ISO 7781:2017

INTERNATIONAL ISO
STANDARD 7781
Fifth edition
2017-11
Styrene-butadiene rubber, raw —
Determination of soap and organic-
acid content
Caoutchouc butadiène-styrène brut — Détermination de la teneur en
savons et acides organiques
Reference number
ISO 7781:2017(E)
©
ISO 2017

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ISO 7781:2017(E)

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ISO 7781:2017(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 2
7 Sampling and sample preparation . 3
8 Procedure. 3
8.1 Preparation of the test solution . 3
8.2 Method A — Titration method using indicator reagent . 3
8.2.1 Procedure for determination of soap content . 3
8.2.2 Procedure for determination of organic-acid content . 3
8.3 Method B — Titration method using automatic potentiometric titrator . 4
8.3.1 Procedure for determination of soap content . 4
8.3.2 Procedure for determination of organic-acid content . 4
8.4 Method C — Back titration method using automatic potentiometric titrator . 4
8.4.1 Preparation . 4
8.4.2 Sample titration procedure. 6
8.4.3 Blank titration procedure. 6
8.4.4 Determination of equivalence points . 6
9 Expression of results . 6
9.1 Soap content . 6
9.2 Organic-acid content . 7
10 Precision data . 8
11 Test report . 8
Annex A (informative) Test for rosin . 9
Annex B (informative) Precision data for method C .10
Bibliography .12
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ISO 7781:2017(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
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URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 45, Rubber and rubber products,
Subcommittee SC 3, Raw materials (including latex) for use in the rubber industry.
This fifth edition cancels and replaces the fourth edition (ISO 7781:2008), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— addition of a new Method C (8.4) for the back titration using automatic potentiometric titrator;
— update of Method B (8.3) for the titration method using automatic potentiometric titrator;
— deletion of buffer solutions of pH 7, pH 4 and pH 9 (former 4.6 to 4.8);
— addition of pipette (6.9) and automatic potentiometric titrator (6.11);
— update of the expression of results (Clause 9);
— addition of precision data in informative Annex B;
— addition of a bibliography.
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INTERNATIONAL STANDARD ISO 7781:2017(E)
Styrene-butadiene rubber, raw — Determination of soap
and organic-acid content
1 Scope
This document specifies three methods for the determination of the soap and organic-acid content of
raw styrene-butadiene rubber (SBR).
— Method A is the titration method using indicator reagent.
— Method B is the titration method using an automatic potentiometric titrator.
— Method C is the back titration method using an automatic potentiometric titrator.
Since the soaps and organic acids present in the rubber are not single chemical compounds, the method
gives only an approximate value for the soap and organic-acid content.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 385, Laboratory glassware — Burettes
ISO 648, Laboratory glassware — Single-volume pipettes
ISO 1042, Laboratory glassware — One-mark volumetric flasks
ISO 1795, Rubber, raw natural and raw synthetic — Sampling and further preparative procedures
ISO 4799, Laboratory glassware — Condensers
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http://www.electropedia.org/
— ISO Online browsing platform: available at https://www.iso.org/obp
4 Principle
A weighed test portion of the rubber, in the form of thin strips, is extracted using an ethanol-toluene
azeotrope, or, for alum-coagulated rubber, using an ethanol-toluene-water mixture. After making up to
a standard volume, an aliquot portion of the extract is withdrawn and titrated with acid-base titration.
— Method A: Titrate with standard acid for the determination of soap and with standard alkali for the
determination of organic acid using indicator reagent.
— Method B: Titrate with standard acid for the determination of soap and with standard alkali for the
determination of organic acid using automatic potentiometric titrator. The titration equivalence
point is determined by inflection point.
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ISO 7781:2017(E)

— Method C: Back titrate with standard alkali for the determination of soap and organic acid after
adding sufficient acid. The titration equivalence point is determined by inflection point.
For methods A and B, there are no precision data available. Therefore their precision is not known. For
method C, if the quantity of soap is very low, the standard deviation of the measurement of soap content
should be looked at carefully.
5 Reagents
During the analysis, use only reagents of recognized analytical grade and only distilled water or water
of equivalent purity.
5.1 Ethanol-toluene azeotrope (ETA).
Mix seven volumes of absolute ethanol with three volumes of toluene. Alternatively, mix seven volumes
of commercial-grade ethanol with three volumes of toluene, and boil the mixture with anhydrous
calcium oxide (quicklime) under reflux for 4 h. Cool to room temperature and decant through No. 42
filter paper.
5.2 Ethanol-toluene-water mixture.
3 3
Mix 95 cm of ETA (5.1) and 5 cm of water.
3
5.3 Sodium hydroxide solution, c(NaOH) = 0,1 mol/dm , accurately standardized.
5.4 Thymol blue indicator.
3 3
Dissolve 0,06 g of thymol blue in 6,45 cm of 0,02 mol/dm sodium hydroxide solution and dilute to
3
50 cm with water.
3
5.5 Hydrochloric acid, c(HCl) = 0,05 mol/dm , accurately standardized.
6 Apparatus
Use ordinary laboratory apparatus and the following.
6.1 Balance, accurate to 1 mg.
6.2 Hotplate.
3 3
6.3 Wide-mouthed conical flask, of capacity 400 cm to 500 cm .
3
6.4 Volumetric flask, of capacity 250 cm , complying with the requirements of ISO 1042.
6.5 Reflux condenser, complying with the requirements of ISO 4799.
3
6.6 Conical flask, of capacity 250 cm .
NOTE Alternatively, a Soxhlet extractor can be used instead of a reflux condenser and a conical flask.
3
6.7 Burette, of capacity 25 cm , complying with the requirements of ISO 385.
3
6.8 Pipette, of capacity 100 cm , complying with the requirements of ISO 648.
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ISO 7781:2017(E)

3
6.9 Pipette, of capacity 2 cm , complying with the requirements of ISO 648.
6.10 Magnetic stirrer, with a polytetrafluoroethylene-coated stirrer bar.
6.11 Automatic potentiometric titrator, including an electrode which is applicable to test the pH of
organic solution.
7 Sampling and sample preparation
Sheet out 2 g to 6 g of rubber, selected and prepared in accordance with ISO 1795. Cut into pieces no
larger than 2 mm × 2 mm or strips no longer than 10 mm and no wider than 5 mm. Weigh a test portion
of approximately 2 g to the nearest 0,001 g.
8 Procedure
8.1 Preparation of the test solution
3
Place a circular filter paper in the bottom of the wide-mouthed conical flask (6.3), and add 100 cm
of ETA extraction solvent (5.1) for all rubbers except alum-coagulated rubbers. For alum-coagulated
rubbers, use an ethanol-toluene-water mixture (5.2).
Introduce the strips of rubber separately into the flask, swirling after each addition so that the strips
are thoroughly wetted with solvent and sticking is
...