ASTM D8031-16

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: D8031 − 16
Standard Test Method for
Acid Number of Terephthalic Acid (TA) by Automatic
Potentiometric Titration
This standard is issued under the fixed designation D8031; 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 E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
1.1 This test method covers the determination of acid
2.2 Other Document:
number in terephthalic acid (TA) by automatic potentiometric
OSHA Regulations, 29 CFR paragraphs 1910.1000 and
titration. As the index of TA purity, acid number is usually
1910.1200
within 674 to 676 mg KOH/g.
1.2 In determining the conformance of the test results using
3. Terminology
this method, results shall be rounded off in accordance with the
3.1 Definitions:
rounding-off method of Practice E29.
3.1.1 acid number, n—the quantity of base, expressed as
1.3 The values stated in SI units are to be regarded as
milligrams of potassium hydroxide per gram of sample that is
standard. No other units of measurement are included in this
required to titrate a sample in a specified solvent to a specified
standard.
end point.
1.4 This standard does not purport to address all of the
4. Summary of Test Method
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 4.1 ATAsample is dissolved in pyridine, diluted with water
and titrated potentiometrically with standard sodium hydroxide
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. solution, using a glass indicating electrode and a reference
electrode or a combination electrode. The meter readings are
2. Referenced Documents
plotted automatically against the respective volumes of titrat-
2 ing solution and the end points are taken only at well-defined
2.1 ASTM Standards:
inflectionsintheresultingcurve.Theacidnumberiscalculated
D664 Test Method for Acid Number of Petroleum Products
as milligrams of KOH per gram of TA sample. Its theoretical
by Potentiometric Titration
value of TA sample is 675.5 mg KOH/g.
D1193 Specification for Reagent Water
D4790 Terminology ofAromatic Hydrocarbons and Related
5. Significance and Use
Chemicals
5.1 An estimate of TApurity can be determined by titrating
D6809 Guide for Quality Control and Quality Assurance
with KOH. As an index of TA purity, the acid number can be
Procedures for Aromatic Hydrocarbons and Related Ma-
used as a guide in the quality control of TA production.
terials
E29 Practice for Using Significant Digits in Test Data to
6. Apparatus
Determine Conformance with Specifications
6.1 Analytical Balance.
E177 Practice for Use of the Terms Precision and Bias in
ASTM Test Methods
6.2 Automatic Potentiometric Titrator—Automatic titration
E300 Practice for Sampling Industrial Chemicals
systems capable of adding fixed increments of titrant at fixed
time intervals (monotonic) or variable titrant increments with
electrode stability between increment additions (dynamic) with
This test method is under the jurisdiction of ASTM Committee D16 on
endpoint seeking capabilities as prescribed in the method.As a
Aromatic Hydrocarbons and Related Chemicals and is the direct responsibility of
minimum, the apparatus shall meet the performance and
Subcommittee D16.02 on Oxygenated Aromatics.
specification requirements as warranted by the manufacturer.
Current edition approved March 1, 2016. Published June 2016. DOI: 10.1520/
D8031-16.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Available from U.S. Government Printing Office, Superintendent of
Standards volume information, refer to the standard’s Document Summary page on Documents, 732 N. Capitol St., NW, Washington, DC 20401-0001, http://
the ASTM website. www.access.gpo.gov.
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D8031 − 16
6.2.1 Amonotonic or dynamic mode of titrant shall be used. 8. Hazards
During the titration, the speed and volume of the addition may
8.1 Consult current federal regulations, supplier’s Safety
vary depending on the rate of change of the system. The
Data Sheets, and local regulations for all materials used in this
recommended maximum volume increment is 0.5 mL and the
test method.
recommended minimum volume increment is 0.05 mL.
9. Sampling, Test Specimens, and Test Units
6.2.2 Buret, capable of delivering titrant in 0.05 mL or
larger increments.The buret tip should be able to deliver titrant 9.1 Use only representative samples obtained as described
directly into the titration vessel without exposure to the in Practice E300, unless otherwise specified.
surrounding air. Burets with total capacities of 5 mL, 10 mL
10. Preparation of Apparatus
and 20 mL are typically available from commercial manufac-
10.1 Adjust the apparatus in accordance with the manufac-
turers of auto titrators and can be used if their specifications
turer’s instructions.
meet the criteria of 6.2.1 and 6.2.2.
6.2.3 Titration Stand, suitable for supporting the electrode, 10.2 Preparation of Electrodes:
stirrer and buret tip.
10.2.1 WhentheAg/AgClreferenceelectrodeisusedforthe
titration and it contains an electrolyte which is not 1M to 3M
6.3 Sensing Electrode,standardpH,suitablefornonaqueous
LiCl in ethanol, replace the electrolyte. Drain the electrolyte
titrations.
from the electrode, wash away all the salt (if present) with
6.4 Reference Electrode, silver/silver chloride (Ag/AgCl)
water and then rinse with ethanol. Rinse several times with the
reference electrode, filled with 1M to 3M LiCl in ethanol. LiCl electrolyte solution. Finally, replace the sleeve and fill the
electrode with the LiCl electrolyte to the filling hole. When
7. Reagents refitting the sleeve, ensure that there will be a free flow of
electrolyte into the system. A combination electrode shall be
7.1 Purity of Reagents—Unless otherwise indicated, it is
prepared in the same manner. The electrolyte in a combination
intended that all reagents shall conform to the reagent grade
electrode can be removed with the aid of a vacuum suction.
specification of the Analytical Reagents of the American
10.3 Testing of Electrodes—Test the meter-electrode com-
Chemical Society, where such specifications are available.
bination when first put into use, or when new electrodes are
Other grades may be used, provided it is first ascertained that
installed, and retest at intervals thereafter. Rinse the electrodes
the reagent is of sufficient high purity to permit its use without
with solvent then with water. Dip them into a pH 4 aqueous
lessening the performance or accuracy of the determination.
buffer solution. Read the mV value after stirring one minute.
Reagent chemicals shall be used for all tests.
Remove the electrodes and rinse with water. Dip the electrodes
7.2 Purity of Water—Unless otherwise indicated, references
in a pH 7 aqueous buffer. Read the mV value after stirring one
to water shall be understood to mean Type III of Specification
minute. Calculate the mV difference. A good electrode system
D1193. Boil the water gently for 5 to 10 min to remove any
will have a difference of at least 158 mV (20 to 25°C). If the
CO and cool the water to room temperature.
difference is less than 158 mV, lift the sleeve of the electrode
and ensure electrolyte flow. Repeat the measurements. If the
7.3 Pyridine—(Warning—Flammable and harmful if
difference is still less than 158 mV, clean or replace the
inhaled, swallowed or absorbed through the skin.).
electrode(s).
7.4 Oxalic Acid (standard reagent, H C O ·2H O)—
2 2 4 2
10.3.1 When the sensing electrode and the reference elec-
(Warning—Harmful if absorbed through the skin.).
trode are separate, one pair of electrodes shall be considered as
one unit. If one or the other is changed, it shall be considered
7.5 Sodium Hydroxide Solution (0.5 M)—Weigh 20 g of
as a different pair and
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