ISO 7725:2020

INTERNATIONAL ISO
STANDARD 7725
Second edition
2020-01
Rubber and rubber products —
Determination of chlorine and
bromine content
Caoutchouc et produits à base de caoutchouc — Détermination de la
teneur en brome et en chlore
Reference number
ISO 7725:2020(E)
©
ISO 2020

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ISO 7725:2020(E)

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© ISO 2020
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Published in Switzerland
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ISO 7725:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Sample preparation . 2
5.1 Sampling . 2
5.2 Sample solutions . 2
6 Method A — Ion chromatography . 2
6.1 Reagents. 2
6.2 Apparatus . 3
6.3 Determination . 3
6.3.1 Setting up the ion chromatograph . 3
6.3.2 Preparation of the calibration curve . 3
6.3.3 Measuring blank solutions and sample solutions . 3
6.4 Calculation . 4
7 Method B — Potentiometric titration . 4
7.1 Reagents. 4
7.2 Apparatus . 5
7.3 Determination . 5
7.4 Calculation . 6
8 Precision . 7
9 Test report . 7
Annex A (normative) Preparation of sample solutions by the tubular furnace combustion
method . 8
Annex B (normative) Preparation of sample solutions by the oxygen combustion flask method .11
Annex C (normative) Preparation of sample solutions by the oxygen combustion bomb method .14
Annex D (informative) Conditions for ion chromatography .16
Annex E (informative) Precision results from an interlaboratory test programme .18
Bibliography .20
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ISO 7725:2020(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 www .iso .org/
iso/ foreword .html.
The committee responsible for this document is ISO/TC 45, Rubber and rubber products, Subcommittee
SC 2, Testing and analysis.
This second edition cancels and replaces the first edition (ISO 7725:1991), which has been technically
revised. The main changes compared to the previous edition are as follows:
— the ion chromatography method has been added in Clause 6;
— the titration procedure has been improved in Clause 7;
— the tubular furnace combustion method for sample preparation has been added in Annex A;
— the oxygen combustion flask method for sample preparation has been improved in Annex B;
— the oxygen combustion bomb method for sample preparation has been added in Annex C;
— mercury nitrate and hydrazine are no longer used due to their hazardous properties.
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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ISO 7725:2020(E)

Introduction
The two test methods in this document for quantitative analysis can be used for any type of rubbers
(i.e. raw rubber, vulcanized rubber or unvulcanized rubber) containing chlorine and/or bromine in any
form of existence, such as an element of polymer chain, chemical additives or a part of contaminations.
Three combustion methods are given for preparation of sample solution, i.e. tubular furnace
combustion method, oxygen combustion flask method and oxygen combustion bomb method. After
sample solutions are prepared, a content determination procedure, i.e. either ion chromatography or
potentiometric titration, follows. The most convenient and efficient method from the testing time and
safeness points of view, is the combination of tubular furnace combustion and ion chromatography, as
ion chromatography is widely used in quality control of polymer/rubber products or in environmental
analysis.
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INTERNATIONAL STANDARD ISO 7725:2020(E)
Rubber and rubber products — Determination of chlorine
and bromine content
WARNING 1 — 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 and to
determine the applicability of any other restrictions.
WARNING 2 — 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 methods for the determination of chlorine and/or bromine present in raw
rubber as well as vulcanized or unvulcanized rubber compounds.
The methods are applicable to natural rubbers and to the following synthetic rubbers: isoprene,
styrene-butadiene, butadiene, butyl, halogenated butyl, nitrile, ethylene-propylene, chloroprene and
epichlorohydrin.
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 1795, Rubber, raw natural and raw synthetic — Sampling and further preparative procedures
ISO 3696, Water for analytical laboratory use — Specification and test methods
ISO 4661-2, Rubber, vulcanized — Preparation of samples and test pieces — Part 2: Chemical tests
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 http:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle
A sample solution is obtained by combustion of a test piece either burnt in a tubular furnace with
a stream of oxygen-containing gas and passed into a prepared solution or burnt in an oxygenic
atmosphere in a flask or a bomb which contains a solution to absorb the combustion gas. The sample
solution is then analysed by ion chromatography (method A) or potentiometric titration (method B) to
determine the content of chlorine and/or bromine in a sample.
For rubber samples of very low halogen content, method A is preferable since the inflection point might
not be obtained by method B.
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ISO 7725:2020(E)

5 Sample preparation
5.1 Sampling
For raw rubber, carry out sampling in accordance with ISO 1795.
For rubber compounds, carry out sampling in accordance with ISO 4661-2.
NOTE Chlorine and bromine contained in additives or contaminants can be detected by these sampling
methods, unless previously removed by extraction.
5.2 Sample solutions
To prepare sample solutions by the tubular furnace combustion method, follow the method specified in
Annex A. Repeat the procedure to create two solutions for each sample.
To prepare sample solutions by the oxygen combustion flask method, follow the method specified in
Annex B. Repeat the procedure to create two solutions for each sample.
To prepare sample solutions by the oxygen combustion bomb method, follow the method specified in
Annex C. Repeat the procedure to create two solutions for each sample.
If incomplete combustion, such as soot generation during combustion, is expected by the oxygen
combustion flask method, it is preferable to use the tubular furnace combustion method or the oxygen
combustion bomb method.
6 Method A — Ion chromatography
6.1 Reagents
6.1.1 Water, of grade 1 or higher. It shall be as specified in ISO 3696.
6.1.2 Chloride ion standard stock solution, of a commercial standard solution with a certified
chloride ion concentration, e.g. 1 000 mg/l of chloride, traceable to national standards. Comply with the
manufacturer’s expiration date or recommended shelf-life.
6.1.3 Chloride ion calibration solution, prepared by diluting the chloride ion standard stock solution
(6.1.2) with water (6.1.1). Prepare at least four solutions of different concentration of chloride ion covering
the expected concentration from the sample. The solutions shall be prepared every analytical day.
6.1.4 Bromide ion standard stock solution, of a commercial standard solution with a certified
bromide ion concentration, e.g. 1 000 mg/l of bromide, traceable to national standards. Comply with the
manufacturer’s expiration date or recommended shelf-life.
6.1.5 Bromide ion calibration solution, prepared by diluting the bromide ion standard stock solution
(6.1.4) with water (6.1.1). Prepare at least four solutions of different concentration of bromide ion covering
the expected concentration from the sample. The solutions shall be prepared every analytical day.
6.1.6 Mixed calibration solution, for measuring chloride ion and bromide ion at the same time,
prepared by mixing chloride ion (6.1.2) and bromide ion (6.1.4) with water (6.1.1). Prepare at least four
solutions of different concentration of chloride ion and bromide ion covering the expected concentration
from the sample. The solutions shall be prepared every analytical day.
6.1.7 Eluent solution, capable of eluting chloride ion and bromide ion to a proper retention time in
selected column. Follow the column manufacturer’s instructions.
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ISO 7725:2020(E)

NOTE Some examples of eluent are provided in Annex D.
6.2 Apparatus
6.2.1 Ion chromatograph, consisting of a pump to supply eluent, a sample injector, a column and a
conductivity detector specified as follows:
— pump: capable of delivering a constant flow within the range of 0,1 ml/min to 2,0 ml/min;
— sample injector: capable of injecting a constant volume of solution;
— column: filled with anion exchange resin suitable for resolving chloride ion and bromide ion from
other inorganic anions;
— suppressor: used for reducing the conductivity of high ionic eluent;
— detector: for detecting conductivity.
NOTE Some examples of commercially available column are provided in Annex D.
An ion chromatograph fitted with a suppressor as Figure 1 is suitable for sensitive analysis. The
suppressor is attached between a column and a detector.
An ion chromatograph without a suppressor as Figure 2 may also be selected.
Figure 1 — Example of basic ion chromatography system with suppressor
Figure 2 — Example of basic ion chromatography system without suppressor
6.3 Determination
6.3.1 Setting up the ion chromatograph
Set up the ion chromatograph in accordance with the manufacturer’s instructions.
6.3.2 Preparation of the calibration curve
Inject the calibration solutions in increasing order of concentration and measure the conductivity of
chloride ion and bromide ion peak area in each solution. Determine the correlation formula (calibration
curve) by plotting the peak area as a function of concentration by means of linear regression.
6.3.3 Measuring blank solutions and sample solutions
Inject the blank solution and sample solutions (created in 5.2) to measure the peak areas in sequence.
Then determine the chloride ion and bromide ion concentration of the blank (C and C , respectively)
B1 B2
as well as the concentration of the individual sample solution (C and C , respectively) by using the
S1 S2
calibration curve determined in 6.3.2.
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ISO 7725:2020(E)

6.4 Calculation
Calculate the chlorine content (w ) and/or bromine content (w ), in percentage by mass, by
1 2
Formulae (1) and (2), using the two determination values in 6.3. Average the results and round to the
second decimal place.
CC− ×V
()
SB11
w = ×100 (1)
1
W×1000
CC− ×V
()
SB22
w = ×100 (2)
2
W×1000
where
W is the mass of the test piece, in mg (see A.4.2, B.4.1 or C.4.1);
C is the chloride ion concentration of the sample solution, in mg/l;
S1
C is the chloride ion concentration of the blank solution, in mg/l;
B1
C is the bromide ion concentration of the sample solution, in mg/l;
S2
C is the bromide ion concentration of the blank solution, in mg/l;
B2
V is the fixed amount of the solution, in ml (see A.4.5, B.4.9 or C.4.8).
7 Method B — Potentiometric titration
7.1 Reagents
7.1.1 Water, of grade 1 or higher. It shall be as specified in ISO 3696.
3
7.1.2 Nitric acid, concentrated, ρ = 1,42 g/cm .
7.1.3 0,5mol/l Nitric acid, c(HNO ) = 0,5 mol/l, obtained by diluting 30 ml of concentrated nitric acid
3
(7.1.2) to 1 000 ml with water (7.1.1).
NOTE Commercially available 0,5 mol/l nitric acid can be used.
7.1.4 Silver nitrate standard volumetric solution, c(AgNO ) = 0,02 mol/l.
3
Standardize a silver nitrate solution obtained by dissolving 3,4 g of silver nitrate in 1 000 ml of water
(7.1.1) as follows:
Take a measured volume (between 1 ml and 5 ml) of sodium chloride solution (7.1.5), dilute with water
(7.1.1) to 25 ml in a 50 ml beaker, and add 2 ml of 0,5 mol/l nitric acid (7.1.3) along with a stirring
bar. Place the solution on a stirrer (7.2.2) and titrate with the silver nitrate solution (7.1.4) using an
automatic titrator (7.2.3). Read the volume from the automatic titrator. Determine the factor ( f) from
the averaged result in duplicating the same procedure.
Commercially available 0,02 mol/l silver nitrate standard volumetric solution and its guaranteed
factor ( f ) can be used.
7.1.5 Sodium chloride solution, traceable to national standards. c(NaCl) = 0,02 mol/l.
Alternatively, dissolve 1,168 g of sodium chloride in 1 000 ml of water (7.1.1).
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ISO 7725:2020(E)

7.1.6 Aluminium nitrate, of analytical reagent grade.
NOTE Aluminium nitrate is used to minimize the mutual interference caused by co-precipitation of chloride
and bromide.
7.1.7 Ethanol, of analytical reagent grade.
Alternatively, 2-propanol may be substituted wherever ethanol is specified.
7.2 Apparatus
7.2.1 Electronic balance, capable of weighing to the nearest 0,01 mg.
7.2.2 Magnetic stirrer and a stirring bar.
7.2.3 Automatic titrator.
7.2.4 Silver electrode, to be used as a measuring electrode.
7.2.5 Reference electrode, of the salt bridge type.
NOTE It is possible to use a combined electrode instead of the silver electrode and the reference electrode.
7.3 Determination
7.3.1 Transfer the blank solutions and sample solutions (created in 5.2) obtained quantitatively to a
300 ml beaker each with several washes water. Place a magnetic stirring bar in the beaker and place the
assembly on the magnetic stirrer (7.2.2).
7.3.2 Add 2 ml of the 0,5 mol/l nitric acid (7.1.3) and 2 g of the aluminium nitrate (7.1.6). Continue
stirring to dissolve the aluminium nitrate, then add 160 ml of ethanol (7.1.7).
7.3.3 Insert the electrode pair (7.2.4 and 7.2.5) into the solution and titrate with the silver nitrate
solution (7.1.4) with an automatic titrator (7.2.3).
NOTE The problem is least when bromine is in excess. At its worst, when chlorine is in excess, the bromine
concentration might appear high, to an extent of 5 % of the chloride ion content. The problem can be minimized
[2]
by carrying out the titration at 60 °C and keeping the rate of addition of titrant to a minimum .
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ISO 7725:2020(E)

7.3.4 The first inflection point on the titration curve represents the bromide ion end point and the
volume of titrant for bromide ion (V ). The second inflection represents the chloride ion end point and
1
the volume of titrant for chloride ion (V ). A typical titration curve is given in Figure 3.
2
Key
X volume of AgNO3 solution (ml)
Y potential difference (mV)
-
1 Br end point
-
2 Cl end point
Figure 3 — Example of titration curve
NOTE The kind of ion can be identified by initially measuring the potential difference at the end point of
each solution which contains chlorine alone and bromine alone.
7.3.5 Determine the volume of the silver nitrate solution required to titrate a blank solution to the
inflection point (V ).
b
7.4 Calculation
Calculate the chlorine content (w ) and/or bromine content (w ), in percentage by mass, by Formulae (3)
3 4
and (4), using the two determination values in 7.3.4 and 7.3.5. Average the results and round to the
second decimal place.
VV−−Vh××f×35,45
()
21 b
w = ×100 (3)
3
W
CC− ×V
()
SB22
w = ×100 (4)
2
W×1000
where
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ISO 7725:2020(E)

V is the volume, in ml, of silver nitrate solution required to titrate to the first inflection point;
1
V is the volume, in ml, of silver nitrate solution required to titrate to the second inflection point;
2
V is the volume, in ml, of silver nitrate solution of blank titration;
b
h is the concentration, in mol/l, of the silver nitrate solution;
f is the factor obtained in 7.1.4;
W is the mass of the test piece, in mg (see A.4.2, B.4.1 or C.4.1).
8 Precision
See Annex E.
9 Test report
The test report shall include the following information:
a) sample details:
1) full description of the sample and its origin;
2) method of preparation of the test piece from the sample, for example moulded or cut;
b) test method:
1) a full reference to the test method used, e.g. method A of ISO 7725:2020;
2) preparation method, i.e. tubular furnace combustion method, oxygen combustion flask method
or oxygen combustion bomb method;
3) determination procedure of halogen content, i.e. method A or method B;
4) details of any procedures not specified in this document;
c) results of the determination, expressed in % and rounded to the second decimal place;
d) date of the test.
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ISO 7725:2020(E)

Annex A
(normative)

Preparation of sample solutions by the tubular furnace
combustion method
A.1 General
This annex gives the preparation procedure of sample solutions by the tubular furnace combustion
method.
A.2 Reagents and materials
A.2.1 Water, of grade 1 or higher. It shall be as specified in ISO 3696.
A.2.2 Hydrogen peroxide, 30 % (m/m) solution, of analytical reagent grade.
A.2.3 Sodium carbonate, of analytical reagent grade.
A.2.4 Sodium hydrogen carbonate, of analytical reagent grade.
A.2.5 Absorbent solution, obtained by dissolving 2,0 ml hydrogen peroxide (A.2.2), 0,286 g sodium
carbonate (A.2.3) and 0,025 g sodium hydrogen carbonate (A.2.4) in 1 000 ml of water (A.2.1).
The absorbent solution shall be prepared every analytical day.
A.2.6 Oxygen, of mass fraction > 99,7 %.
A.2.7 Argon, of mass fraction > 99,98 %.
A.2.8 Air, of certified quality in contamination and purity.
NOTE Air is an alternative to the combustion gas of oxygen mixed with argon. The certification of the air
quality can be judged by individual laboratories in so far as it does not affect the test result.
A.2.9 Membranous filter, of pore size 0,45 µm.
A.3 Apparatus
Apparatus consists of a combustion furnace, a quartz tube and an absorbing vessel, as shown in
Figure A.1. Other types of apparatus than those specified in A.3.1 to A.3.4 may be used when they are
proven to give the same result.
A.3.1 Combustion furnace, with an electric heater capable of heating at a temperature of 1 000 °C and
maintaining the temperature for a certain period of time.
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ISO 7725:2020(E)

A.3.2 Quartz tube, having heat-resistance enough to be heated at the maximum temperature (e.g.
1 000 °C) and capable of holding a sample boat inside.
A gas of oxygen mixed with argon, or air which is supplied by the system, passes through the tube.
A.3.3 Sample boat, made of quartz, ceramic, or platinum and capable of carrying a test piece.
It shall have heat-resistance enough to be heated by the electric heater at the maximum temperature
(e.g. 1 000 °C).
A.3.4 Absorption vessel, made of borosilicate glass with sufficient volume to allow the gas to bubble
through the absorbent solution in it when it is filled at a certain level.
A.3.5 Electronic balance, capable of weighing to the nearest 0,01 mg.
Key
1 flow of combustion gas
2 quartz tube
3 sample boat
4 absorption vessel
5 combustion furnace
Figure A.1 — Example of the tubular combustion furnace
A.4 Procedure
A.4.1 Burn the sample boat at the combustion maximum temperature and weigh it to the nearest
0,1 mg after cooling it down.
A.4.2 Weigh out a test piece of 10 mg to 150 mg taken from the test sample prepared in 5.1 and place
it in the sample boat. Weigh the boat with the test piece to the nearest 0,1 mg and determine the mass of
the test piece, W, by subtracting the mass measured in A.4.1.
A.4.3 Follow the procedure a) to c) below unless manufacturer's instructions are available:
a) Insert the sample boat into the combustion furnace.
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ISO 7725:2020(E)

b) Put 15 ml of the absorbent solution (A.2.5) in the absorption vessel and connect it to the combustion
furnace.
c) Start the flow of the argon and oxygen gas, or air, and turn on the combustion furnace to burn the
test piece completely. An example of the combustion condition is as follows:
— temperature: 1 000 °C;
— flow rate of argon gas: 200 ml/min;
— flow rate of oxygen gas: 400 ml/min.
NOTE If the conditions are specified in manufacturer's instructions, the amount of the absorbed liquid, the
combustion temperature and the gas flow rate can be set accordingly.
A.4.4 After the completion of burning, wash through the whole tube and collect the washes into the
absorption vessel together with the absorbent solution to use as a sample solution.
A.4.5 For potentiometric titration, use the solution (A.4.4).
For ion chromatography analysis, adjust the solution volume (A.4.4) to a fixed amount, V, using a
specified volume vessel. If necessary, filter out a particulate matter in the sample solution with a
membranous filter (A.2.9) before injecting the solution into the ion chromatograph. To avoid possible
contamination, wash the membranous filter with a small quantity of the absorbent solution (A.2.5).
NOTE If any specific conditions are provided by manufacturer's instructions, the fi
...