ISO 19051:2015
(Main)Rubber, raw natural, and rubber latex, natural — Determination of nitrogen content by Micro Dumas combustion method
Rubber, raw natural, and rubber latex, natural — Determination of nitrogen content by Micro Dumas combustion method
ISO 19051:2015 specifies a test method for the determination of nitrogen content of raw natural rubber using the Micro Dumas combustion method. This method is also applicable to natural rubber latex.
Latex de caoutchouc brut — Détermination de la teneur en azote par la méthode de combustion de Micro Duma
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 19051
First edition
2015-05-01
Rubber, raw natural, and rubber latex,
natural — Determination of nitrogen
content by Micro Dumas combustion
method
Latex de caoutchouc brut — Détermination de la teneur en azote par
la méthode de combustion de Micro Duma
Reference number
©
ISO 2015
© ISO 2015, Published in Switzerland
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ii © ISO 2015 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Principle . 1
4 Reagents . 2
5 Apparatus . 2
6 Calibration . 3
7 Sampling and preparation of test piece . 3
8 Procedure. 4
9 Test report . 5
Annex A (normative) Calibration schedule . 7
Annex B (informative) Technical justification of Micro Dumas combustion process .8
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
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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).
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on the ISO list of patent declarations received (see www.iso.org/patents).
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assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information.
The committee responsible for this document is ISO/TC 45, Rubber and rubber products, Subcommittee
SC 2, Testing and analysis.
iv © ISO 2015 – All rights reserved
Introduction
The Dumas combustion method has become the most frequently used method worldwide for the accurate
and fast determination of nitrogen. Compared to the wet chemical Kjeldahl method, it is superior in
terms of speed, safety, and environmental friendliness. The representative analysis of a natural product
requires a larger sample size of up to 1 g or more.
The nitrogen content of natural rubber is related to the protein level. The protein content of natural
rubber varies depending upon its source and methods used in its processing. Generally, raw natural
rubber is expected to have a nitrogen content in the range of 0,3 % to 0,6 %. The normal latex grades
have lower levels of nitrogen than the “dry” rubbers, with values around 0,2 %. However, “skim” rubber,
with its high protein content, will have appreciably higher values, in the range of 1,5 % to 2,5 %.
This test method will help determine the nitrogen content of the raw natural rubber in the shortest
possible time and will be helpful for laboratory quality control testing.
INTERNATIONAL STANDARD ISO 19051:2015(E)
Rubber, raw natural, and rubber latex, natural —
Determination of nitrogen content by Micro Dumas
combustion method
WARNING 1 — Persons using this International Standard should be familiar with normal
laboratory practice. This International Standard 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 and to ensure compliance with any national regulatory conditions.
WARNING 2 — Certain procedures specified in this International Standard 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 International Standard specifies a test method for the determination of nitrogen content of raw
natural rubber using the Micro Dumas combustion method. This method is also applicable to natural
rubber latex.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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 124:2014, Latex, rubber — Determination of total solids content
ISO 1795, Rubber, raw natural and raw synthetic — Sampling and further preparative procedures
ISO 18899:2013, Rubber — Guide to the calibration of test equipment
3 Principle
In the combustion process (furnace at ca. 1 000 °C), nitrogen is converted to nitrogen gas/oxides. If
other elements are present, they will also be converted to different combustion products. A variety of
absorbents are used to remove these additional combustion products.
The combustion products are swept out of the combustion chamber by an inert carrier gas such as
helium and passed over heated (about 600 °C) high purity copper. This copper can be situated at the
base of the combustion chamber or in a separate furnace. The function of this copper is to remove any
oxygen not consumed in the initial combustion and to convert any oxides of nitrogen to nitrogen gas.
The gases are then passed through the absorbent traps.
Detection of the gases can be carried out in a variety of ways, including the following:
a) GC separation followed by quantification using thermal conductivity detection;
b) partial separation by GC (“frontal chromatography”) followed by thermal conductivity detection;
c) series of separate infrared and thermal conductivity cells for detection of individual compounds.
Quantification of nitrogen requires calibration with high purity “micro-analytical standard” standard
reference compounds with known nitrogen content.
During the combustion process, organic compounds are oxidized at high temperature to yield carbon
dioxide, water, and oxides of nitrogen:
O;1000°C
CHNCompoundC→ OH++ONO
22 x
The oxides of nitrogen are then converted to nitrogen gas in the presence of metallic copper:
Cu;600°C
NO → N
x 2
4 Reagents
4.1 Helium (99,99 %), as carrier gas and high purity (99,99 %) oxygen as combustion gas.
4.2 Analytical standards, as per Table 1.
Table 1 — Reference standards
Sample Molecular formula Theoretical %C Theoretical %H Theoretical %N Theoretical %S
Tryptophan C H N O 64,69 % 5,92 % 13,72 % 0 %
11 12 2 2
Imidazole C H N 52,93 % 5,92 % 41,15 % 0 %
3 4 2
Isatin C H NO 65,31 % 3,43 % 9,52 % 0 %
8 5 2
Alanine C H NO 40,44 % 7,92 % 15,72 % 0 %
3 7 2
Nicotinamide C H N O 59,01 % 4,95 % 22,94 % 0 %
6 6 2
Lysine C H N O 49,30 % 9,65 % 19,16 % 0 %
6 14 2 2
Cyclohexanone C H O 73,43 % 10,27 % 0 % 0 %
6 10
Acetanilide C H NO 71,09 % 6,71 % 10,36 % 0 %
8 9
Urea CH N O 20,00 % 6,71 % 46,65 % 0 %
4 2
Atropine C H NO 70,56 % 8,01 % 4,84 % 0 %
17 23 3
Cystine C H N O S 29,99 % 5,03 % 11,66 % 26,69 %
6 12 2 4 2
Sulphanilamide C H N O S 41,84 % 4,68 % 16,27 % 18,62 %
6 8 2 2
BBOT C H N O S 72,53 % 6,09 % 6,51 % 7,44 %
26 26 2 2
5 Apparatus
5.1 Combustion elemental analysers are manufactured in a variety of configurations to suit specific
applications and the choice will depend on the elements of interest, the sample type and size, and the
concentration of the analyte.
All instruments require two gas supplies: an inert carrier gas (helium recommended) and high purity
oxygen (minimum 99,99 %). The strict specification for oxygen and the carrier gas is associated with
the need to reduce the nitrogen “blank” contribution to an inconsequential level. Additionally, GC-type
gas filters are also usually fitted to prevent trace organic species and water entering the combustion
system.
The choice of test piece introduction system will depend on the application and the nature of the material.
For solids or viscous liquids, the test pieces are weighed out into tin capsules. For liquids, the test pieces
can be sealed in individual aluminium vials or introduced through a liquid auto-sampler. Both capsules
2 © ISO 2015 – All rights reserved
and vials are pre-cleaned and dried to avoid trace contamination from oils and water acquired during
their manufacture. Instruments are marketed with either simple “one shot” introduction interfaces or a
carousel type auto-sampler. In some instances, a microbalance is directly interfaced with the analyser
to allow the automatic recording of the weight of each test piece.
The combustion section of the analyser is designed to achieve both complete combustion of the test
piece and conversion of oxides of nitrogen to nitrogen gas (N ). Although different approaches have been
chosen by different manufacturers, the use of high purity copper is universal for the reduction stage.
In some instruments, the combustion and reduction stages are housed in separate furnaces. In others,
the reactions are combined in a single two-tier furnace. Catalysts are usually added to the combustion
section to aid complete combustion along with absorbents to remove potential contaminants. Both the
catalysts/absorbents and copper metal are packed into readily exchangeable tubes made of ceramic
material or high quality silica.
The detection system within the analyser can take several forms depending on the combustion mode
and test piece size. With small test pieces, the combustion gases can be separated on a GC column and
quantified using a thermal conductivity detector. A schematic diagram of such a system is shown in
Figure 1. If larger test pieces are required, an instrument employing “frontal” chromatography can be
chosen. The latter approach employs a GC column with thermal conductivity detection but provides a
step-wise profile for integration. Other detection approaches do not require a separation step but use
separate infrared and thermal conductivity cells to respond to individual elements.
Figure 1 — Different components of elemental analyser (Micro Dumas method)
5.2 Analytical balance, capable of weighing to the nearest 0,000 01 g.
5.3 Micro syringe, calibrated.
5.4 Test piece holder, generally, a tin capsule.
6 Calibration
The instrument shall be calibrated in accordance with the schedule given in Annex A.
7 Sampling and preparation of test piece
7.1 For the determination of nitrogen in raw solid rubber, take the laboratory sample in accordance
with the method specified in ISO 1795 and prepare the tes
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