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ISO 16995:2015 — Determination of Water Soluble Chloride, Sodium and Potassium in Solid Biofuels
A comprehensive guide to the international standard for analysing key ionic species in solid biofuels to assess fouling and corrosion potential
ISO 16995:2015 specifies a method for the determination of the water-soluble chloride, sodium and potassium content in solid biofuels. These elements are critical indicators of the fouling and corrosion behaviour of biomass during thermal conversion. The standard is part of the ISO 17225 series on solid biofuels and is widely adopted, including as the Canadian national standard CAN/CSA ISO 16995-15. This article provides an in-depth review of the scope, technical requirements, implementation considerations and compliance aspects of the standard.
Scope and Application
ISO 16995:2015 applies to all solid biofuels such as wood chips, pellets, briquettes, agricultural residues, and non-woody biomass. The method quantitatively determines the mass fraction of chloride (as Cl–), sodium (as Na) and potassium (as K) that are extractable with water at ambient temperature. The water-soluble fraction is directly relevant to corrosion potential in combustion systems and to the formation of low-melting ash compounds that cause slagging and fouling.
Tip: For samples with very low moisture content, moisture correction as per ISO 18134 should be applied before calculation of dry-basis results.
The standard is intended for use by laboratories, fuel producers, and power plant operators who need reliable data on ionic species for fuel quality assessment, blending decisions, or environmental compliance. It is applicable to concentration ranges typically encountered in solid biofuels, from a few mg/kg up to several percent by mass on a dry basis.
Technical Requirements
Sample Preparation
Proper sample preparation is essential for reproducible results. The sample must be reduced to a particle size below 1 mm as specified in ISO 14780. A test portion of approximately 0.5–1 g (recorded to the nearest 0.1 mg) is used. The moisture content of the sample must be determined simultaneously on a separate portion according to ISO 18134-1 or ISO 18134-2 to permit dry-basis reporting.
Extraction Procedure
The water-soluble ions are extracted by shaking the test portion with 50 mL of distilled or deionised water (conductivity ≤ 0.1 mS/m) at room temperature for 60 ± 5 minutes using an end-over-end shaker. After extraction, the suspension is filtered through a 0.45 μm membrane filter. The filtrate is collected in a clean container and may be diluted further if needed to bring concentrations within the calibration range of the analytical instrument.
Warning: Glassware and filtration equipment must be thoroughly cleaned with dilute nitric acid and rinsed with deionised water to avoid contamination from previous samples or detergents.
Analytical Methods
The standard prescribes the following analytical techniques for the filtrate:
- Chloride: Ion chromatography (IC) with suppressed conductivity detection is the reference method. Alternatively, potentiometric titration may be used when IC is not available, but the user must demonstrate equivalence.
- Sodium and Potassium: Flame photometry (FP) or inductively coupled plasma optical emission spectrometry (ICP-OES) are recommended. Atomic absorption spectrometry (AAS) is also acceptable provided the calibration range is appropriate.
Calibration standards must be prepared from certified reference materials and matrix-matched where possible. The laboratory must verify linearity, detection limits, and repeatability before routine analysis.
| Analyte | Recommended Method | Working Range (mg/L in extract) | Limit of Quantification (dry basis, % w/w) |
|---|---|---|---|
| Chloride (Cl–) | Ion chromatography | 0.1 – 50 | 0.001 |
| Sodium (Na) | Flame photometry / ICP-OES | 0.1 – 20 | 0.001 |
| Potassium (K) | Flame photometry / ICP-OES | 0.1 – 40 | 0.001 |
Calculation and Expression of Results
The concentration of each analyte in the sample is calculated from the extract concentration, taking into account the dilution factor and the test portion mass. Results are expressed as mass percentage on a dry basis (%, w/w) using the moisture content determined on a separate test portion. The standard requires reporting to at least three significant figures unless the value is below the limit of quantification, in which case the result is reported as “< LOQ”.
Implementation Highlights
Implementing ISO 16995:2015 in a laboratory requires a modest investment in analytical equipment if only one of the three analytes is needed. However, for comprehensive fuel characterisation, ion chromatographs and ICP-OES are already present in most biomass testing facilities. Key points for smooth implementation include:
- Quality control: Run a certified reference material (e.g., a known biomass CRM) with every batch of 10 samples to verify accuracy.
- Blanks: Extract a blank (deionised water only) with each batch and subtract any background signal.
- Duplicate analyses: The standard recommends duplicate determinations; the relative standard deviation should not exceed 10 % for concentrations above 0.01 %.
- Instrument maintenance: For IC, ensure the column is properly conditioned and the suppressor is regenerated according to the manufacturer’s instructions.
Success: Laboratories that follow the quality assurance guidelines of ISO 16995:2015 typically achieve inter-laboratory reproducibility within 15 % for all three analytes at concentrations above 0.01 % (w/w).
Compliance and Quality Assurance Notes
Adherence to ISO 16995:2015 is often required for certification to the solid biofuels quality standards ISO 17225-1 to ISO 17225-7. When used in regulatory contexts, the laboratory must be accredited to ISO/IEC 17025 and include this method in its scope of accreditation. The standard does not specify acceptance limits; these are defined by the fuel specification or contract.
Important: The method only determines water-soluble species. For total chloride or total alkali content (e.g., from silicates or insoluble salts), alternative methods such as combustion ion chromatography or microwave acid digestion must be used.
Compliance also requires reporting the moisture content of the sample, the analytical method used for each element, and the limit of quantification achieved. The standard explicitly recommends that results be reported with an indication of the expanded measurement uncertainty (coverage factor k=2, 95 % confidence).
For users in Canada, the national adoption CAN/CSA ISO 16995-15 is technically identical to the ISO version. Canadian laboratories should verify any additional requirements from the Standards Council of Canada or the specific regulatory body.
Q: What is the difference between water-soluble and total chloride in biofuels?
A: Water-soluble chloride is the fraction that dissolves in water at room temperature, which is directly responsible for corrosion during combustion. Total chloride includes organically bound and insoluble forms (e.g., in some agricultural residues) and requires more aggressive digestion or combustion methods. ISO 16995:2015 only addresses the water-soluble fraction.
A: Water-soluble chloride is the fraction that dissolves in water at room temperature, which is directly responsible for corrosion during combustion. Total chloride includes organically bound and insoluble forms (e.g., in some agricultural residues) and requires more aggressive digestion or combustion methods. ISO 16995:2015 only addresses the water-soluble fraction.
Q: Can I use the same extract for all three analytes?
A: Yes. The standard procedure produces a single aqueous extract that can be analysed for chloride, sodium, and potassium. However, if the chloride concentration is very high, dilution may be needed for IC, while the same dilution may not be suitable for trace sodium. In such cases, prepare separate dilutions from the original extract.
A: Yes. The standard procedure produces a single aqueous extract that can be analysed for chloride, sodium, and potassium. However, if the chloride concentration is very high, dilution may be needed for IC, while the same dilution may not be suitable for trace sodium. In such cases, prepare separate dilutions from the original extract.
Q: Is the method applicable to torrefied biomass or biochar?
A: The standard is primarily intended for raw and processed solid biofuels. Torrefied materials and biochars may have altered solubility behaviour. The user should validate the method for such matrices by performing spike recovery tests. The extraction conditions (time, temperature, particle size) may need to be re-optimised.
A: The standard is primarily intended for raw and processed solid biofuels. Torrefied materials and biochars may have altered solubility behaviour. The user should validate the method for such matrices by performing spike recovery tests. The extraction conditions (time, temperature, particle size) may need to be re-optimised.
Q: What is the typical analysis time?
A: Excluding sample grinding and drying, the extraction step takes 60 minutes. Instrumental analysis for chloride (IC) is about 10–15 minutes per injection; sodium and potassium by flame photometry can be measured in less than 2 minutes per element. A batch of 10 samples (including blanks and QC) can be completed in a single working day.
A: Excluding sample grinding and drying, the extraction step takes 60 minutes. Instrumental analysis for chloride (IC) is about 10–15 minutes per injection; sodium and potassium by flame photometry can be measured in less than 2 minutes per element. A batch of 10 samples (including blanks and QC) can be completed in a single working day.
© 2026 — This article is provided for informational purposes and does not replace the official text of ISO 16995:2015 or CAN/CSA ISO 16995-15. Laboratory personnel should refer to the published standard for authoritative requirements.