ISO 625:2025

Coal and coke - Determination of carbon and hydrogen - Liebig method

This document specifies a method of determining the total carbon and the total hydrogen in coal and coke, by the Liebig method.

Charbon et coke — Dosage du carbone et de l'hydrogène — Méthode de Liebig

General Information

• Status: Published
• Publication Date: 07-Aug-2025
• ICS: 73.040 - Coals
• ICS: 75.160.10 - Solid fuels
• Technical Committee: ISO/TC 27/SC 5 - Methods of analysis
• Drafting Committee: ISO/TC 27/SC 5 - Methods of analysis
• Current Stage: 6060 - International Standard published
• Start Date: 08-Aug-2025
• Due Date: 20-May-2026
• Completion Date: 08-Aug-2025

Relations

• Revises: ISO 625:1996 - Solid mineral fuels - Determination of carbon and hydrogen - Liebig method
• Effective Date: 27-May-2023
• Revises: ISO 625:1996/Cor 1:1996 - Solid mineral fuels - Determination of carbon and hydrogen - Liebig method - Technical Corrigendum 1
• Effective Date: 27-May-2023

Overview

ISO 625:2025 - "Coal and coke - Determination of carbon and hydrogen - Liebig method" specifies a laboratory procedure for measuring total carbon and total hydrogen in coal and coke by combustion (Liebig method). The method combusts a known mass of sample in oxygen, oxidizes incomplete combustion products over copper oxide, and collects the resulting water and carbon dioxide for gravimetric determination. The standard replaces ISO 625:1996 and updates apparatus, reagents and procedural details.

Key technical topics and requirements

• Principle: Complete combustion in oxygen, secondary oxidation over copper oxide, absorption of products and gravimetric measurement. Corrections are applied for moisture and carbonate carbon when required.
• Apparatus: Detailed requirements for combustion and absorption systems, including:
  • Combustion tube (fused silica or hard glass), typical length 1.25 m and diameter 12–15 mm.
  • Three furnaces (approx. heated zones described) to achieve required temperatures (e.g., boat region ~925 °C, copper oxide region ~800 °C, lead chromate/Ag gauze area ~500 °C).
  • Combustion boat (platinum/porcelain/fused silica) and analytical balance (0.1 mg resolution).
  • Purification train for oxygen (U-tubes with magnesium perchlorate and sodium hydroxide).
  • Absorption train using Midvale tubes and a guard tube with reagents in sequence: magnesium perchlorate, manganese dioxide, sodium hydroxide, etc.
• Reagents and packing: Specifications for magnesium perchlorate, sodium hydroxide on inert base (self-indicating preferred), granular manganese dioxide, copper oxide wire, lead chromate, and pure silver gauze.
• Preparation and procedure: Clauses cover conditioning of tubes, packing of absorbents, sample preparation, blank tests, and expression of results (total carbon, organic carbon, total hydrogen, hydrogen corrected for moisture).
• Safety & handling: Warnings on toxic reagents and disposal (e.g., do not attempt to regenerate magnesium perchlorate due to explosion risk).
• Quality aspects: Clauses on precision (repeatability and reproducibility limits) and required test report content.

Practical applications and who uses it

• Laboratories performing proximate/ultimate analysis of coal and coke for quality control, fuel specification and research.
• Coal producers, coke plants, power generation and metallurgical industries monitoring fuel composition.
• Environmental and regulatory testing labs requiring accurate carbon and hydrogen determinations for emissions accounting or material characterization.
• Standards bodies and test method developers comparing combustion-based methods (ISO 625 vs alternative methods such as ISO 609).

Related standards

• ISO 609 - alternative method for C and H determination (referenced in the introduction).
• Normative references that support sampling and moisture corrections: ISO 687, ISO 925, ISO 5068-2, ISO 11722, ISO 13909-4/6, ISO 18283.

Keywords: ISO 625:2025, Liebig method, coal and coke analysis, determination of carbon and hydrogen, combustion analysis, absorption train, Midvale tube, magnesium perchlorate.

Standards Content (Sample)

International
Standard
ISO 625
Third edition
Coal and coke — Determination
2025-08
of carbon and hydrogen —
Liebig method
Charbon et coke — Dosage du carbone et de l'hydrogène —
Méthode de Liebig
Reference number
© ISO 2025
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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or ISO’s member body in the country of the requester.
ISO copyright office
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents and materials . 2
6 Apparatus . 2
7 Preparation of the apparatus . 7
7.1 Preparation of the combustion tube .7
7.2 Conditioning of the combustion tube .8
7.3 Conditioning of the absorption train .8
8 Preparation of test sample . 9
9 Procedure . 9
10 Blank test . 9
11 Expression of results . 10
11.1 Total carbon .10
11.2 Organic carbon .10
11.3 Total hydrogen .10
11.4 Total hydrogen mass fraction, less that present as moisture .11
11.5 Additional information.11
12 Precision .11
12.1 Repeatability limit .11
12.2 Reproducibility limit .11
13 Test report .11
Annex A (informative) Derivation of factors used in calculations .12
Bibliography . 14

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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related to conformity assessment, as well as information about ISO's adherence to the World Trade
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This document was prepared by Technical Committee ISO/TC 27, Coal and coke, Subcommittee SC 5, Methods
of analysis.
This third edition cancels and replaces the second edition (ISO 625:1996), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 625:1996/Cor. 1:1996.
The main changes are as follows:
— the normative references have been updated;
— the mandatory terms and definitions clause (Clause 3) has been added and subsequent clauses have been
renumbered;
— Clause 5 has been updated;
— Figures 1, 2, 3 and 4 have been updated;
— Clause 9 has been updated;
— Clause 11 has been revised;
— the test report has been updated;
— Annex A has been revised.
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
An alternative method to the one specified in this document is given in ISO 609.

v
International Standard ISO 625:2025(en)
Coal and coke — Determination of carbon and hydrogen —
Liebig method
1 Scope
This document specifies a method of determining the total carbon and the total hydrogen in coal and coke,
by the Liebig method.
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 925, Solid mineral fuels — Determination of carbonate carbon content — Gravimetric method
ISO 5068-2, Brown coals and lignites — Determination of moisture — 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
ISO 13909-4, Coal and coke — Mechanical sampling — Part 4: Preparation of test samples of coal
ISO 13909-6, Coal and coke — Mechanical sampling — Part 6: Preparation of test samples of coke
ISO 18283, Coal and coke — Manual sampling
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
A known mass of coal or coke is burnt in a current of oxygen in a tube impervious to gases, the products of
the incomplete combustion being further burnt over copper oxide; all the hydrogen is converted to water
and all the carbon to carbon dioxide. These products are absorbed by suitable reagents and determined
gravimetrically. Oxides of sulfur are retained by lead chromate, chlorine by a silver gauze roll and oxides of
nitrogen by granular manganese dioxide.
The results include the carbon in the carbonates and the hydrogen combined in the moisture and in the water
of constitution of silicates. A determination of moisture is carried out at the same time, and an appropriate
correction is applied to the hydrogen value obtained by combustion. A determination of carbon dioxide may
also be made and the total carbon value corrected for the presence of mineral carbonates.

5 Reagents and materials
WARNING — Care shall be exercised when handling reagents, many of which are toxic.
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
distilled water or water of equivalent purity.
5.1 Magnesium perchlorate, anhydrous, with a particle size of less than 1,2 mm and preferably in the
size range from 1,2 mm to 0,7 mm.
WARNING — Determine any local regulations when disposing of exhausted magnesium perchlorate
prior to use. Regeneration of magnesium perchlorate shall not be attempted, owing to the risk of
explosion.
5.2 Sodium hydroxide on an inert base, preferably of a coarse grading, for example 3,0 mm to 1,5 mm,
but not finer than the grading 1,2 mm to 0,7 mm, and preferably of the self-indicating type.
5.3 Manganese dioxide, granular, 1,2 mm to 0,7 mm.
Manganese dioxide in the granular form and the size required can be prepared as follows.
Dissolve manganese sulfate in water and boil the solution. Make the solution alkaline with ammonium
hydroxide (5.10) and add solid ammonium persulfate (5.11), in small portions, to the boiling solution until
precipitation is complete. Filter through a hardened fast-filter paper, wash with water by decantation, then
with dilute sulfuric acid (5.12) and finally with water until acid-free. Transfer the moist precipitate to a
mortar (6.11) and place in an oven until most of the water has evaporated, but the powder is still damp.
Press the mass into a cake with a pestle (6.11), using firm pressure. Complete the drying, break up the cake
cautiously and sieve to separate the 1,2 mm to 0,7 mm size.
5.4 Copper gauze, of mesh approximately 1 mm and 10 mm wide.
5.5 Copper oxide, wire form, chopped to particles approximately 3 mm long with a diameter of
approximately 0,2 mm.
5.6 Lead chromate, fused, size range 2,4 mm to 1,2 mm.
5.7 Pure silver gauze, of mesh approximately 1 mm, made of wire approximately 0,3 mm in diameter.
5.8 Oxygen, hydrogen-free, preferably prepared from liquid air and not by electrolysis. Electrolytically
prepared oxygen shall be passed over red-hot copper oxide before use to remove any trace of hydrogen.
5.9 Glass wool.
5.10 Ammonium hydroxide, concentrated solution, mass fraction not less than 25 %.
5.11 Ammonium persulfate, solid.
5.12 Dilute sulfuric acid, with volume fraction of 50 %.
6 Apparatus
6.1 Analytical balance, capable of determining the mass to the nearest 0,1 mg.

6.2 Purification train, for absorbing water vapour and carbon dioxide from the oxygen used for the
combustion. Assemble the train using a series of U-tubes containing the following reagents in the order
stated, in the direction of flow:
a) magnesium perchlorate (5.1) for absorbing water;
b) sodium hydroxide on an inert base (5.2) for absorbing carbon dioxide;
c) magnesium perchlorate (5.1) for absorbing the water evolved in the reaction between carbon dioxide
and sodium hydroxide.
The purification train shall be large enough to render frequent recharging unnecessary, even with
continuous use.
6.3 Combustion assembly
6.3.1 Furnaces. The combustion tube is heated by three furnaces. For the 1,25 m combustion tube
described in Clause 7, the following approximate lengths are appropriate:
a) furnace No. 1 (to heat the boat and its contents to 925 °C) — 250 mm;
b) furnace No. 2 (to keep the entire copper oxide section of the tube heated to 800 °C) — 500 mm;
c) furnace No. 3 (to cover the lead chromate and the roll of pure silver gauze and to heat the former to
about 500 °C) — 200 mm.
6.3.2 Combustion tube, of fused silica or suitable hard glass. The diameter of the tube shall be 12 mm to
15 mm. A suitable length is 1,25 m.
6.3.3 Combustion boat, of platinum, porcelain or fused silica, approximately 70 mm long.
6.4 Absorption train, for absorbing the water and carbon dioxide evolved by the combustion of the
sample. Assemble the train using the following reagents in the order stated, in the direction of flow.
a) magnesium perchlorate (5.1) for absorbing the water evolved during the combustion;
b) granular manganese dioxide (5.3) for absorbing oxides of nitrogen;
c) magnesium perchlorate (5.1) for absorbing the water evolved from the manganese dioxide;
d) sodium hydroxide on an inert base (5.2) for absorbing carbon dioxide;
e) magnesium perchlorate (5.1) for absorbing the water produced in the reaction between carbon dioxide
and sodium hydroxide.
Midvale tubes (Figure 1), which provide a large area of reaction, are used for all the reagents except
manganese dioxide, which is contained in a guard tube (Figure 2), providing a long contact time with
minimum mass.
A typical absorption train, with details of the packing, is shown in Figure 3. A is the absorber for water, В is a
guard-tube absorber for oxides of nitrogen, and С is the absorber for any water evolved from the manganese
dioxide. Carbon dioxide is absorbed in D, the magnesium perchlorate in the upper portion absorbing any
water produced in the reaction between carbon dioxide and sodium hydroxide. A second carbon dioxide
absorber, E, shall be added as a precautionary measure.

Dimensions in millimetres
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