ASTM D7946-19

Standard Test Method for Initial pH (i-pH)-Value of Petroleum Products

SIGNIFICANCE AND USE
5.1 New and used petroleum products may contain acidic constituents that are present as additives, degradation products formed during service, such as oxidation products or components formed from combustion gases. The i-pH-value is a measure of the amount of acidic substances and their acidity defined under the conditions of test. The i-pH-value is used as a measure of lubricant degradation in service.  
5.2 The corrosiveness of acidic components depends on their concentration and acidity. The i-pH-value is a measure of the amount of dissociated acidic components with the potential of corrosiveness towards metals.  
5.3 Since a variety of oxidation and blow-by products contribute to the i-pH-value, this test method cannot be used to predict corrosiveness of oil or blends under service conditions against metallic components. No general correlation is known between i-pH-value and the corrosive tendency of blends or oils toward metals.
SCOPE
1.1 This test method covers procedures for the determination of initial pH (i-pH) in new and in-service lubricants.
Note 1: In new and used oils, the constituents that may be considered to have characteristics influencing the i-pH value 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. “Initial” is used to differentiate from aqueous systems. The analysis is terminated after a defined time interval whenever equilibrium conditions, as known for pH measurements in aqueous systems, are not reached (see 3.1.1.2)  
1.2 This test method is used to indicate relative changes that occur in oil during use under oxidizing conditions or due to contamination by blow-by gases of combustion processes of biogases regardless of the color or other properties of the in-service lubricants. Although the initial pH 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 correlation between corrosion of non-ferrous bearing metals and initial pH value is known.  
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: 30-Nov-2019
• ICS: 75.120 - Hydraulic fluids
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.06 - Analysis of Liquid Fuels and Lubricants

Relations

• Replaces: ASTM D7946-14 - Standard Test Method for Initial pH (i-pH)-Value of Petroleum Products
• Effective Date: 01-Dec-2019
• Refers: ASTM D4057-06(2011) - Standard Practice for Manual Sampling of Petroleum and Petroleum Products
• Effective Date: 01-Jun-2011
• Refers: ASTM D1193-06 - Standard Specification for Reagent Water
• Effective Date: 01-Mar-2006
• Refers: ASTM D4057-95(2000) - Standard Practice for Manual Sampling of Petroleum and Petroleum Products
• Effective Date: 10-Apr-2000
• Refers: ASTM D1193-99e1 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999
• Refers: ASTM D1193-99 - Standard Specification for Reagent Water
• Effective Date: 10-Feb-1999

Overview

ASTM D7946-19 is the globally recognized standard test method for determining the initial pH (i-pH) value of new and in-service petroleum products, specifically lubricants. Developed by ASTM International, this procedure provides a means to assess the amount and strength of acidic constituents present in petroleum-based oils. The i-pH value is crucial for monitoring lubricant degradation and understanding chemical changes caused by oxidation and contamination during the product's lifecycle.

This standard applies to both fresh and used lubricants, offering industry professionals a reliable metric for tracking lubricant health. While i-pH provides insights into acidic content, it is not intended to predict the corrosiveness of oils under operational conditions.

Key Topics

• Definition of i-pH: The initial pH (i-pH) is the measure of the hydrogen ion concentration in an oil sample, obtained under standardized, non-aqueous conditions. It reflects the presence of dissociated acidic components which might have corrosive potential.

• Acidic Constituents: Petroleum products may contain organic and inorganic acids, esters, phenolic compounds, resins, metal salts, inhibitors, and detergents. The i-pH value provides a relative gauge of their overall acidity.

• Measurement Procedure: The test involves dissolving a fixed amount of lubricant in a solvent mixture, followed by pH measurement using suitable glass and reference electrodes. The outcome reflects the acidity level at a specific test interval.

• Significance for Lubricant Degradation: The i-pH value changes as lubricants degrade due to oxidation, exposure to combustion gases (blow-by), or other contaminants during operation.

• Limitations: The test is not designed to predict metal corrosion in service, as no general correlation exists between i-pH values and corrosive tendencies toward metals.

Applications

• Condition Monitoring: Regular i-pH testing provides valuable information for the maintenance of engines, turbines, and industrial machinery by helping detect lubricant deterioration.

• Quality Control: Refineries, lubricant manufacturers, and service laboratories use ASTM D7946-19 to monitor product quality both at the production stage and during field use.

• Failure Analysis: Determining changes in i-pH can aid in diagnosing the causes of lubricant breakdown, helping optimize oil change intervals and maintenance cycles.

• Research and Development: The method supports formulation improvements by enabling researchers to track the stability of additives and degradation patterns in new lubricant compositions.

Related Standards

• ASTM D1193: Specification for reagent water, crucial for accurate test results.
• ASTM D4057: Practice for manual sampling of petroleum and petroleum products, ensuring representative samples.
• ASTM D4177: Practice for automatic sampling of petroleum and petroleum products, facilitating reliable and efficient sample collection.
• ASTM D974: Standard Test Method for Acid and Base Number by Color-Indicator Titration, offering alternative acidity determination techniques.

Practical Value

• Early Detection: Enables identification of lubricant issues before they result in equipment damage.
• Cost Savings: Helps extend oil life, reduce unnecessary oil changes, and lower maintenance costs by pinpointing actual lubricant degradation.
• Compliance: Aligns with international standards required by quality management systems and regulatory frameworks.
• Versatility: Suitable for use on a variety of petroleum-based lubricants across numerous industries, regardless of oil color or other physical properties.

ASTM D7946-19 provides a standardized, reproducible method for evaluating the initial acidity of lubricants, supporting reliable oil analysis programs in automotive, industrial, and energy sectors.

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.
Designation: D7946 − 19
Standard Test Method for
Initial pH (i-pH)-Value of Petroleum Products
This standard is issued under the fixed designation D7946; 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.
1. Scope* 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers procedures for the determina-
D1193Specification for Reagent Water
tion of initial pH (i-pH) in new and in-service lubricants.
D4057Practice for Manual Sampling of Petroleum and
NOTE 1—In new and used oils, the constituents that may be considered
Petroleum Products
to have characteristics influencing the i-pH value include organic and
inorganic acids, esters, phenolic compounds, lactones, resins, salts of D4177Practice for Automatic Sampling of Petroleum and
heavy metals, salts of ammonia and other weak bases, acid salts of
Petroleum Products
polybasic acids, and addition agents such as inhibitors and detergents.
“Initial” is used to differentiate from aqueous systems. The analysis is
3. Terminology
terminated after a defined time interval whenever equilibrium conditions,
3.1 Definitions:
as known for pH measurements in aqueous systems, are not reached (see
3.1.1.2)
3.1.1 initial pH (i-pH), n—the measure of the hydrogen ion
(H+) concentration when a fixed sample amount of lubrication
1.2 Thistestmethodisusedtoindicaterelativechangesthat
oil is dissolved into a fixed volume of solvent and using a
occur in oil during use under oxidizing conditions or due to
specified detection system.
contamination by blow-by gases of combustion processes of
3.1.1.1 Discussion—This test method expresses the amount
biogases regardless of the color or other properties of the
of dissociated acidic components as i-pH of a sample in a
in-service lubricants. Although the initial pH is made under
mixtureoftolueneandpropan-2-oltowhichasmallamountof
definite equilibrium conditions, the test method is not intended
water has been added.
to measure an absolute acidic property that can be used to
3.1.1.2 Discussion—The measured i-pH is called initial
predictperformanceofoilunderserviceconditions.Nogeneral
since the original definition of the pH-scale depends on
correlation between corrosion of non-ferrous bearing metals
aqueous solutions. Its application to organic solutions will not
and initial pH value is known.
give stable electrode potential for all types of samples. A
1.3 The values stated in SI units are to be regarded as potential reading after a defined time is used in those cases
standard. No other units of measurement are included in this
instead.
standard.
4. Summary of Test Method
1.4 This standard does not purport to address all of the
4.1 The oil sample is dissolved in solvent mixture and the
safety concerns, if any, associated with its use. It is the
i-pH value is measured using a glass indicating electrode and
responsibility of the user of this standard to establish appro-
a reference electrode or a combination electrode. The meter
priate safety, health, and environmental practices and deter-
readings are measured manually or automatically.
mine the applicability of regulatory limitations prior to use.
1.5 This international standard was developed in accor-
5. Significance and Use
dance with internationally recognized principles on standard-
5.1 New and used petroleum products may contain acidic
ization established in the Decision on Principles for the
constituents that are present as additives, degradation products
Development of International Standards, Guides and Recom-
formed during service, such as oxidation products or compo-
mendations issued by the World Trade Organization Technical
nents formed from combustion gases. The i-pH-value is a
Barriers to Trade (TBT) Committee.
measure of the amount of acidic substances and their acidity
defined under the conditions of test. The i-pH-value is used as
a measure of lubricant degradation in service.
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. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Dec. 1, 2019. Published January 2020. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2014. Last previous edition approved in 2014 as D7946–14. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D7946-19. 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
D7946 − 19
5.2 The corrosiveness of acidic components depends on 6.7 Beaker, 100 mL, or suitable capacity, made of borosili-
their concentration and acidity.The i-pH-value is a measure of cate glass or other suitable material.
theamountofdissociatedacidiccomponentswiththepotential
6.8 Stand, suitable for supporting the electrodes, and stirrer.
of corrosiveness towards metals.
NOTE3—Anarrangementthatallowstheremovalofthebeakerwithout
disturbing the electrodes and stirrer is desirable.
5.3 Since a variety of oxidation and blow-by products
contributetothei-pH-value,thistestmethodcannotbeusedto 6.9 Graduated Cylinder, 100 mL, or dispensing device
capable of delivering 70mL 6 0.5 mL.
predict corrosiveness of oil or blends under service conditions
against metallic components. No general correlation is known
6.10 Pipette, 2.0 mL, Class A.
between i-pH-value and the corrosive tendency of blends or
oils toward metals.
7. Reagents
7.1 Purity of Reagents—Reagent grade chemicals shall be
6. Apparatus
used in all tests. Unless otherwise indicated, it is intended that
6.1 pH-Meter—Meter, a voltmeter or a potentiometer, that
all reagents shall conform to the specifications of the commit-
will operate with an accuracy of 60.005Vand a sensitivity of
tee onAnalytical Reagents of theAmerican Chemical Society,
60.002 V over a range of at least 6 0.5 V when the meter is
where such specifications are available. Other grades may be
used with the electrodes specified in 6.2 to 6.4 and when the
used, provided it is first ascertained that the reagent is of
resistance between the electrodes falls within the range from
sufficiently high purity to permit its use without lessening the
0.2MΩ to 20 MΩ. The meter shall be protected from stray
accuracy of the determination.
electrostatic fields so that no permanent change in the meter
7.2 Commercially available solutions may be used in place
readings over the entire operating range is produced by
of laboratory preparations provided the solutions have been
touching,withagroundedlead,anypartoftheexposedsurface
certified as being equivalent.
oftheglasselectrode,theglasselectrodelead,thestirrer,orthe
7.3 Alternative volumes of the solutions may be prepared,
meter.
provided the final solution concentration is equivalent.
NOTE 2—A suitable apparatus could consist of a continuous-reading
electronic voltmeter designed to operate on an input of less than 5 ×
7.4 Purity of Water—Unless otherwise indicated, reference
-12
10 A, when an electrode system having a resistance of 1000MΩ is
to water shall be understood to mean reagent water that meets
connected across the meter terminals and provided with a metal shield
connected to the ground, as well as a satisfactory terminal to connect the the requirements of either Type I, II, or III of Specification
shielded connection wire from the glass electrode to the meter without
D1193.
interference from any external electrostatic field.
7.5 Ethanol, (Warning—Flammable and toxic, especially
6.2 Sensing Electrode, Standard pH, suitable for nonaque-
when denatured.)
ous media.
7.6 Lithium Chloride, LiCl.
6.3 Reference Electrode, Silver/Silver Chloride (Ag/AgCl)
7.7 Lithium Chloride Electrolyte—Prepare a 1 mol/L to
Reference Electrode, filled with 1 mol/L to 3mol⁄L LiCl in
3mol⁄L solution of lithium chloride (LiCl) in ethanol.
ethanol.
7.8 Propan-2-ol, Anhydrous, (less than 0.1 % H O).
6.4 Combination Electrodes—Sensing electrodes may have
(Warning—Flammable.) If adequately dry reagent cannot be
the Ag/AgCl reference electrode built into the same electrode
procured,itcanbedriedbydistillationthroughamultipleplate
body, which offers the convenience of working with and
column, discarding the first5%of material distilling overhead
maintaining only one electrode. The combination electrode
and using the 95 % remaining. Drying can also be accom-
shall have a sleeve junction on the reference compartment and
plished using molecular sieves such as Linde Type 4A, by
shall use an inert ethanol electrolyte, for example, 1mol⁄L to
passing the solvent upward through a molecular sieve column
3mol⁄L LiCl in ethanol. These combination electrodes shall
using one part of molecular sieve per ten parts of solvent.
havethesameresponseorbetterresponsethanadualelectrode
NOTE 4—It has been reported that, if not originally inhibited against it,
system. They shall have movable sleeves for easy rinsing and
propan-2-ol can contain peroxides. When this occurs, an explosion is
addition of electrolyte.
possible when the storage of the vessel or other equipment such as a
dispensing bottle, is near empty and approaching dryness.
6.5 Variable-Speed Mechanical Stirrer, a suitable type,
7.9 Toluene, (Warning—Flammable.)
equipped with a propeller-type stirring paddle. The rate of
stirringshallbesufficienttoproducevigorousagitationwithout
7.10 Commercial Aqueous pH 4 and, pH 7 Buffer
spattering and without stirring air into the solution.Apropeller
Solutions—These solutions shall be replaced at regular inter-
with blades 6 mm in radius and set at a pitch of 30° to 45° is
vals consistent with their stability or when contamination is
satisfactory. A magnetic stirrer is also satisfactory. If an
electrical stirring apparatus is used, it shall be electrically
correct and grounded so that connecting or disconnecting the
ACS Reagent Chemicals, Specifications and Procedures for Reagents and
powertothemotorwillnotproduceapermanentchangeinthe
Standard-Grade Reference Materials, American Chemical Society, Washington,
meter reading during the course of the determination.
DC. For suggestions on the testing of reagents not listed by theAmerican Chemical
Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
6.6 Beaker, 250 mL capacity, made of borosilicate glass or
U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
other suitable material. copeial Convention, Inc. (USPC), Rockville, MD.
D7946 − 19
suspected. Information relating to their stability should be electrodes with propan-2-ol immediately before use, and then
obtained from the manufacturer. with the dilution solvent.
8.7 Whennotinuse,theglassindicatorelectrodemembrane
8. Electrode System
should be kept hydrated and stored according to the manufac-
turers’recommendation.
8.1 Preparation of Electrodes—When a Ag/AgCl reference
electrode is used for the determination and it contains an
8.8 Electrode Life—Typically, electrode usage is limited to
electrolyte which is not 1mol⁄L to 3mol⁄L LiCl in ethanol,
3months to 6 months depending, upon usage. Electrodes have
replacetheelectrolyte.Draintheelectrolytefromtheelectrode,
a limited shelf life and shall be tested before use (see 8.2).
wash away all the salt (if present) with water and then rinse
9. Standardization of Apparatus
with ethanol. Rinse several times with the LiCl electrolyte
solution. Finally, replace the sleeve and fill the electrode with
9.1 Calibration of the pH-meter with the Aq
...

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: D7946 − 14 D7946 − 19
Standard Test Method for
Initial pH (i-pH)-Value of Petroleum Products
This standard is issued under the fixed designation D7946; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope Scope*
1.1 This test method covers procedures for the determination of initial pH (i-pH) in new and in-service lubricants.
NOTE 1—In new and used oils, the constituents that may be considered to have characteristics influencing the i-pH value 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. “Initial” is used to differentiate from aqueous systems. The analysis is terminated after a defined time
interval whenever equilibrium conditions, as known for pH measurements in aqueous systems, are not reached (see 3.1.1.2)
1.2 This test method is used to indicate relative changes that occur in oil during use under oxidizing conditions or due to
contamination by blow-by gases of combustion processes of biogases regardless of the color or other properties of the in-service
lubricants. Although the initial pH 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 correlation between
corrosion of non-ferrous bearing metals and initial pH value is known.
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 safety, health, and healthenvironmental 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.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
3. Terminology
3.1 Definitions:
3.1.1 initial pH (i-pH), n—the measure of the hydrogen ion (H+) concentration when a fixed sample amount of lubrication oil
is dissolved into a fixed volume of solvent and using a specified detection system.
3.1.1.1 Discussion—
This test method expresses the amount of dissociated acidic components as i-pH of a sample in a mixture of toluene and
propan-2-ol to which a small amount of water has been added.
3.1.1.2 Discussion—
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 July 1, 2014Dec. 1, 2019. Published August 2014January 2020. Originally approved in 2014. Last previous edition approved in 2014 as
D7946 – 14. DOI: 10.1520/D7946-14.10.1520/D7946-19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7946 − 19
The measured i-pH is called initial since the original definition of the pH-scale depends on aqueous solutions. Its application to
organic solutions will not give stable electrode potential for all types of samples. A potential reading after a defined time is used
in those cases instead.
4. Summary of Test Method
4.1 The oil sample is dissolved in solvent mixture and the i-pH value is measured using a glass indicating electrode and a
reference electrode or a combination electrode. The meter readings are measured manually or automatically.
5. Significance and Use
5.1 New and used petroleum products may contain acidic constituents that are present as additives, degradation products formed
during service, such as oxidation products or components formed from combustion gases. The i-pH-value is a measure of the
amount of acidic substances and their acidity defined under the conditions of test. The i-pH-value is used as a measure of lubricant
degradation in service.
5.2 The corrosiveness of acidic components depends on their concentration and acidity. The i-pH-value is a measure of the
amount of dissociated acidic components with the potential of corrosiveness towards metals.
5.3 Since a variety of oxidation and blow-by products contribute to the i-pH-value, this test method cannot be used to predict
corrosiveness of oil or blends under service conditions against metallic components. No general correlation is known between
i-pH-value and the corrosive tendency of blends or oils toward metals.
6. Apparatus
6.1 pH-Meter—Meter, a voltmeter or a potentiometer, that will operate with an accuracy of 60.005 V and a sensitivity of
60.002 V over a range of at least 6 0.5 V when the meter is used with the electrodes specified in 6.2 to 6.4 and when the resistance
between the electrodes falls within the range from 0.2 MΩ0.2 MΩ to 20 MΩ.MΩ. The meter shall be protected from stray
electrostatic fields so that no permanent change in the meter readings over the entire operating range is produced by touching, with
a grounded lead, any part of the exposed surface of the glass electrode, the glass electrode lead, the stirrer, or the meter.
-12
NOTE 2—A suitable apparatus could consist of a continuous-reading electronic voltmeter designed to operate on an input of less than 5 × 10 A, A,
when an electrode system having a resistance of 1000-MΩ1000 MΩ is connected across the meter terminals and provided with a metal shield connected
to the ground, as well as a satisfactory terminal to connect the shielded connection wire from the glass electrode to the meter without interference from
any external electrostatic field.
6.2 Sensing Electrode, Standard pH, suitable for nonaqueous media.
6.3 Reference Electrode, Silver/Silver Chloride (Ag/AgCl) Reference Electrode, filled with 1 mol/L to 3 mol ⁄L LiCl in ethanol.
6.4 Combination Electrodes—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
junction on the reference compartment and shall use an inert ethanol electrolyte, for example, 11 mol mol/Lto⁄L to 3 mol ⁄L LiCl
in ethanol. These combination electrodes shall have the same response or better response than a dual electrode system. They shall
have movable sleeves for easy rinsing and addition of electrolyte.
6.5 Variable-Speed Mechanical Stirrer, a suitable type, equipped with a propeller-type stirring paddle. The rate of stirring shall
be sufficient to produce vigorous agitation without spattering and without stirring air into the solution. A propeller with blades 6
mm in radius and set at a pitch of 30° to 45° is satisfactory. A magnetic stirrer is also satisfactory. If an electrical stirring apparatus
is used, it shall be electrically correct and grounded so that connecting or disconnecting the power to the motor will not produce
a permanent change in the meter reading during the course of the determination.
6.6 Beaker, 250 mL capacity, made of borosilicate glass or other suitable material.
6.7 Beaker, 100 mL, or suitable capacity, made of borosilicate glass or other suitable material.
6.8 Stand, suitable for supporting the electrodes, and stirrer.
NOTE 3—An arrangement that allows the removal of the beaker without disturbing the electrodes and stirrer is desirable.
6.9 Graduated Cylinder, 100 mL, or dispensing device capable of delivering 70 mL 6 0.5 mL.
6.10 Pipette, 2.0 mL, Class A.
D7946 − 19
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 Commercially available solutions may be used in place of laboratory preparations provided the solutions have been certified
as being equivalent.
7.3 Alternative volumes of the solutions may be prepared, provided the final solution concentration is equivalent.
7.4 Purity of Water—Unless otherwise indicated, reference to water shall be understood to mean reagent water that meets the
requirements of either Type I, II, or III of Specification D1193.
7.5 Ethanol, (Warning—Flammable and toxic, especially when denatured.)
7.6 Lithium Chloride, LiCl.
7.7 Lithium Chloride Electrolyte—Prepare a 1 mol/L to 3 mol ⁄L solution of lithium chloride (LiCl) in ethanol.
7.8 Propan-2-ol, Anhydrous, (less than 0.1 % H O). (Warning—Flammable.) If adequately dry reagent cannot be procured, it
can be dried by distillation through a multiple plate column, discarding the first 5 % of material distilling overhead and using the
95 % remaining. Drying can also be accomplished using molecular sieves such as Linde Type 4A, by passing the solvent upward
through a molecular sieve column using one part of molecular sieve per ten parts of solvent.
NOTE 4—It has been reported that, if not originally inhibited against it, propan-2-ol can contain peroxides. When this occurs, an explosion is possible
when the storage of the vessel or other equipment such as a dispensing bottle, is near empty and approaching dryness.
7.9 Toluene, (Warning—Flammable.)
7.10 Commercial Aqueous pH 4 and, pH 7 Buffer Solutions—These solutions shall be replaced at regular intervals consistent
with their stability or when contamination is suspected. Information relating to their stability should be obtained from the
manufacturer.
8. Electrode System
8.1 Preparation of Electrodes—When a Ag/AgCl reference electrode is used for the determination and it contains an electrolyte
which is not 1 mol ⁄L to 3 mol ⁄L 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 several times with the LiCl electrolyte solution. Finally, replace
the sleeve and fill the electrode with the LiCl electrolyte to the filling hole. When refitting the sleeve, ensure that there will be a
free flow of electrolyte into the system. A combination electrode shall be prepared in the same manner. The electrolyte in a
combination electrode can be removed with the aid of a vacuum suction.
8.2 Testing of Electrodes—Test the meter-electrode combination when first put into use, or when new electrodes are installed,
and retest at intervals thereafter. Rinse the electrodes with solvent then with water, and dip them into a pH 4 aqueous buffer
solution. Read the mV value after stirring 1 min. Remove the electrodes and rinse with water. Dip the electrodes into a pH 7
aqueous buffer. Read the mV value after stirring 1 min. Calculate the mV difference. A good electrode system will have a difference
of at least 162 mV (20 °C to 25 °C). If the difference is less than 162 mV, lift the sleeve of the electrode and insure electrolyte
flow. Repeat the measurements. If the difference is still less than 162 mV, clean or replace the electrode(s).
8.3 When the sensing electrode and the reference electrode are separate, one pair of electrodes shall be considered as one unit.
If one or the other is changed, it shall be considered as different pair and shall be retested.
8.4 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 important in obtaining repeatable potentials,
...