ASTM D7253-22

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of the standard as published by ASTM is to be considered the official document.
Designation: D7253 − 16 D7253 − 22
Standard Test Method for
Polyurethane Raw Materials: Determination of Acidity as
Acid Number for Polyether Polyols
This standard is issued under the fixed designation D7253; 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*
1.1 This test method measures the acidic constituents in polyether polyols and reports the results as acid number. The typical acid
number range is 0-0.1 mg KOH/g sample. (See Note 1.)
1.2 The values stated in SI units are to be regarded as standard.
1.3 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.
NOTE 1—There is no known ISO equivalent to this standard.
1.4 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:
D883 Terminology Relating to Plastics
E456 Terminology Relating to Quality and Statistics
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
E2935 Practice for Evaluating Equivalence of Two Testing Processes
3. Terminology
3.1 Definitions—For definitions of terms used in this test method see Terminology D883, unless otherwise specified. For terms
relating to precision and bias and associated issues, the term used in this standard are defined in accordance with Terminology
E456.
3.2 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.22 on Cellular Materials - Plastics
and Elastomers.
Current edition approved Sept. 1, 2016March 15, 2022. Published September 2016March 2022. Originally approved in 2006. Last previous edition approved in 20112016
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as D7253 - 11D7253 - 16.(2011) . DOI:10.1520/D7253-16. DOI:10.1520/D7253-22.
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 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
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D7253 − 22
3.2.1 acid number—the quantity of base, expressed in milligrams of potassium hydroxide, that is required to titrate acidic
constituents in 1 g of sample.
4. Summary of Test Method
4.1 The sample is dissolved in 2-propanol. The resulting single-phase solution is titrated at room temperature with 0.02 N
methanolic potassium hydroxide solution to an end point indicated by the color change (pink endpoint) of the added
phenolphthalein.
NOTE 2—It is permissible to use automatic titrators using a potentiometric endpoint for the titration portion of this test provided the automatic titration
method is tested to obtain at least equivalent results to the manual titration.
NOTE 3—Phenolphthalein is the indicator of choice based on published hydroxyl number methods that include an acid number correction. Other indicators
can be are chosen if specific acids are of interest. Bromothymol blue (green endpoint) can be is used for stronger acids (pKa’s < ~4) and thymolphthalein
(blue endpoint) can be is used for weak acids (pKa’s > ~7).
5. Significance and Use
5.1 This test method is suitable for quality control, as a specification test, and for research. Acid numbers indicate the extent of
any neutralization reaction of the polyol with acids. The results of this method measure batch-to-batch uniformity and are used as
correction factors in calculating true hydroxyl numbers.
6. Interferences
6.1 Samples or constituents that are highly-colored will interfere with or prevent the use of this test method. In this case, a
potentiometric titration using an autotitrator is recommended, see Note 2.
7. Apparatus
7.1 Buret, 10 mL, can be manual or automatic.
7.2 Graduated cylinder, 10 mL, maximum.
7.3 Balance, analytical with sensitivity of at least 60.0001 g.
7.4 Erlenmeyer flask, at least 250 mL capacity.
7.5 Stirring bars or stirring rods.
8. Reagents and Materials
8.1 Purity of Reagents—Use reagent-grade chemicals in all tests. Unless otherwise indicated, it is intended that all reagents
conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such
specifications are available. Other grades can be used are acceptable provided it is first ascertained that the reagent is of
sufficiently high purity to permit its use without lessening the accuracy of the determination.
8.2 Phenolphthalein, Indicator Grade.
8.3 Phenolphthalein Indicator Solution—Dissolve 1 g of phenolphthalein in 100 mL of methanol.
8.4 Potassium Hydroxide Standard Solution (0.02 N)—Prepare and standardize a 0.02 N methanolic potassium hydroxide solution.
Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing of reagents not listed by
the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K. and the United States Phamacopeia and National Formulary,
U.S. Pharmaceutical Convention, Inc. (USPC), Rockville, MD.
D7253 − 22
TABLE 1 Round Robin Data for Acid Number (mg KOH/g) in
Accordance with Practice E691
A B C D E
Material Average S S r
...