Overview
ISO 21436:2020 - "Pulps - Determination of lignin content - Acid hydrolysis method" specifies a laboratory procedure to measure total lignin in wood pulps using a two‑step sulfuric acid hydrolysis. The method is applicable to unbleached, bleached and semi‑bleached wood pulp with lignin content above 1% and is not generally intended for fully bleached chemical pulps (lignin too low for accurate determination). The standard defines sample preparation, hydrolysis procedures, filtration, gravimetric determination of acid‑insoluble (Klason) lignin, and spectrophotometric measurement of acid‑soluble lignin (UV absorbance at 205 nm).
Key Topics
- Principle: Two‑step sulfuric acid hydrolysis dissolves carbohydrates; the insoluble residue (Klason lignin) is filtered, dried and weighed. Soluble lignin is measured spectrophotometrically and summed with the insoluble fraction to give total lignin.
- Hydrolysis procedures:
- Procedure A (preferred): Primary hydrolysis with 72% H2SO4 at 30 °C for 1 h, dilute to ~4% and secondary hydrolysis in an autoclave at 120 °C for 1 h - faster and suitable when carbohydrate analysis is also required.
- Procedure B: Primary hydrolysis at 15–20 °C for 2 h, secondary hydrolysis in 3% H2SO4 at 100 °C for 4 h.
- Apparatus & reagents: filtration crucibles (fritted glass), autoclave (Procedure A), water bath (Procedure B), drying oven (105 °C), analytical balance (0.1 mg), UV‑Vis spectrophotometer with quartz cuvettes, 72% sulfuric acid and distilled water.
- Sampling & sample prep: Representative sampling per ISO 7213; grind/disintegrate to pass 0.85 mm; extract resins with acetone (ISO 14453) where required; determine moisture by ISO 638.
- Quality & reporting: Tests run in duplicate; calculate total lignin = acid‑insoluble + acid‑soluble lignin; precision and detailed test report requirements are included in the standard.
- Safety: Use caution - hazardous chemicals (concentrated sulfuric acid, acetone) and high temperatures are involved.
Applications
ISO 21436:2020 is used by:
- Quality control and process laboratories in pulp and paper mills for monitoring delignification and bleach demand.
- R&D and academic researchers studying pulping, bleaching and lignin chemistry.
- Contract testing laboratories providing standardized lignin content results for specifications, acceptance testing and process optimization.
Practical uses include assessing pulping efficiency, estimating bleaching chemical requirements, comparing processing routes, and supporting product specification and compliance.
Related Standards
- ISO 638 - Determination of dry matter (oven‑drying)
- ISO 7213 - Pulps - Sampling for testing
- ISO 14453 - Pulps - Acetone‑soluble matter (resin extraction)
- ISO 1762 - Determination of ash content
Keywords: ISO 21436:2020, lignin content, pulps, acid hydrolysis method, Klason lignin, acid‑insoluble lignin, acid‑soluble lignin, sulfuric acid hydrolysis, pulp testing, Procedure A, Procedure B.
