ISO 21436:2020

INTERNATIONAL ISO
STANDARD 21436
First edition
2020-12
Pulps — Determination of lignin
content — Acid hydrolysis method
Pâtes — Détermination de la teneur en lignine — Méthode
d’hydrolyse acide
Reference number
ISO 21436:2020(E)
©
ISO 2020

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ISO 21436:2020(E)

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© ISO 2020
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ISO 21436:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Apparatus . 2
6 Reagents . 2
7 Sampling . 3
8 Test Specimens . 3
9 Procedure. 3
9.1 Hydrolysis . 3
9.1.1 General. 3
9.1.2 Hydrolysis procedure A. 4
9.1.3 Hydrolysis procedure B . . 4
9.2 Filtration . 5
9.3 Acid-insoluble lignin determination . 5
9.4 Acid-soluble lignin determination . 5
10 Calculation . 5
11 Precision . 6
12 Test Report . 6
Annex A (informative) Precision . 8
Bibliography .12
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ISO 21436:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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This document was prepared by Technical Committee ISO/TC 6, Paper, board and pulps.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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ISO 21436:2020(E)

Introduction
The main objective of measuring lignin in pulp is to assess the effect of a particular pulping or bleaching
process on the degree of delignification. In chemical pulping, the goal is to remove lignin from wood
with minimum degradation of the carbohydrates. The higher the level of residual lignin in any type of
unbleached pulp, the greater the amount of bleaching chemicals that is applied in order to achieve a
target brightness.
[1]-[3]
Comprehensive textbooks and reviews have been written on methods of lignin determination .
This document specifies one such method, commonly used for the determination of the total lignin
content of pulp. In this method, a pulp sample is treated with sulfuric acid, in a two-step (primary and
secondary) hydrolysis process, to solubilize the carbohydrates. Most of the lignin remains insoluble at
the end of the treatment and is filtered off, dried and weighed. This acid-insoluble lignin is also referred
to as “Klason lignin”.
A small portion of lignin is dissolved during acid hydrolysis of the pulp. This so-called acid-soluble
lignin is determined spectrophotometrically, from the UV absorbance at 205 nm of the filtrate from the
[4]-[6]
acid-insoluble lignin determination . The total lignin content is determined as the sum of the acid-
insoluble and acid-soluble lignin.
[7]-[9]
Two hydrolysis procedures are described in this document. In procedure A , the primary hydrolysis
is performed with 72 % sulfuric acid at 30 °C for one hour, followed by dilution with water to 4 %
[10][11]
sulfuric acid, and secondary hydrolysis in an autoclave at 120 °C for one hour. In procedure B , the
primary hydrolysis is done at 15-20 °C for two hours, followed by secondary hydrolysis at 3 % sulfuric
acid in a water bath at 100 °C for four hours. In procedure A, the use of 4 % sulfuric acid, instead of 3 %,
for secondary hydrolysis has no impact on the lignin analysis and is accepted when both lignin and
carbohydrates need to be analysed in the same sample.
Both procedures have been shown to give the same results; thus, either one can be used for determining
acid-insoluble lignin. However, procedure A is considerably more rapid, and the use of an autoclave
allows multiple samples to be hydrolysed simultaneously with minimum supervision. As such, it is now
more commonly used in laboratories equipped with an autoclave. It is therefore the preferred method
and should be used when analysis of carbohydrates is required in addition to the determination of lignin.
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INTERNATIONAL STANDARD ISO 21436:2020(E)
Pulps — Determination of lignin content — Acid
hydrolysis method
WARNING — This method involves the use of hazardous chemicals. Care should be taken to
ensure that the relevant precautions are taken.
1 Scope
The method is applicable to unbleached, bleached and semi-bleached wood pulp with a lignin content
above 1 %. It is not generally intended for fully bleached chemical pulp, because the lignin content in
these pulps is too low to be determined accurately.
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 638, Paper, board and pulps — Determination of dry matter content — Oven-drying method
ISO 1762, Paper, board, pulps and cellulose nanomaterials — Determination of residue (ash content) on
ignition at 525 °C
ISO 7213, Pulps — Sampling for testing
ISO 14453, Pulps — Determination of acetone-soluble matter
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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 http:// www .electropedia .org/
3.1
lignins
class of complex, organic macromolecules, containing aromatic sub-units, that play a key role in the
formation of cell walls in wood and bark, conferring mechanical strength and rigidity to cell walls and
to plants as a whole
Note 1 to entry: Lignin is the main non-carbohydrate constitutent of wood.
3.2
acid-insoluble (Klason) lignin
residue after treating wood or pulp with sulfuric acid in a two-step hydrolysis procedure to solubilize
the carbohydrates into monosaccharides
3.3
acid-soluble lignin
portion of lignin (3.1) which is soluble during the acid-insoluble lignin determination
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ISO 21436:2020(E)

3.4
acetone-soluble matter
amount of material that can be extracted with acetone from a sample of pulp by the method specified
in ISO 14453
4 Principle
A pulp sample is treated with sulfuric acid in a two-step (primary and secondary) hydrolysis process to
dissolve the carbohydrates. The residue after hydrolysis is filtered off, dried, and weighed, and referred
to as acid-insoluble or Klason lignin. A small amount of lignin is dissolved during acid hydrolysis. This
so-called acid-soluble lignin is determined by measuring the absorbance at 205 nm of the filtrate from
the acid-insoluble lignin determination. The total lignin content is determined as the sum of the acid-
insoluble lignin and acid-soluble lignin.
5 Apparatus
5.1 Filtration equipment: filtering flask; filtering crucible, fritted glass, medium or fine porosity,
30 ml; adapter for the filtering crucible, siphon tube (optional).
NOTE The choice of fritted glass porosity depends on the rate of filtration of the particular type of sample.
For slow filtering samples, the use of medium (M) porosity is preferable. In low-yield sulfite pulps especially,
lignin forms a fine dispersion and clogs the pores of the filter. Filtration can be facilitated by using a medium
porosity crucible with a disc of fine porosity glass-fibre filter paper fitted over the sintered glass in the crucible.
Other types of filtering crucibles, such as alundum or porous porcelain crucibles lined with a mat of
glass fibres can also be used.
5.2 Constant temperature water bath, capable of maintaining a temperature of 30 ± 1 °C
(Procedure A); or 20 ± 1 °C (Procedure B).
5.3 Flask, Erlenmeyer, 1 000 ml, or smaller depending on the size of the pulp specimen.
5.4 Procedure A only: Autoclave, capable of maintaining a temperature of 120 ± 3 °C.
5.5 Drying oven, convection type, maintained at 105 ± 2 °C.
5.6 Analytical balance, accurate to 0,1 mg.
5.7 Spectrophotometer, UV-visible, diode array or simple wavelength, with high purity quartz
cuvettes of pathlength 1 cm.
6 Reagents
6.1 Water, distilled or deionized.
6.2 Sulfuric acid, 72 % w/w (specific gravity 1,633 8 at 20 °C). 72 % sulfuric acid is available
commercially. It can also be prepared from concentrated sulfuric acid as follows:
Slowly add 650 ml of concentrated sulfuric acid (H SO sp gr 1,84) to 400 ml of water, while cooling
2 4
under a cold water trap. When the temperature has reached equilibrium with the ambient temperature,
adjust the specific gravity of the sulfuric acid solution to 1,633 8, with the use of a hydrometer, by
careful addition of concentrated sulfuric acid or water.
6.3 Acetone, only if extraction of the sample is required prior to hydrolysis with sulfuric acid.
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ISO 21436:2020(E)

7 Sampling
If the analysis is being made to evaluate a lot of a consignment of pulp, the sample shall be taken in
accordance with ISO 7213. If the analysis is made on another type of sample, report the origin of the
sample, and if possible the sampling procedure.
Obtain a representative sample of pulp equivalent to about 10 g moisture-free pulp. Air dry the pulp
and disintegrate in a household blender, or grind in a Wiley mill to pass a No. 20 (0,85 mm) screen.
Groundwood and high yield pulps containing a significant amount of resins shall be extracted with
acetone (6.3) according to ISO 14453 before testing.
NOTE Resins, if not extracted from the pulp prior to analysis, would remain insoluble in acid and be weighed
as lignin.
NOTE Acetone is considered an effective solvent for extracting resin from pulp. Dichloromethane and
ethanol/benzene (1:2), as specified in other methods, are not recommended due to health hazards. In particular,
benzene is a confirmed carcinogen.
Determine the moisture content of the pulp according to ISO 638 by drying a 2-3 g specimen in an
oven at 105 ± 3 °C to constant weight. If the pulp shall be pre-extracted, the moisture content shall be
determined on the extracted pulp.
8 Test Specimens
Weigh (5.6) a test specimen, equivalent to 300 mg to 1,0 g of oven-dried mass of pulp to the nearest
0,1 mg (see NOTE) and transfer to a 200 ml beaker. Make sure that the test specimens taken are
representative of the sample received.
The mass of the test specimen is such as to provide a minimum of 20 mg of lignin in the residue
remaining after acid hydrolysis, for accurate weighing. The following can be used as a guide to the
amount of pulp that should be selected for analysis, based on the lignin content:
— >10 % lignin: 300 mg pulp
— 5-10 % lignin: 500 mg pulp
— <5 % lignin: 1,0 g pulp
NOTE The amount of pulp selected for analysis also depends on whether hydrolysis procedure A or
hydrolysis procedure B is used.
9 Procedure
Run the entire procedure in duplicate.
9.1 Hydrolysis
9.1.1 General
[7]-[9]
Two hydrolysis procedures are described in this Standard. In procedure A , the primary hydrolysis
is performed at 30 °C for 1 h, followed by secondary hydrolysis in an autoclave at 120 °C for 1 h. In
[10][11]
procedure
...