ASTM D7881-20e1

Standard Test Method for Determination of 4-Carboxybenzaldehyde and <emph type="bdit" >p</emph>-Toluic Acid in Purified Terephthalic Acid by Capillary Electrophoresis with Reverse Voltage Mode

SIGNIFICANCE AND USE
5.1 The presence of 4-CBA and p-TOL in PTA used for the production of polyester is undesirable because they can slow down the polymerization process; and 4-CBA is also imparting coloration to the polymer due to thermal instability.  
5.2 Determining the amount of 4-CBA and p-TOL remaining from the manufacture of PTA is often required. This test method is suitable for setting specifications and for use as an internal quality control where these products are produced or used.  
5.3 This test method is intended as an alternative to the HPLC method for the determination of 4-CBA and p-TOL in PTA. The major benefits of CE are speed, simplicity, reduced reagent consumption, and operating costs.
SCOPE
1.1 This test method2 covers the determination of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid (p-TOL) in purified terephthalic acid (PTA) by capillary electrophoresis (CE) with reverse voltage mode and UV detection. It is applicable for 4-CBA from 3 to 400 mg/kg and for p-TOL from 8 to 400 mg/kg, respectively.  
1.2 In determining the conformance of the test results using this method to applicable specification, results shall be rounded off in accordance with the rounding-off method of Practice E29.  
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-Sep-2020
• ICS: 71.080.40 - Organic acids
• Technical Committee: D16 - Aromatic, Industrial, Specialty and Related Chemicals
• Drafting Committee: D16.02 - Oxygenated Aromatics

Relations

• Refers: ASTM D6809-02(2016) - Standard Guide for Quality Control and Quality Assurance Procedures for Aromatic Hydrocarbons and Related Materials
• Effective Date: 01-Nov-2016
• Refers: ASTM E691-13 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-May-2013
• Refers: ASTM E691-11 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Nov-2011
• Refers: ASTM E29-08 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Oct-2008
• Refers: ASTM E691-08 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Oct-2008
• Refers: ASTM D6809-02(2007) - Standard Guide for Quality Control and Quality Assurance Procedures for Aromatic Hydrocarbons and Related Materials
• Effective Date: 01-Jun-2007
• Refers: ASTM E29-06b - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Nov-2006
• Refers: ASTM E29-06a - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 15-Sep-2006
• Refers: ASTM E29-06 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-May-2006
• Refers: ASTM D1193-06 - Standard Specification for Reagent Water
• Effective Date: 01-Mar-2006
• Refers: ASTM E691-05 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 01-Nov-2005
• Refers: ASTM E29-04 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 01-Dec-2004
• Refers: ASTM D6809-02 - Standard Guide for Quality Control and Quality Assurance Procedures for Aromatic Hydrocarbons and Related Materials
• Effective Date: 10-Jun-2002
• Refers: ASTM E29-02e1 - Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
• Effective Date: 10-May-2002
• Refers: ASTM E691-99 - Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
• Effective Date: 10-May-1999

Overview

ASTM D7881-20e1 is a standard test method developed by ASTM International for the determination of 4-carboxybenzaldehyde (4-CBA) and p-toluic acid (p-TOL) in purified terephthalic acid (PTA) using capillary electrophoresis (CE) with reverse voltage mode and UV detection. This method is applicable to a concentration range of 4-CBA from 3 to 400 mg/kg and p-TOL from 8 to 400 mg/kg.

PTA is a key raw material in polyester production, and residual 4-CBA and p-TOL can negatively affect the polymerization process and the quality of polyester, including imparting unwanted coloration. Monitoring and controlling these impurities is critical for ensuring product quality and meeting both internal and customer specifications.

Key Topics

• Impurity Impact: Both 4-CBA and p-TOL are undesirable in PTA as they can slow down polymerization and affect physical properties and appearance of polyester.
• Alternative to HPLC: Capillary electrophoresis, as described in ASTM D7881, offers advantages over traditional HPLC methods, including faster analysis, greater simplicity, and lower reagent costs.
• Reverse Voltage Mode: This method utilizes reverse voltage in capillary electrophoresis, improving separation and detection of low-level impurities by UV absorbance.
• Applicable Range: Tested for 4-CBA (3–400 mg/kg) and p-TOL (8–400 mg/kg), ensuring suitability for typical quality control needs in PTA manufacturing.
• Quality Control and Specification Setting: The method supports both routine internal quality checks and compliance with customer or regulatory requirements.
• Result Handling: Results are to be reported in SI units and rounded in accordance with ASTM E29 guidelines.

Applications

• PTA Manufacturing: Routine testing of PTA for residual 4-CBA and p-TOL during and after production, ensuring product is within acceptable limits for use in polyester production.
• Polyester Industry: Quality control in polyester manufacturing, detecting impurities that affect processing and product appearance.
• Specification Testing: Used by producers and buyers to set and verify impurity specifications for PTA shipments.
• Research and Development: Facilitates process optimization and innovation by providing rapid, reliable data on impurity content.
• Compliance Verification: Ensures products satisfy international trade and regulatory standards as recommended by the World Trade Organization’s TBT Committee.

Practical Value

• Speed and Efficiency: Delivers faster results compared to HPLC, supporting high-throughput industrial environments.
• Cost-Effectiveness: Requires less reagent consumption and lower operating costs, important for labs managing large sample volumes.
• Sensitivity and Accuracy: Effective for both trace-level detection and high-concentration monitoring in diverse sample matrices.
• Instrument Versatility: The method’s use of capillary electrophoresis with UV detection is compatible with widely available lab instruments.

Related Standards

• ASTM D1193 - Specification for Reagent Water, ensuring purity of water used in test preparations.
• ASTM D6809 - Guide for Quality Control and Quality Assurance Procedures for Aromatic Hydrocarbons and Related Materials.
• ASTM E29 - Practice for Using Significant Digits in Test Data.
• ASTM E691 - Practice for Conducting Interlaboratory Studies.
• EN ISO 8213 - Chemical products sampling techniques.
• OSHA 29 CFR 1910 - Provides regulatory context for safety and handling in the laboratory environment.

Keywords

capillary electrophoresis, purified terephthalic acid, PTA, 4-carboxybenzaldehyde, p-toluic acid, reverse voltage, impurity analysis, polyester production, quality control, ASTM D7881

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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.
´1
Designation:D7881 −20
Standard Test Method for
Determination of 4-Carboxybenzaldehyde and p-Toluic Acid
in Purified Terephthalic Acid by Capillary Electrophoresis
with Reverse Voltage Mode
This standard is issued under the fixed designation D7881; 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.
ε NOTE—Research report information in Section 14 was updated editorially in February 2021.
1. Scope* 2. Referenced Documents
2.1 ASTM Standards:
1.1 This test method covers the determination of
D1193 Specification for Reagent Water
4-carboxybenzaldehyde (4-CBA) and p-toluic acid (p-TOL) in
D6809 Guide for Quality Control and Quality Assurance
purified terephthalic acid (PTA) by capillary electrophoresis
Procedures for Aromatic Hydrocarbons and Related Ma-
(CE) with reverse voltage mode and UV detection. It is
terials
applicablefor4-CBAfrom3to400mg/kgandforp-TOLfrom
E29 Practice for Using Significant Digits in Test Data to
8 to 400 mg/kg, respectively.
Determine Conformance with Specifications
1.2 In determining the conformance of the test results using
E691 Practice for Conducting an Interlaboratory Study to
thismethodtoapplicablespecification,resultsshallberounded
Determine the Precision of a Test Method
off in accordance with the rounding-off method of Practice
2.2 ISO Documents:
E29.
EN ISO 8213 Chemical Products for Industrial Use—
Sampling Techniques—Solid Chemical Products in the
1.3 The values stated in SI units are to be regarded as
Form of ParticlesVarying from Powders to Coarse Lumps
standard. No other units of measurement are included in this
2.3 Other Document:
standard.
OSHA Regulations, 29 CFR paragraphs 1910.1000 and
1.4 This standard does not purport to address all of the
1910.1200
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 3. Terminology
priate safety, health, and environmental practices and deter-
3.1 DefinitionsofTermsSpecifictoReverseVoltageModein
mine the applicability of regulatory limitations prior to use.
this Standard:
1.5 This international standard was developed in accor-
3.1.1 capillary electrophoresis, n—an electrophoretic tech-
dance with internationally recognized principles on standard-
nique in which a sample is introduced into a 50 µm to 100 µm
ization established in the Decision on Principles for the
i. d. fused-silica capillary filled with electrolyte solution and
Development of International Standards, Guides and Recom- subjected to high voltage for separation.
mendations issued by the World Trade Organization Technical 3.1.1.1 Discussion—Reverse voltage, with the cathode on
theinjectionsideandtheanodeonthedetectionside,isapplied
Barriers to Trade (TBT) Committee.
across the capillary causing electrolyte and analytes to migrate
towards the anode and through the capillary’s UV detector
window. Analytes are separated based upon the differential
This test method is under the jurisdiction of ASTM Committee D16 on
Aromatic, Industrial, Specialty and Related Chemicals and is the direct responsi-
bility of Subcommittee D16.02 on Oxygenated Aromatics.
Current edition approved Oct. 1, 2020. Published December 2020. Originally For referenced ASTM standards, visit the ASTM website, www.astm.org, or
approved in 2013. Last previous edition approved in 2019 as D7881 – 19. DOI: contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
10.1520/D7881-20E01. Standards volume information, refer to the standard’s Document Summary page on
This standard is based on SH/T 1687–2000 Purified terephthalic acid for the ASTM website.
industrial use—determination of 4-Carboxybenzaldehyde and p-Toluic Acid-High Available from Available from American National Standards Institute (ANSI),
performance capillary electrophoresis (HPCE), copyright SINOPEC, 22 Chaoyang- 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
men North Street, Chaoyang District, Beijing, China 100728. A copy of SH/T AvailablefromU.S.GovernmentPrintingOfficeSuperintendentofDocuments,
1687–2000 may be obtained from China Petrochemical Press, www.sinopec- 732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401, http://
press.com. www.access.gpo.gov.
*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
´1
D7881−20
rates of migration in the electrical field.Analyte detection and 3.1.8 electrokinetic sampling, n—a sample introduction
quantitation are based upon the principles of UV detection. technique in which the injection side of the capillary with
electrolyte and the electrode are inserted into the sample
3.1.2 electrolyte, n—a combination of a buffer reagent, an
solution.Avoltageisthenapplied,causinganalytestoenterthe
ion-pair reagent and an electroosmotic-flow modifier placed
capillary by electrophoretic migration and EOF.
inside the capillary, used as a carrier for the analytes, and for
detection and quantitation.
3.1.8.1 Discussion—Nanolitres of sample are injected into
3.1.3 electroosmotic flow (EOF), n—the directional velocity the capillary with analyte bias effects. It has considerable
of electrolyte-solution flow within the capillary under an benefit in terms of increased sensitivity when attempting to
applied voltage; the velocity and direction of flow are deter- quantify trace levels of analytes.
mined by electrolyte chemistry, capillary-wall chemistry, and 3.1.9 migration time, n—the time required for a specific
applied voltage. analyte to migrate through the capillary to the detector.
3.1.4 electroosmotic-flow modifier (OFM), n—a cationic
3.1.9.1 Discussion—Themigrationtimeincapillaryelectro-
quaternary amine in the electrolyte that dynamically coats the
phoresis is analogous to retention time in chromatography.
negatively charged silica wall giving it a net positive charge.
4. Summary of Test Method
3.1.4.1 Discussion—This modifier reverses the direction of
4.1 A PTA sample is dissolved in ammonium hydroxide
the electrolyte’s natural electroosmotic flow and directs it
solution. The 4-CBA, p-TOL and PTA dissociate and become
towards the anode and detector. This modifier augments the
homologous ions under basic conditions. A fixed amount of
analyte’smigrationandenhancesspeedofanalysis(seeFig.1).
this solution is introduced into the capillary using hydrody-
3.1.5 electropherogram, n—a graphical presentation of UV
namic sampling or electrokinetic sampling. A voltage is ap-
detector response versus time of analysis; the x-axis is migra-
plied to the capillary to separate the impurities, 4-CBA and
tion time, which is used to identify the analyte qualitatively,
p-TOL, from PTA. External standard calibration is used for
and the y-axis is UVresponse, which can be converted to peak
quantification.
area or height for quantitation.
3.1.6 electrophoretic mobility, n—the specific velocity of a 5. Significance and Use
charged analyte in the electrolyte under specific
5.1 The presence of 4-CBAand p-TOL in PTAused for the
electroosmotic-flow conditions.
production of polyester is undesirable because they can slow
down the polymerization process; and 4-CBAis also imparting
3.1.6.1 Discussion—The mobility of an analyte is directly
coloration to the polymer due to thermal instability.
related to the analyte’s equivalent ionic conductance and
applied voltage, and is the primary mechanism of separation.
5.2 Determining the amount of 4-CBA and p-TOL remain-
3.1.7 hydrodynamic sampling, n—a sample introduction
ing from the manufacture of PTA is often required. This test
technique in which the injection side of the capillary with
method is suitable for setting specifications and for use as an
electrolyteisimmersedintosamplesolutionandthenapositive
internal quality control where these products are produced or
pressure difference is applied.
used.
3.1.7.1 Discussion—Nanolitres of sample are introduced 5.3 This test method is intended as an alternative to the
into the capillary without analyte bias effects. HPLC method for the determination of 4-CBA and p-TOL in
FIG. 1Pictorial Diagram of Anion Mobility and Electroosmotic Flow Modifier
´1
D7881−20
PTA. The major benefits of CE are speed, simplicity, reduced 7. Reagents and Materials
reagent consumption, and operating costs.
7.1 Purity of Reagents—Unless otherwise indicated, it is
intended that all reagents shall conform to the reagent grade
6. Apparatus
specification for analytical reagents of theAmerican Chemical
6.1 Capillary Electrophoresis System—The system consists
Society, where such specifications are available. Other grades
ofthefollowingcomponents,asshowninFig.2,orequivalent:
may be used, provided it is first ascertained that the reagent is
6.1.1 High Voltage Power Supply, capable of generating
of sufficiently high purity to permit its use without lessening
voltage between 0 and 30 kV with the capability of working in
the performance or accuracy the determination. Reagent
a constant voltage mode.
chemicals shall be used for all tests.
6.1.2 Covered Sample Carousel, to prevent environmental
NOTE 1—Calibration and detection limits of this test method can be
biased by the purity of the reagents.
contamination of the samples and electrolytes during a multi-
sample batch analysis.
7.2 TetradecyltrimethylAmmonium Chloride.
6.1.3 Sample Introduction Mechanism, capable of hydrody-
7.3 Sodium 1-hexanesulfonate.
namic sampling or electrokinetic sampling technique.
7.4 3-(Cyclohexylamino)-1-propanesulfonic Acid.
6.1.4 Capillary Purge Mechanism, to purge the capillary
after every analysis with fresh electrolyte to eliminate any
7.5 Sodium Phosphate Dibasic Dodecahydrate.
interference from the previous sample matrix, and to clean the
7.6 Sodium Hydroxide.
capillary with sodium hydroxide solution and water.
7.7 25 % (w/w) Ammonium Hydroxide Solution.
6.1.5 UV Detector, having the capability of monitoring 200
nm, or equivalent.
7.8 Purity of Water—Unless otherwise indicated, references
6.1.6 Fused Silica Capillary, a 50 to 100 µm (inner diam-
to water shall be understood to mean Type 1 reagent water
eter) by 375 µm (outer diameter) by 60 cm (length) having a
conforming or exceeding Specification D1193. Freshly drawn
polymer coating for flexibility, with an uncoated section to act
water should be used for preparation of all stock and working
as the cell window for UV detection.
standards, electrolytes, and solutions.
6.1.7 Constant Temperature Compartment, to keep the
7.9 PTA Standard for Calibrations—A certified PTA cali-
samples, capillary, and electrolytes at constant temperature.
bration standard with known amounts of 4-CBAand p-TOL is
6.2 Data System, a computer system that can acquire data at
required. If it is not commercially available, please refer to
20 points/s minimum, express migration time in minutes to
Annex A1 for determining the concentrations of 4-CBA and
three decimal places.
p-TOL in a PTA sample. The calibrated PTA sample can be
served as a PTA calibration standard.
6.3 Sample Filter, a disposable syringe filter made of
cellulose acetate, with a pore size between 0.22 and 0.45 µm,
7.10 OFMConcentrateSolution(0.05mol/Ltetradecyltrim-
and is chemically inert to aqueous solutions, is recommended
ethyl ammonium chloride)—Dissolve approximately 0.75 g of
for the removal of particulate matter from the sample solution.
tetradecyltrimethyl ammonium chloride (TTAC) in a 50-mL
volumetric flask and dilute to 50 mL with water.
6.4 pH Meter, consisting of a glass-calomel double
electrode, used to determine pH values of the solutions.
7.11 Sodium Hydroxide Solution (0.5 mol/L sodium
hydroxide)—Dissolveapproximately20gofsodiumhydroxide
ina1L plastic volumetric flask and dilute to 1 L with water.
7.12 Ammonium Hydroxide Solution (2.5 % (m/m) ammo-
nium hydroxide solution)—Add approximately 50 mL 25 %
(m/m) ammonium hydroxide solution in a 500-mL volumetric
flask and dilute to 500 mL with water.
7.13 Electrolyte Solution, working in reverse voltage mode.
7.13.1 Electrolyte solution A (50 mm sodium
1-hexanesulfonate,10mMdisodiumhydrogenphosphateand1
mM OFM)—Weigh approximately 0.50 g sodium
1-hexanesulfonate and 0.18 g sodium phosphate dibasic do-
decahydrate in a 50-mL volumetric flask, add 1 mL OFM
concentrate solution and dilute to 50 mLwith water.Adjust the
solution pH to 10.5 to 11.0 using 7.11 sodium hydroxide
solution. Filter and degas the solution before use.
ACS Reagent Chemicals, Specifications and Procedures for Reagents and
Standard-Grade Reference Materials, American Chemical Society, Washington,
DC. For suggestions on the testing of reagents not listed by theAmerican Chemical
Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,
U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-
FIG. 2Typical Instrumental Setup copeial Convention, Inc. (USPC), Rockville, MD.
´1
D7881−20
7.13.2 Electrolyte Solution B (50 mM sodium 10.9 Program the data system for an acquisition rate of at
1-hexanesulfonate, 5 mM CAPS and 1 mM OFM)—Weigh least 20 poi
...