Overview - ISO 6350:2024 (Lignins, dry matter content)
ISO 6350:2024 specifies laboratory methods for determining the dry matter content of lignins using two validated drying techniques: oven‑drying and freeze‑drying (lyophilization). The standard applies to solid lignins from kraft, organosolv, soda and sulfite pulping processes, and to lignins recovered after enzymatic or acid hydrolysis of biomass. Important: oven‑drying is not applicable to kraft lignins in the base (sodium) form. Freeze‑drying is recommended when preserving lignin integrity or when preparing samples for additional analyses.
Key topics and technical requirements
- Scope and applicability
- Applies only to lignins in solid form (not slurries unless frozen and then lyophilized).
- Methods cover kraft, organosolv, soda, sulfite and hydrolysis lignins.
- Sampling and sample handling
- Representative sample: about 2 g to 3 g (air‑dry basis) for sampling; test specimen typically 1 g to 2 g.
- Store airtight; if stored >2–3 days, refrigerate at 5 °C ± 1 °C; bring to room temperature before opening.
- Precondition samples: air‑dry to >75% solids to minimize degradation during final drying.
- Oven‑drying method
- Use water‑vapour‑proof containers, previously dried to constant mass.
- Dry at 105 °C ± 2 °C: initial 4 h, then repeat 1 h increments until constant mass (difference ≤ 0.1% of pre‑drying test piece mass). Total drying not to exceed 7 h.
- Not suitable for kraft lignins in sodium/base form.
- Freeze‑drying (lyophilization)
- Freeze specimen (overnight or ≥4 h) then lyophilize (typical conditions –80 °C, ~25 mTorr) for 24 h increments until constant mass (≤ 0.1% change).
- Preferred method to prevent thermal degradation and loss of VOCs; required when drying prior to other lignin analyses.
- Apparatus and accuracy
- Analytical balance accuracy: 0.1 mg.
- Desiccator, freeze‑dryer, ventilated oven and suitable containers specified.
- Calculation and precision
- Dry matter (%) = (mass after drying / mass before drying) × 100.
- Precision data (repeatability and reproducibility) are reported from a round‑robin study (see Annex A).
Applications and users
- Laboratories in pulp & paper, biorefineries and biomass characterization.
- Quality control (QC) for lignin production and product specification.
- Research groups studying lignin composition, thermal stability, or downstream conversion (chemistry, materials, adhesives).
- Regulatory and procurement testing where standardized solids content reporting is required.
Related standards and references
- ISO 6350 complements existing biomass compositional and solids/extractives procedures (for example, documented NREL methods referenced in the introduction).
- Prepared by ISO/TC 6 (Paper, board and pulps); users should consult Annex A for precision data and the ISO Online Browsing Platform for related terminology.
Keywords: ISO 6350:2024, lignins dry matter content, oven‑drying, freeze‑drying, lyophilization, kraft lignin, organosolv, biomass analysis.