignedtooperateonaninputoflessthan5 310
against the respective volumes of titrating solution and the end
A, when an electrode system having 1000-MV resistance is connected
pointsaretakenonlyatwelldefinedinflectionsintheresulting
across the meter terminals and provided with a metal shield connected to
curve.Whennodefiniteinflectionsareobtained,endpointsare the ground, as well as a satisfactory terminal to connect the shielded
connectionwirefromtheglasselectrodetothemeterwithoutinterference
taken at meter readings corresponding to those found for
from any external electrostatic field.
freshly prepared nonaqueous acidic and basic buffer solutions.
6.2.2 Glass Electrode, pencil type, 125 to 180 mm in length
5. Significance and Use
and 8 to 14 mm in diameter (C in Fig. 1).
6.2.2.1 The body of the electrode shall be made of a
5.1 New and used petroleum products may contain acidic
chemically resistant glass tube with a wall thickness of 1 to 3
constituents that are present as additives or as degradation
mm.
products formed during service, such as oxidation products.
6.2.2.2 The end dipping into the solution shall be closed
The relative amount of these materials can be determined by
with a hemisphere of glass sealed on to the electrode tube and
titrating with bases. The acid number is a measure of this
the radius of this hemisphere shall be about 7 mm. The
amount of acidic substance, in the oil-always under the
thickness of the glass in the hemisphere shall be great enough
conditionsofthetest.Theacidnumberisusedasaguideinthe
so that the resistance of the hemisphere is 100 to 1000 MV at
quality control of lubricating oil formulations. It is also
25°C.
sometimes used as a measure of lubricant degradation in
6.2.2.3 The electrode shall contain a reproducible, perma-
service. Any condemning limits must be empirically estab-
nently sealed liquid cell for making electrical connection with
lished.
the inner surface of the hemisphere.
5.2 Since a variety of oxidation products contribute to the
6.2.2.4 The entire electrical connection from the sealed
acid number and the organic acids vary widely in corrosion
contact cell to the meter terminal shall be surrounded by an
properties,thetestmethodcannotbeusedtopredictcorrosive-
electrical shield that will prevent electrostatic interference
ness of an oil under service conditions. No general correlation
when the shield is grounded.
is known between acid number and the corrosive tendency of
6.2.2.5 The shield shall be insulated from the electrical
oils toward metals.
connectionbyinsulatingmaterialofthehighestquality,suchas
rubber and glass, so that the resistance between the shield and
6. Apparatus
the entire length of the electrical connection is greater than
6.1 Thecellassemblyusedforthepotentiometrictitrationis
50000 MV.
shown in Fig. 1.
6.2.3 Calomel Reference Electrode, pencil type, 125 to 180
6.2 Manual Titration Apparatus:
mm in length and 8 to 14 mm in diameter (B in Fig. 1).
6.2.1 Meter,avoltmeterorapotentiometerthatwilloperate 6.2.3.1 This electrode shall be made of glass and shall be
with an accuracy of 60.005 V and a sensitivity of 60.002 V providedwithanexternal,removableglasssleeveonthesealed
overarangeofatleast 60.5Vwhenthemeterisusedwiththe end that is dipped into the titration solution.
electrodes specified in 6.1.2 and 6.1.3 and when the resistance 6.2.3.2 The glass sleeve shall be 8 to 25 mm in length, shall
between the electrodes falls within the range from 0.2 to 20 be slightly tapered, and shall be ground to fit the electrode so
MV.Themetershallbeprotectedfromstrayelectrostaticfields that the sealed end of the electrode protrudes 2 to 20 mm
so that no permanent change in the meter readings over the beyond the sleeve.The ground surface shall be continuous and
entire operating range is produced by touching, with a free of smooth spots.
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn. Contact ASTM
International (www.astm.org) for the latest information.
e1
D664–95 (2001)
6.2.3.3 At a point between the extremities of the ground 7. Reagents
surface, the electrode tube shall be pierced by a hole or holes
7.1 Purity of Reagents—Reagent grade chemicals shall be
1 mm in diameter. The electrode shall contain the necessary
used in all tests. Unless otherwise indicated, it is intended that
mercury, calomel, and electrical connection to the mercury, all
all reagents shall conform to the specifications of the commit-
arranged in a permanent manner.
tee onAnalytical Reagents of theAmerican Chemical Society,
6.2.3.4 The electrode shall be filled almost to capacity with
where such specifications are available. Other grades may be
saturated KCl electrolyte and shall be equipped with a stop-
used, provided it is first ascertained that the reagent is of
pered port through which the electrolyte may be replenished.
sufficiently high purity to permit its use without lessening the
6.2.3.5 When suspended in the air and with the sleeve in accuracy of the determination.
place, the electrode shall not leak electrolyte at a rate greater
7.2 Purity of Water—Unlessotherwiseindicated,references
than one drop in 10 min. to water shall be understood to mean reagent water as defined
by Type III of Specification D1193.
NOTE 4—Certain alternative electrode-electrolyte combinations have
7.3 Hydrochloric Acid (HCl)—Relative density 1.19
been shown to give satisfactory results although the precision using these
(Warning—Corrosive, causes burns.).
alternatives has not been determined. Combination electrodes can be used
7.4 Propan-2-ol, Anhydrous, (less than 0.1 % H O)
for this test method provided they have sufficiently fast response time. 2
(Warning—Flammable.). If adequately dry reagent cannot be
6.2.4 Variable-Speed Mechanical Stirrer, a suitable type,
procured,itcanbedriedbydistillationthroughamultipleplate
equipped with a glass, propeller-type stirring paddle (D in Fig.
column, discarding the first 5% of material distilling overhead
1).Apropeller with blades 6 mm in radius and set at a pitch of
and using the 95% remaining. Drying can also be accom-
30 to 45° is satisfactory.Amagnetic stirrer is also satisfactory.
plished using molecular sieves such as Linde Type 4A, by
6.2.4.1 If electrical stirring apparatus is used, it shall be
passing the solvent upward through a molecular sieve column
electrically correct and grounded so that connecting or discon-
using one part of molecular sieve per ten parts of solvent.
necting the power to the motor will not produce a permanent
7.5 2,4,6 Trimethyl Pyridine (g Collidine)—((CH ) C H N)
3 3 5 2
change in the meter reading during the course of the titration.
—(mol weight 121.18), (Warning—2,4,6-Trimethyl Pyridine
6.2.5 Burette, 10-mL capacity, graduated in 0.05-mL divi-
(g collidine) is hazardous if swallowed, breathed, or spilled on
sions and calibrated with an accuracy of6 0.02 mL (E in Fig.
skin or eyes. Warning—Wear chemical safety goggles, neo-
1).The burette shall have a glass stopcock and shall have a tip
preneorrubberglovesandapron.Useonlyinawell-ventilated
that extends 100 to 130 mm beyond the stopcock. The burette
hood, or wear an approved respirator for organic vapor or a
for KOH shall have a guard tube containing soda lime or other
supplied-air respirator. Do not take internally.) conforming to
CO -absorbing substance.
the following requirements:
6.2.6 Titration Beaker, 250-mL capacity, made of borosili-
Boiling range 168 to 170°C
Refractive index, n 1 498 2 6 0 000 5
cate glass (A in Fig. 1). D
Color colorless
6.2.7 Titration Stand, suitable for supporting the electrodes,
7.5.1 Storethereagentoveractivatedaluminaandkeepina
stirrer, and burette in the positions shown in Fig. 1.
brown glass bottle.
NOTE 5—Anarrangementthatallowstheremovalofthebeakerwithout
7.6 m-Nitrophenol—(NO C H OH)—(molweight139.11),
2 6 4
disturbing the electrodes, burette, and stirrer is desirable.
conforming to the following requirements (Warning—m-
Nitrophenol can be hazardous if swallowed, breathed, or
6.3 Automatic Titration Apparatus:
spilled on skin or eyes. Wear chemical-safety goggles, neo-
6.3.1 Automatic titration systems shall be generally in
prene or rubber gloves, and apron. Use only in a well-
accordance with 6.2 and provide the following technical
ventilated hood, or wear an approved respirator for organic
performance characteristics or features.
vapor or a supplied-air respirator. Do not take internally.):
6.3.1.1 Automatic adaptation of the titration speed in the
Melting point 96 to 97°C
continuous titrant delivery mode to the slope of the titration
Color pale yellow
curve with the capability of complying with the potential
7.6.1 Store the reagent in a brown glass bottle.
equilibrium specified and providing titration rates of less than
7.7 Potassium Chloride Electrolyte—Prepare a saturated
0.2 mL/min during titration and preferably 0.05 mL/min at
solution potassium chloride (KCl) in water.
inflections and at nonaqueous acid and base end points.
7.8 Potassium Hydroxide—(Warning—Causes severe
6.3.1.2 Interchangeable precision motor-driven burettes
burns.).
with a volume dispensing accuracy of 60.
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