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 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.