ISO 24698-2:2018

INTERNATIONAL ISO
STANDARD 24698-2
Second edition
2018-10
Rubber, raw — Determination of bound
acrylonitrile content in acrylonitrile-
butadiene rubber (NBR) —
Part 2:
Kjeldahl method
Caoutchouc brut — Détermination du contenu en acrylonitrile lié
dans le caoutchouc acrylonitrile-butadiène (NBR) —
Partie 2: Méthode Kjeldahl
Reference number
ISO 24698-2:2018(E)
©
ISO 2018

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ISO 24698-2:2018(E)

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© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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Published in Switzerland
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ISO 24698-2:2018(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents and materials . 2
6 Apparatus . 2
7 Sampling, preparation of the sample for the determination and preparation of reagents .3
7.1 Raw NBR, XNBR or NBIR rubber . 3
7.2 NBR Latex . 3
7.3 Preparation of reagents . 4
8 Procedure. 4
9 Calculation of bound acrylonitrile content . 5
3
9.1 When sulfuric acid (0,05 mol/dm ) is used as the standard volumetric solution . 5
3
9.2 When hydrochloric acid (0,05 mol/dm ) is used as the standard volumetric solution . 5
10 Precision . 5
11 Test report . 6
Annex A (informative) Examples of automatic analysers based on the Kjeldahl method .7
Annex B (informative) Precision on NBR . 9
Annex C (informative) Precision on NBR Latex.10
Bibliography .11
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ISO 24698-2:2018(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
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
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
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
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 second edition cancels and replaces the first edition (ISO 24698-2:2008), which has been
technically revised.
The main changes compared to the previous edition are as follows:
— addition in the scope of two types of raw rubbers, XNBR and NBIR, that contain acrylonitrile;
— addition of NBR latex to the scope and in sample preparation (7.2);
— amendment of of finishing condition of sample weight in 7.1;
— addition of ITP results of NBR latex in a new Annex C.
A list of all the parts in the ISO 24698 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
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INTERNATIONAL STANDARD ISO 24698-2:2018(E)
Rubber, raw — Determination of bound acrylonitrile
content in acrylonitrile-butadiene rubber (NBR) —
Part 2:
Kjeldahl method
WARNING — Persons using this document should be familiar with normal laboratory practice.
This document does not purport to address all of the safety problems, if any, associated with its
use. It is the responsibility of the user to establish appropriate safety and health practices.
CAUTION — Certain procedures specified in this document might involve the use or generation
of substances, or the generation of waste, that could constitute a local environmental hazard.
Reference should be made to appropriate documentation on safe handling and disposal after use.
1 Scope
This document specifies a method for the determination of the bound acrylonitrile content in NBR by an
automatic analyser which uses the Kjeldahl method. The method is also applicable to XNBR (carboxylic
acrylonitrile-butadiene rubber) and NBIR (acrylonitrile- butadiene-isoprene rubber) as well as NBR latex.
NOTE This document and ISO 24698-1 can give different results on the same rubber sample.
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 123, Rubber latex — Sampling
ISO 1407:2011, Rubber — Determination of solvent extract
ISO 1795, Rubber, raw natural and raw synthetic — Sampling and further preparative procedures
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:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at https: //www .electropedia .org/
4 Principle
The nitrogen in a sample of raw rubber is converted into an ammonium salt by digestion with potassium
sulfate, sulfuric acid and a copper sulfate catalyst. Ammonia is then released by the addition of strong
alkali and steam-distilled into boric acid solution and titrated with standard volumetric acid solution.
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ISO 24698-2:2018(E)

5 Reagents and materials
5.1 Reference material: ammonium sulfate, purity ≥99,5 %.
5.2 Standard volumetric acid solutions:
3
— sulfuric acid solution, 0,05 mol/dm ;
3
— hydrochloric acid solution, 0,05 mol/dm .
3
5.3 Sulfuric acid, ρ = 1,84 g/cm .
3
5.4 Sodium hydroxide solution, 10 mol/dm .
5.5 Potassium sulfate (K SO ).
2 4
5.6 Copper sulfate (CuSO ·5H O).
4 2
5.7 Indicators, in accordance with the analyser manufacturer's instructions:
— methyl red;
— methylene blue;
— bromophenol blue.
5.8 Boric acid solution, in accordance with the analyser manufacturer's instructions (commonly, 3 %
or 4 % solution).
5.9 Ethanol, purity ≥95 % by volume.
5.10 Methanol, purity ≥99,8 % by volume.
6 Apparatus
6.1 Automatic analyser.
6.1.1 General
The automatic analyser consists of the following components:
a) a digestion unit, capable of maintaining a minimum operating temperature in accordance with the
manufacturer's instructions for digestion of the sample;
b) a digestion tube, capable of being used for both digestion and distillation;
c) a distillation unit, capable of introducing a fixed volume of sodium hydroxide into the digestion
tube when it is placed in the unit and of steam-distilling, for a fixed time, the liberated NH and
3
condensing it into a fixed volume of ammonia-absorbing boric acid solution in a titration vessel;
d) a titration unit, capable of introducing the boric acid solution into the titration vessel before
distillation and of titrating the distillate with standard volumetric acid solution photometrically
using a photo-cell or potentiometrically;
e) a microprocessor, capable of calibrating the instrument with a reference material and of converting
the titration result into mass % of nitrogen in the sample.
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ISO 24698-2:2018(E)

6.1.2 Performance requirements
The accuracy of the system shall be demonstrated by performing ten successive determinations using
a reference material such as ammonium sulfate. The mean of the ten determinations with the reference
material shall be within ±0,2 percentage points of the theoretical value. The relative standard deviation
shall be within 0,5 % by mass of nitrogen for the reference material.
NOTE The relative standard deviation, (r), is given by Formula (1):
s
r =×100 (1)
()
w
N
where
(r) is the relative standard deviation, in %;
s is the standard deviation;
w is the mean nitrogen content, in mass %.
N
6.2 Balance, weighing to the nearest 0,1 mg.
6.3 Extraction apparatus, as specified for method C in ISO 1407:2011.
3
6.4 Beaker, capacity 300 cm .
6.5 Stirrer.
6.6 Sieve, aperture 150 μm.
6.7 Absorbent tissue.
6.8 Drying oven, capable of maintaining a temperature of 100 °C ± 2 °C.
6.9 Roll mill, the temperature of whose rolls can be maintained at 100 °C ± 5 °C.
7 Sampling, preparation of the sample for the determination and preparation of
reagents
7.1 Raw NBR, XNBR or NBIR rubber
Take a sample of 10 g to 50 g in accordance with the method specified in ISO 1795, and pass the sample
between the surfaces of the mill rolls with the nip set at 0,2 mm ± 0,05 mm and with the surface
temperature of the rolls maintained at 100 °C ± 5 °C.
Take a portion of about 3 g to 5 g from the sample prepared as specified above.
Extract this portion with ethanol by method C in ISO 1407:2011 and rinse the extracted material twice
with a small amount of fresh ethanol. Pour the contents of the flask onto a clean 150 μm sieve to recover
the extracted material. Gently blot the extracted material with absorbent tissue to remove excess
solvent, and dry the pieces of extracted rubber, separated from each other, in the oven at 100 °C ± 2 °C
until the mass does not change by more than 2,0 mg over a period of 10 min.
7.2 NBR Latex
Take about 500 g of NBR latex sample by the appropriate method in ISO 123.
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ISO 24698-2:2018(E)

In order to coagulate the latex, add dropwise a small portion of the NBR latex sample, containing
3 3
approximately 5 g of solid NBR, to 150 cm of ethan
...