ISO 20336:2025
(Main)Coal and coke — Determination of total sulfur by Coulomb titration method
Coal and coke — Determination of total sulfur by Coulomb titration method
This document specifies an alternative method of determining the total sulfur mass fraction of coal and coke by high-temperature combustion by Coulomb titration.
Charbon et coke — Détermination de la teneur totale en soufre par la méthode de titrage de Coulomb
General Information
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Standards Content (Sample)
International
Standard
ISO 20336
Second edition
Coal and coke — Determination
2025-02
of total sulfur by Coulomb
titration method
Charbon et coke — Détermination de la teneur totale en soufre
par la méthode de titrage de Coulomb
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagent and materials . 1
6 Apparatus . 2
7 Preparation of test sample . 4
8 Procedure . 5
8.1 Preparation of test .5
8.2 Calibration .5
8.3 Calibration procedure .5
8.4 Procedure .6
8.5 Calibration check .6
9 Expression of result . 6
10 Precision . 7
10.1 Repeatability limit .7
10.2 Reproducibility limit .7
11 Test report . 7
iii
Foreword
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This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5, Methods
of analysis.
This second edition cancels and replaces the first edition (ISO 20336:2017), which has been technically
revised.
The main changes are as follows:
— replaced “solid mineral fuels” with “coal and coke” and “content” with “mass fraction”;
— added Figure 2 of a purified air stream supply system;
— added a note to 8.1.4.
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
iv
Introduction
The objective of this document is to provide an alternative method for determining the total sulfur mass
fraction in coal and coke with high-temperature combustion by Coulomb titration using automated
instruments.
v
International Standard ISO 20336:2025(en)
Coal and coke — Determination of total sulfur by Coulomb
titration method
1 Scope
This document specifies an alternative method of determining the total sulfur mass fraction of coal and
coke by high-temperature combustion by Coulomb titration.
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 687, Coke — Determination of moisture in the general analysis test sample
ISO 5068-2, Brown coals and lignites — Determination of moisture content — Part 2: Indirect gravimetric
method for moisture in the analysis sample
ISO 11722, Solid mineral fuels — Hard coal — Determination of moisture in the general analysis test sample by
drying in nitrogen
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology 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
In the presence of the catalyst, a sample of known mass is combusted at 1 150 °C in a stream of purified air.
The sulfur oxides are formed and dissolved in the electrolytic solution. The sulfur dioxides are titrated by the
iodine and bromine librated from the electrolysis of the potassium iodide and potassium bromide solution.
The quantity of the electricity consumed by electrolysis is integrated and used to calculate the mass fraction
of sulfur dioxides. The instrument is calibrated with certified reference materials. The percentage of total
sulfur in the sample is calculated according to the sulfur dioxides mass fraction and the prior calibration.
5 Reagent and materials
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
distilled water or water of equivalent purity.
5.1 Potassium iodide.
5.2 Potassium bromide.
5.3 Glacial acetic acid.
5.4 Self-indicating silica gel, industrial grade.
5.5 Sodium hydroxide, chemical grade.
5.6 Electrolytic solution, composed of 5 g of potassium iodide (5.1) and 5 g of potassium bromide (5.2)
dissolved in (250 to 300) ml of water, then add 10 ml of glacial acetic acid.
5.7 Glass wool.
5.8 Aluminosilicate wool.
5.9 Catalyst, tungsten trioxide.
5.10 Certified reference materials (CRMs), coal or coke.
6 Apparatus
6.1 Instrument, for determining sulfur by Coulomb titration, commercially available, consisting of the
following (see Figure 1).
6.2 Tube resistance furnace and combustion tube, capable of maintaining a temperature of
(1 150 ± 10) °C in the combustion zone which is at least 70 mm long, with the Pt-Rh-Pt thermocouple and
the temperature controller. The combustion tube is made of silicon or corundum, capable of withstanding
temperatures greater than 1 300 °C.
6.3 Electrolytic cell and magnetic stirrer.
The electrolytic cell consists of a cylindrical glass cup of over 400 ml capacity, with (120 to 180) mm high,
in which there are a pair of Pt electrolytic electrodes each of 150 mm area and a pair of Pt indicating
electrodes each of 15 mm area. The response time of indicating electrodes should be less than 1 s. The
magnetic stirrer with speed of 500 r/min and regulator is also provided.
6.4 Coulomb integrator.
The integration linearity error is less than 0,1 % at (0 to 350) mA of electrolyzing current, with a (4 to
6 digit) digital display or printer to give the mass fraction of the sulfur in the sample.
6.5 Program controller.
The sample can be pushed into and withdrawn from the combustion tube according to the specified program.
6.6 Purified air stream supply system, consisting of magnetic pumps and purifying tubes and a
flowmeter.
Supply an air stream at the rate of 1 500 ml/min and aspirating it at 1 000 ml/min (indicating by a flowmeter),
which is purified and dried by the sodium hydroxide and self-indicating silica gel packed in the purifying
tubes respectively (see Figure 2).
NOTE The sodium hydroxide is not necessary for the laboratories in which the air is not polluted by acid gas.
6.7 Combustion boat, made of unglazed porcelain or quartz, about 60 mm long at the part of packing the
sample, capable of withstanding over 1 200 °C.
6.8 Quartz tray, capable of withstanding over 1 200 °C and supporting the combustion boat moving in
and out the combustion tube.
6.9 Analytical balance, having a resolution of 0,1 mg.
Key
1 program controller
2 pulling stem
3 combustion tube
4 purified device for air stream
5 resistance furnace
6 sample boat
7 quartz tray
8 electrolytic cell
9 stirrer stem
10 magnetic stirrer
11 gas filter
Figure 1 — Equipment for determining sulfur by Coulomb titration
Key
1 tube resistance furnace and combustion tube
2 electrolytic cell
3 silica gel
4 flow meter, 1 000 mL/min
5 regulator
6 pump
7 air
8 silica gel
9 NaOH
10 pump
11 regulator
Figure 2 — Purified air stream supply system
7 Preparation of test sample
The coal or coke used for the determination of total sulfur is the general analysis test sample ground to pass
a sieve of 212 µm aperture, and for the coal or coke with sulfur higher than 5 % of the sulfur, the general
analysis test sample is ground to pass a sieve of 100 µm aperture.
Expose the sample in a thin layer for the minimum time required for the moisture mass fraction to reach
to approximate equilibrium with the laboratory atmosphere. Bef
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