ISO 5073:2021

Brown coals and lignites — Determination of humic acids

This document specifies volumetric methods for the determination of total humic acids and free humic acid of brown coals and lignites.

Charbons bruns et lignites — Dosage des acides humiques

General Information

• Status: Published
• Publication Date: 08-Dec-2021
• ICS: 73.040 - Coals
• Technical Committee: ISO/TC 27/SC 5 - Methods of analysis
• Drafting Committee: ISO/TC 27/SC 5/WG 7 - Brown coals and lignites
• Current Stage: 6060 - International Standard published
• Start Date: 09-Dec-2021
• Due Date: 17-Oct-2022
• Completion Date: 09-Dec-2021

Relations

• Revises: ISO 5073:2013 - Brown coals and lignites — Determination of humic acids
• Effective Date: 23-Apr-2020

Overview - ISO 5073:2021 (Brown coals and lignites - Determination of humic acids)

ISO 5073:2021 specifies volumetric methods for determining total humic acids and free humic acid in brown coals and lignites. The standard defines sample preparation, extraction, oxidation and titration steps using established reagents and apparatus, and provides the calculation and reporting rules needed for repeatable analytical results. This fourth edition updates terms, extraction and calculation details and includes Annex A for site‑specific calibration of the carbon-to-humic-acid ratio.

Key topics and technical requirements

• Scope: Volumetric determination of humic acids (total and free) in brown coals and lignites.
• Principle: Alkaline extraction (sodium pyrophosphate for total; sodium hydroxide for free), oxidation of carbon in extracts with potassium dichromate, and back‑titration of excess dichromate with ammonium ferrous sulfate using 1,10‑phenanthroline indicator.
• Reagents & concentrations: Defined preparation of sodium pyrophosphate extraction solution, NaOH extraction solution, K2Cr2O7 oxidizing solutions and standardized ammonium ferrous sulfate titrant (standardization against 0.1 mol/L dichromate).
• Apparatus & conditions: Thermostatically controlled water bath at (100 ± 1) °C, precision balance (0.1 mg), specified flask and filtration practice.
• Sample preparation: Test sample sized to nominal top size 212 µm prepared per ISO 13909‑4 or ISO 18283; moisture determination as per ISO 5068‑2; samples equilibrated to laboratory atmosphere and mixed thoroughly.
• Procedure details: Typical bench parameters given (e.g., ~0.200 g sample, 150 mL extraction solution, 2 h extraction at 100 °C, 5 mL aliquots for oxidation/titration).
• Calculation: Mass fraction calculation converts titration volumes to humic acid percent using an average carbon-to-humic-acid ratio of 0.59 (Annex A describes redetermination for local applicability).
• Quality: Includes precision statements and repeatability considerations; full test report requirements are specified.

Applications and who uses ISO 5073:2021

• Coal and lignite testing laboratories performing routine quality and compositional analyses.
• Power generation and mining companies monitoring fuel quality and organic matter composition.
• Environmental and soil scientists assessing humic substances released from coals or for comparative studies.
• Standards bodies and regulatory labs that require a validated method for reporting humic acid content.
• Research institutions investigating brown coal chemistry or humic acid extraction techniques.

Related standards (referenced)

• ISO 1171 (ash determination)
• ISO 1213‑2 (vocabulary)
• ISO 5068‑2 (moisture in analysis sample)
• ISO 13909‑4 and ISO 18283 (sampling and sample preparation)
• Annex A of ISO 5073:2021 for local carbon-to-humic-acid ratio calibration

ISO 5073:2021 is a practical, repeatable volumetric method tailored for reliable humic acid measurement in brown coals and lignites, supporting quality control, regulatory compliance and research needs in the coal sector.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 5073
Fourth edition
2021-12
Brown coals and lignites —
Determination of humic acids
Charbons bruns et lignites — Dosage des acides humiques
Reference number
© ISO 2021
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
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
4.1 Total humic acids . 2
4.2 Free humic acid . 2
5 Reagents . 2
6 Apparatus . 3
7 Sample . 3
8 Procedure .3
8.1 Extraction of total humic acids . 3
8.2 Extraction of free humic acid . 3
8.3 Determination of humic acids in extracts . 4
8.3.1 For total humic acids . 4
8.3.2 For free humic acids . 4
9 Calculation and expression of results . 4
10 Precision . 5
10.1 Repeatability limit . 5
11 Test report . 5
Annex A (normative) Determination of the ratio of carbon mass fraction to humic acids
mass fraction of brown coals and lignites . 6
iii
Foreword
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www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 27, Coal and Coke, Subcommittee SC 5,
Methods of analysis.
This fourth edition cancels and replaces the third edition (ISO 5073:2013), of which it constitutes a
minor revision. The changes compared to the previous edition are as follows:
— referenced documents have been updated;
— terms and definitions have been added;
— sample has been added;
— determination of humic acids in extracts has been amended;
— calculation and expression of results have been amended;
— precision has been amended;
— test report has been amended;
— Annex A has been amended.
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
In this document, humic acids are determined by the volumetric method with titration of extracts.
The test is empirical and, in order to ensure reproducible results, it is essential that the composition of
the extraction solution, the temperature and the time of extraction be carefully controlled. The value
0,59, which is the average ratio of carbon content of humic acids for many brown coals and lignites, has
been proved to be applicable. Another value may also be used, predetermined as described in Annex A,
and applicable to different countries or locations.
v
INTERNATIONAL STANDARD ISO 5073:2021(E)
Brown coals and lignites — Determination of humic acids
1 Scope
This document specifies volumetric methods for the determination of total humic acids and free humic
acid of brown coals and lignites.
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 1171, Solid mineral fuels — Determination of ash
ISO 1213-2, Solid mineral fuels — Vocabulary — Part 2: Terms relating to sampling, testing and analysis
ISO 5068-2, Brown coals and lignites — Determination of moisture content — Part 2: Indirect gravimetric
method for moisture in the analysis sample
ISO 13909-4, Hard coal and coke — Mechanical sampling — Part 4: Coal — Preparation of test samples
ISO 18283, Coal and coke — Manual sampling
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1213-2 and the following
apply.
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/
3.1
humic acids
group of complex organic, amorphous compounds of high relative molecular mass which occur as free
acid and as metal salts (humates)
3.2
total humic acids
humic acids (3.1) extracted by an alkaline sodium pyrophosphate solution
3.3
free humic acid
humic acid (3.1) extracted by a sodium hydroxide solution
4 Principle
4.1 Total humic acids
Extraction of an analysis sample of coal is carried out with an alkaline sodium pyrophosphate solution.
Carbon in the humic acid extracts is oxidized with potassium dichromate followed by titration of the
excess dichromate with ammonium ferrous sulfate standard solution.
4.2 Free humic acid
Extraction of an analysis sample of coal is carried out with a sodium hydroxide solution. Carbon in
the humic acid extracts is oxidized with potassium dichromate followed by titration of the excess
dichromate with ammonium ferrous sulfate standard solution.
5 Reagents
5.1 Sodium pyrophosphate, alkaline extraction solution.
Dissolve 15 g of crystalline sodium pyrophosphate (Na P O ·10H O) and 7 g of sodium hydroxide in 1 l
4 2 7 2
of water.
5.2 Sodium hydroxide solution.
Dissolve 10 g of sodium hydroxide in 1 l of water.
5.3 Potassium dichromate standard solution, c(1/6 K Cr O ) = 0,1 mol/l.
2 2 7
Dissolve 4,903 6 g of potassium dichromate, previously dried at 130 °C, in water. Transfer to a 1 l
volumetric flask and dilute to the mark with water.
5.4 Potassium dichromate oxidizing solution, c(1/6 K Cr O ) = 0,4 mol/l.
2 2 7
Dissolve 20 g of potassium dichromate in 1 l of water.
5.5 Ammo
...